Uzi!…. For years the mere utterance of the word has sent the heart pounding and the pulse racing in the minds of soldiers, Hollywood producers, and homegrown Walter Mitty types alike. This near mythical creation out of the brilliant mind of Uziel Gal (Galilei) has risen from the depths of despair of a nearly stillborn young nation, to its current status as THE defining image of at least three generations. There is almost no place on earth that the name, or image, of the Uzi Submachine gun cannot be found such that this universal familiarity has led to its being one of the most popular firearms on the collectors market today.

Available Guns

Unfortunately for the NFA weapons collector in these United States, the Uzi SMG is not as readily available as its worldwide status would suggest. Due to our controlling firearms laws and the market demands of past years, the collectible versions of the Uzi have come to be defined by three distinct variations.

Any fully transferable factory produced, foreign made gun (SMG) would have to have been imported prior to 1968 and the enactment of the Gun Control Act. In this category there exist two variations of the type: the Israeli/IMI produced originals, and the license built clones produced in Belgium by FN-Herstal. But before you start looking for one of those be aware that very few of either origin were ever imported prior to 1968 and they command a premium price!

The only other option for a Factory produced SMG is either the large number of dealer sample/law enforcement guns that were imported between 1968-1986 (which are restricted to acquisition by civilians who are Special Occupational Tax Payers only), or the US manufactured SMGs produced in the middle 1980s by Group Industries Inc. of Louisville, Ky. The Group guns are factory produced SMGs, accurate and exact in every detail, as they were manufactured on jigs and tooling obtained from FN, often using original FN produced parts obtained in the deal. They are also all fully transferable. Group guns are available for approximately less than half the cost of a Pre-68, fully-transferable, foreign produced, original gun and they are just as good as the originals. The differences are in finish only, and as a bonus the Group guns were offered from the factory in .45ACP and .22LR calibers, in addition to the standard 9mm caliber. It has been reported that very few of the non-9mm calibers were delivered from the factory as such. Group also sold caliber conversion kits as an accessory item. There have been a few problems with later Group Industries Uzis, mostly related to the bolts. Installing a factory Uzi bolt seems to alleviate the problems.

The last, and most common, version of the NFA collectible Uzi SMG are those guns that originated as Title I semi-auto carbines, and were later converted to select fire and registered with the NFA (National Firearms Act Branch of BATF) as transferable SMGs. These guns come in two distinct flavors. First is the “registered receiver conversion”, whose registration status allows the receiver to be modified to the factory original SMG configuration. The second is the “registered bolt conversion”, which employs as the registered item a special bolt that has been manufactured to fit and function in the receiver of a standard, unmodified, semi-auto carbine but in the select fire, open bolt, mode of operation. This type of conversion cannot legally employ some of the standard SMG features that would require receiver modifications, as discussed below.

These two types of conversion guns are our topic of investigation, as there are many variations. As with all conversions, there exist guns that were converted correctly and safely, and there are those that bear watching out for. A thorough understanding of the accepted practices used in a proper conversion of each type will allow the prospective buyer or current owner to gain maximum enjoyment from this gun, while avoiding a potentially bad experience.

The Semi-Auto Carbine

It must be first understood that Action Arms Inc., over the course of the years they were importing the semi-auto guns, imported two different versions the Uzi semi-auto carbine. These versions were officially known as the Model A and the Model B, and they were essentially identical mechanically, except for certain small differences, some of which were also being incorporated into the factory SMGs at this time too. The most noticeable difference was the arrangement of their sighting systems. On the Model A the sights are identical to the original SMG design, in which all adjustability for windage and elevation is accomplished by movement of the front sight elements. The rear sight is a fixed peep, adjustable only for two range settings of 50m and 100m. Part of the problem was not taking into account the change from a 10 inch barrel on the SMG, to the 16 inch barrel on the semi-auto. The Model B incorporated a new sighting system which allowed elevation adjustments on the front sight post, and windage from the rear sight peep, in addition to the range selections. The operational difference is that the Model B does not require any special tool to make adjustments, whereas the Model A requires a dedicated sight tool. Also, on the Model B, the front sling swivel rotates a full 360 degrees, while on the Model A it cannot rotate through a full arc. On early versions of the Model A the bolt face was also slightly different. Early imports had a full cartridge seating bottom rim, whereas later Model A (and all Model B guns) incorporate a relieved lower cartridge seat face to hinder easy conversion to full-auto fire. This became an important distinction when legally converting these guns, as discussed below. There is a perception that the Model A was a better candidate for conversions, as it was closer to the original SMG and could be converted more easily, in some cases. On a few of the very early examples of the Model A, the blocking rail was not adequately welded, which may be the root of that bit of MG Lore.

As an aside it must be pointed out, that while all Israeli manufactured semi-auto carbines were imported by Action Arms Inc., there was also a clone of the semi-auto carbine made by Group Industries here in the USA Group Industries started making SMGs and when the 1986 making ban occurred, they tooled up for the semi-auto guns, of which there are a small number about, but nowhere near as many as of the Action Arms imports. And though it has never been reported that the Group semis were available in time to be receiver registered, as transferable SMG conversions prior to the 1986 making ban, many have apparently been used as the host guns when installing NFA registered, conversion bolts. The design and quality of Group Industries guns rivals that of the Israeli originals, and there should be no hesitation in their employment as host guns for bolt conversions. For our purposes here we are only concerned with original Israeli-made guns, and possibly these Group Industries clones when used as the basis for a conversion with a Registered Bolt. It should be noted that there were imported at differing times, and by differing importers, unlicensed, cheap copies of the semi-auto carbine made by Norinco in China. These poor quality guns made it in under the name of “Officers’ 9”, long before the semi-auto import ban, and long before the ‘current’ rash of Norinco Uzi Carbine clones, imported as sporter rifles commonly seen with thumbhole, sporter style fixed stocks. Despite when (and under which name) these Chinese copies are encountered, it must be very clearly stated that they are extremely poor copies, with regard to metallurgy, fit, and finish. They are so poor as to preclude discussion here. Due to the timing involved, very few of the Chinese guns would seem to have been in-country and able to be registered prior to the 1986 making ban, but some may have been used for an SMG conversion with one of the many legally registered conversion bolts. Under no circumstances could this be recommended, as these guns are so poorly made as to be potentially impossible to make them function reliably with a conversion bolt installed.

What’s So Different About a Semi-auto Carbine?

Like other semi-auto derivatives of ex-military pattern machine-guns, the Uzi semi-auto carbine had to undergo a significant redesign from the SMG parent in order to pass the BATF’s measuring stick to allow importation and sale to U.S. citizens as a Title I firearm. Obviously, such changes are made with the intention of NOT allowing an easy conversion into a machine gun, and it is important to fully understand these engineering changes. A proper, safe, and legal conversion will have retained the inherent safety features of the parent SMG design, while rendering as close as mechanically possible a virtual copy of the SMG functionally and aesthetically. Israel Military Industries (IMI), unfortunately for the NFA collector community, went far beyond the absolute minimum changes required from the SMG design to allow importation and sale in this country. These extra semi-auto only features are the central focus of most of the complaints leveled at conversions of these guns, with regard to function and user-friendliness.

The differences between a functional conversion (one that merely duplicates functionally, full-auto only, or selective fire), and a complete conversion (one that virtually replicates the original SMG in all aspects including function, parts interchangeability, and appearance), are significant. They can cause extreme variation in the fair price range of differing guns. To gain a complete understanding of the selection and desirability of available guns out there, let’s first take a look at the basic mechanical differences between the SMG and the semi-auto carbine (as originally imported).

The single most important difference between the original SMG and the semi-auto carbine is in the respective methods of operation. The SMG fires from the open-bolt position using a fixed firing pin. The carbine had to be redesigned to fire from the closed-bolt position, utilizing a striker. This was solely to pass importation restrictions based upon a pending ruling prohibiting the manufacture of semi-auto Title I guns that fired from an open bolt. To this end there were several significant design changes made. The first was the installation on the rear upper right side of the sheet metal receiver of a long piece of rectangular shaped metal bar (known as a ‘blocking rail) that prevented the drop-in installation of the SMG type, fixed firing pin, Open Bolt. In order to accommodate the blocking rail inside the receiver the semi-auto bolt has a full -length notch cut in its upper right side to allow passage over the blocking rail. Since the gun could not use a fixed firing pin a striker mechanism was incorporated into the bolt group, which now comprised a slightly shorter Bolt with a full length hole drilled through its center to accommodate a moving firing pin, this pin came forward upon, sear release, to strike the cartridge primer. The bolt itself now merely reciprocated within the length of the receiver housing, with each shot closing upon the freshly chambered round. The the striker assembly stayed caught by the sear in the same rear position of the former SMG open bolt. The striker assembly itself comprised the long firing pin and square section of steel that had a sear holding notch cut into its bottom surface, along with a separate spring to provide the striking energy. The semi-auto now had two separate spring assemblies; the main recoil spring (attached in the familiar place on the bolt itself), and the smaller striker spring. There is an interconnecting slot cut into the left side bottom of the semi-auto bolt to mate with the long arm of the striker assembly. This assures proper alignment during movement. Due to the fact that the striker arm (contained the single sear notch) the right bottom ridge of the semi-auto bolt that would normally contain a sear notch in the SMG bolt is milled open from the rear of the ejection opening, to slightly ahead of it. The SMG bolt is solid on the top and side faces, except for the sear holding notch, and the ejection port opening. To finish out the bolt group changes, the SMG has a different type of extractor than the semi-auto bolt. The lip of the semi-auto extractor is considerably thinner and shallower than that installed on the SMG bolt, for unknown reasons. It is clearly desirable to have the SMG version installed in a full-auto gun, and probably any version of the gun. They are completely interchangeable. Lastly, a note is in order on the two different kinds of semi-auto bolts that were installed on the Model A guns, as this has an important bearing on how these guns may have been converted to the full-auto fire mode. When IMI first designed the Model A semi-auto bolt the bolt face was identical to the SMG open-bolt design (save for the deletion of the fixed-firing pin) in that it incorporated a full-circumference cartridge holding rim. This cartridge holding rim was designed to snap around the base rim of the cartridge and hold it in position as it entered the chamber, just prior to contact with the fixed firing pin. Later Model A, and all Model B, guns have the lower section of this rim machined off, as another disabling design feature, to preclude easy modification to full-auto fire, as will now be discussed below.

Now, with the mode of fire changed, the fire controls had to be similarly altered. All fire-control parts are contained in a separate housing attached centrally below the receiver of the gun, and which also comprises the pistol grip and the magazine well. The SMG fire-control assembly allows for three control positions, safe, semi-auto, and full-auto. The selector levers have a small right angle, finger-like, bent piece of metal which, in a SMG installation, moves forward and bypasses the disconnector function in the fully forward full-auto position. For semi-auto fire to occur it is placed in the middle position, where it can function the disconnector, releasing the sear after the trigger nose drops. To force the condition of semi-auto only operation, whereby the disconnector is activated continuously, it would be required to mechanically preclude the selector from moving forward past this point. The alterations were made to the semi-auto grip housing by adding a small block of metal inside the front center shelf of this housing to preclude the selector lever from moving forward enough to engage the full-auto position on the trigger nose, and by-pass the disconnector. The selector levers themselves are the same except for deletion of the third select position notch. Very early semi-auto selectors were identical to the SMG versions, and had all three control position notches already cut. Later versions deleted the third position. Concurrent with the changes in the semi-auto guns which resulted in their being redesignated as Model B, all versions of Uzi selector levers had a vertical safety tang added to the upper surface of the lever, which prevented the sear from dropping (by blocking the left underside sear finger, in the same mechanical fashion as the right underside finger is blocked by the vertical tang of the grip safety) until the selector switch was moved into one of the Fire positions.

The only other difference in the fire-control parts relates to the sear itself. The SMG sear is quite noticeably larger on the fingers that protrude up into the receiver to catch the bolt. In comparison, the semi sear, because it only had to restrain the much lighter striker mass, has smaller fingers. The smaller semi sear will work but is NOT recommended, as excessive wear can result. A proper conversion will have the sear projection holes in the bottom of the receiver milled out to the correct dimensions to allow the factory SMG sear to be installed and function. This was not always done, and on conversion guns utilizing a Registered Bolt it may be looked upon as an illegal receiver modification by BATF, unless the bolt was permanently married to the receiver by serial number on the transfer form. (A note on all Uzi sears: the sears, by design, are made to a less hardened surface treatment than the bolt so that when wear does occur, and it will, the comparatively cheaper sear can be replaced rather than the entire expensive bolt. A highly worn sear can allow runaway fire, in slips over the rounded, worn, sear fingers so always check the sear condition on regular basis!)

Lastly, on the fire control group, the axis pins that hold the lower receiver control group onto the upper receiver have two different size mounting pins/holes, again to preclude a direct swapping of the SMG group onto the Semi-auto receiver. The SMG uses a 5mm pins and receiver holes, while the semi-auto guns use 7mm pins and receiver holes. This prevents an SMG lower from being pinned on without enlarging the pinholes in the SMG trigger housing. You will still have to use 7mm pins to mount it. With the availability of parts kit guns now so abundant many people have chosen to install a real SMG lower, either for increased reliability, or just to get the Hebrew markings of the Israeli originals.

Jumping back up into the front of the receiver, we discover yet another joyous collection of maddening alterations that hinder our journey back to the world of the original SMG configuration. The two most often heard complaints about owning a conversion SMG relate to the barrel selection availability and mounting problems. IMI thoughtfully left no stone unturned when redesigning the Uzi for semi-auto sale in the U.S. Their most fondly remembered alterations prevent the installation and usage of cheap and plentiful SMG short barrels, instead forcing the owner of an SMG conversion that has not been fully SMG configured to resort to modifying and cutting down semi-auto barrels. Thankfully there are on the aftermarket barrels that will interchange. (It should be noted that possession of one of these short barrels that will drop into a semi-auto Uzi and the semi-auto Uzi itself would comprise possession of a short barreled rifle, requiring registration under Title II of the 1968 Gun Control Act). The semi-auto is different from the SMG as regards barrel mounting in two important aspects. First, the actual barrel trunnion, which forms the heart of the forward part of the upper receiver by being welded into place, has a smaller diameter passage hole for the barrel flanges than on the SMG. This prevents an SMG barrel from being slipped into the trunnion. On the front of the magazine well, inside the bottom of the receiver, is welded on both SMG and semi-auto versions a cartridge feed ramp to guide the nose of the bullet into the chamber of the barrel. On the original SMG version that is all that it is, a cartridge guide. On the semi-auto version it also contains a thick ring which serves to hold the rear end of the barrel in position on the feed ramp, but more importantly it prevents the larger rear diameter of a standard SMG barrel from being inserted and utilized in the semi-auto guns. By virtue of the SMG barrel flanges being of a larger diameter, they cannot be directly inserted and used in a semi-auto conversion that does not have these two features fixed. So one has to either cut and recrown the semi-auto barrels, or turn down the flanges on the SMG versions. A proper and complete conversion will have had the trunnion passage hole bored out to SMG spec, and the barrel ring milled off the feed ramp.

The last important difference between the SMG and the semi-auto carbine is in the design of the top covers. The SMG cover has an extra mechanism in the cocking track designed to prevent inadvertent discharge of the weapon if the cocking knob is accidentally released prior to full rearward travel being reached and sear lock-up of the bolt. This is commonly called a ratcheting top cover, due to the small ratchet mechanism which will catch and hold the bolt. This is only a feature in the open- bolt guns. It is not found, or needed, in a closed bolt firing weapon. The visible tip off to identification is the row of ratchet teeth along the side of the top cover bottom track. Semi-auto tracks are smooth for their entire length. There is also a slight difference in the length of the cut opening between the SMG and semi-auto carbine top cover tracks, which will be explained below. A proper conversion need not have this ratcheting top cover to function correctly, but anything designed, and available to the owner/operator, for safety reasons should be utilized. It is not possible to modify the semi-auto top cover for this ratcheting mechanism (for all practical purposes at least), so most complete conversions will have this entire assembly exchanged for a standard SMG unit. The other bonus benefit to this exchange of top covers is that the semi-auto carbine has a lengthy and annoying warning against illegal conversions stamped into the cocking knob slide, and since we’re discussing a legal NFA registered weapon, it’s only fitting to eliminate such aggravating visible verbage on the exterior of the weapon. (See Photo on page 73.)

There is one more aesthetic difference between the semi-auto carbine and the SMG. Both guns could utilize either a folding metal stock or a fixed wooden one. The folders attach semi-permanently in the same fashion on either gun, but on the SMG the wooden stock is provisioned for quick detachment by a release lever on the stock underside. On the semi-auto carbine, the wooden stock is semi-permanently attached. The SMG quick-detach wooden stock will interchange for those who so desire.

The Three Conversion Methods

Now, with an understanding of the various mechanical differences between the SMG and the semi-auto carbine, it is relevant to discuss the actual mechanical methods of conversion that will be found on a NFA registered Uzi carbine SMG conversion.

It did not take long after Action Arms Inc. began importing the first carbines to the USA in 1980. Before some very industrious minds began to figure out how to convert the gun into a full-auto firing (versions in full-auto only, and selective full-auto fire) submachine gun.

The first type of conversion took clever advantage of the design of the striker-fired, closed-bolt, semi-auto mechanism in spite of all that IMI had done to prevent easy conversion. In fact, some of the IMI changes actually facilitated this type of conversion! Recall from above, that the first Model A guns to come in were slightly different than all those that followed. IMI had forgotten a few very important details it seems! For the open-bolt mode of fire, the full-circumference rim on the bolt face, designed to guide the cartridge into the chamber prior to the fixed firing pin striking the primer cap, is critical. Without this lower rim on the bolt face the cartridge bottom rim will hang up on the firing pin, causing jamming as it enters the chamber. The cartridge rim will climb into the bolt face too soon. On a closed-bolt weapon there is no firing pin in the way, until sear release of the striker after bolt closure and chambering, so the elimination of that lower lip is of no consequence unless somehow, that fixed firing pin, or its equivalent, suddenly reappeares! That is exactly what those industrious minds figured out how to do. If the striker could be attached to the rear of the bolt it would act as an open bolt, with a fixed firing pin protruding at the bolt face. Even simpler, it was noticed that the striker spring was strong enough to carry the striker in place against the rear of the bolt during chambering, enough to cause reliable primer ignition! Now you were half way there. The earliest Model A bolts had the full-circumference cartridge holding rim already, remember? Two thirds of the way there. If you could somehow allow the selector lever (early Model A select levers had three detent notches) to move forward far enough to interrupt the disconnector function, the bolt would continue to reciprocate until trigger release…. full-auto (only)! Method one done.

In finer detail, these were mostly all registered receiver conversions for two basic reasons. Reason one, because at this time there was no economic reason NOT to register the whole gun as an SMG, and reason two, the ATF rulings which brought about the so-called “machine gun conversion parts” distinction was just beginning to get started with the “AR15 drop-in auto sear” mania and the like, with the Uzi conversion scene adding to the fire for the following reason. From the just discussed conversion method above, it becomes apparent that one could convert a semi-auto carbine to full-auto with no receiver modifications. If a new selector lever that bypassed the disconnector function could be made that could drop-in, and replace the semi-auto one directly. That was exactly what was done in some cases, with a “Conversion Selector Lever” being NFA registered as the machine gun itself, and either installed in a gun by a Class II Manufacturer or owner (on Form 1), or sold by itself. ATF promptly ruled this lever was a machine gun with the usual warning against unregistered possession, etc.

So, early Model As (with the full SMG style lower bolt face rim) can be found with this NFA registered lever as the only difference, but they will be able to fire full-auto only, not selectively. For later bolts with the lower rim machined off, it was necessary to reinstall that rim as in an SMG bolt face. This was usually done by welding a machine contoured ‘half rim ring’ back onto the bolt face itself. For the selector lever itself to NOT be considered a registerable part under the NFA, it had to be of the original design with the right-angle finger attached that engaged the disconnector (and blocking piece welded onto the front lower shelf of the trigger grip frame housing). Early Model A select levers with all three detent positions could be used just as a regular SMG part if the blocking piece could be removed, but doing so altered the ‘as manufactured’ status of the gun, and was disallowed by ATF, unless the receiver was NFA registered, and this was the only way to remain legal with a conversion of this type early on. The proper conversion method then had the blocking piece removed to replicate the SMG control housing function. Later semi-auto select levers were devoid of the third detent notch, so that had it to be reconfigured on later guns undergoing conversion. This describes the various NFA registered guns that can be encountered from the earliest conversions done. This phase lasted only about a year or so, and changed very quickly when those industrious minds went to work again, this time on the bolt itself.

Since to modify the relieved bolt face seen on most Model A guns required accurate welding and reheat treating of the bolt, it seemed easier to start with a bolt already manufactured in the full SMG configuration as regards open bolt firing requirements. If you started with a SMG type bolt the only major modification needed would be to allow placement inside the semi receiver with its blocking rail. A simple job of milling a slot in the correct location to mimic the design of the semi-auto bolt, and the gun would function in the open bolt mode. And so the now famous “slotted bolt” was born. The first versions sold openly were designed to accommodate the normal closed bolt recoil spring and buffer arrangement which was slightly longer in depth on the semi gun because the semi closed bolt did not need to reciprocate as far back into the receiver because of the striker assembly riding behind it. To this end, the cut track opening for the cocking slide in the top cover had a slightly shorter track, as the cocking slide did not need to be drawn as far back with the striker engaging the sear instead of the bolt. To retain as much of the original SMG operating characteristics as possible, it was soon seen that by replacing the semi recoil and buffer assemblies with a standard SMG version the slotted bolt could attain all the relevant dimensions of the SMG, save for the milled slot in the side. By increasing the length of the slotted conversion bolt to normal SMG spec it also restored the normal cyclic rate by virtue of the increased mass and recoil travel. The only down side to this bolt, and all future NFA registered conversion bolts of this description, is that it now required the full travel of the cocking slide to engage the bolt notch with the sear while cocking the weapon, such that it became necessary to now mill open the length of the cocking track opening to normal SMG specs too. It was often easier to just replace the whole top cover, with the added benefit of the SMG ratcheting mechanism coming along for free.

This all came to an immediate halt in mid 1982, when ATF ruled that such fixed firing pin, slotted bolts, like the drop-in selector or lever, were machine guns in and of themselves, so future manufacture, sale, and possession was to be in full compliance with the NFA. There were though a small number of legal registered receiver conversions made by using these unrestricted, restricted, conversion bolts prior to that ruling, and they were allowed (grandfathered), as they were previously lawfully, registered in full compliance with the existing laws, though the conversion bolt now needed to be married to that particular serially numbered receiver. If it ever needed replacement you were out of luck. With this restriction some of these registered receiver guns simply ditched (read: destroyed) the UN-registered conversion bolt and went ahead and finished the receiver modifications that allowed utilization of an original, solid SMG open-bolt, as these were the only bolts that remained legal for unrestricted sale and transfer after the ruling.

This is the basis then for the second most common method of conversion, that of using an NFA registered, slotted conversion bolt, in an otherwise unmodified Title 1 semi-auto receiver. These slotted conversion bolts were still made and marketed, but they now had to be individually NFA registered, and a lot of them were! The slotted conversion bolt” is operationally identical to the standard SMG open bolt, except for the milled slot in the right upper side face. All other relevant dimensions and operating characteristics are the same, with one small nod to the inescapable reality that it was to be used in an unmodified semi-auto carbine receiver. All semi-auto carbines incorporate the barrel modifications as described above such that a normal SMG bolt face, if used in an unmodified semi-auto receiver, would not properly close against the rear of the barrel chamber, as the restrictor ring which surrounds the rear of the semi-auto barrel would interfere. So, NFA registered slotted conversion bolts are slightly relieved at the forward upper bolt face to accommodate the normal semi-auto barrel and mounting setup.

With a registered conversion bolt installed, it was deemed acceptable by ATF to allow modification of the lower receiver trigger housing fire control unit any way desired, and most units were simply modified from the semi-auto to selective mode of fire, as described above. It is also perfectly acceptable to swap out the semi-auto unit for a standard factory original SMG lower, as long as the SMG trigger housing is modified (by opening up the mounting pin holes to 7mm), and not the upper receiver center block! Even with a registered slotted conversion bolt installed, upper receiver modifications are generally disapproved of by ATF, although the only specifically illegal modification is removal of the blocking rail on the right upper inside of the receiver. The caveat against modifying the lower receiver push pin mounting hole to bring it back to the 5mm SMG standard is that it may be viewed as evidence of intent to illegally convert the semi-auto upper receiver (by allowing pin-on conversion of an SMG lower housing combined with the first bolt modification method described above) if the NFA-registered bolt is ever separated. It is inadvisable to make this modification in light of that distinction. Guns found with the remaining upper receiver modifications, such as barrel trunnion and feed ramp alterations, are not illegal and add to the perceptible value of the gun if done in a professional manner. An SMG conversion using a NFA-registered slotted conversion, bolt is an excellent method of conversion as it does not overly hinder one in obtaining a near copy of the factory SMG original, and in most cases the absolute differences between a bolt gun and registered receiver conversion are merely aesthetic, with the spare parts scenario being the most common complaint. And, if you already have a semi-auto gun in your collection, the acquisition of an NFA-registered conversion bolt may allow an inexpensive changeover into the full-auto realm of the design, as it was originally intended.

The only remaining point to discuss about these registered conversion bolts is as regards their inherent value and weaknesses. All of these bolts were NOT created equal, either in materials or workmanship! If it is decided that a conversion bolt is the way to proceed a careful examination of the intended bolt is in order if you wish to maximize the usage and enjoyment of the converted gun. slotted conversion bolts were made from two distinct sources, each with its own pluses and minuses. The first and most obvious source was to simply take a factory original SMG open-bolt, and then make the two modifications to adapt it to conversion use in the semi-auto carbine, namely milling the clearance slot, and the bolt face relief cut for the rear barrel support ring. The main concern here is: Did the manufacturer of these bolts properly account for the re-heat treatment of the finished bolt. Since these bolts are normally hardened, it should have been annealed prior to machining, and then heat treated again to proper finish hardness. A finish hardness of at least Rockwell 55-56 on the C (hard, but able to be cut with a file) scale is desired to prevent premature wear of the sear holding notches and bolt face. A tungsten carbide-cutter mill may have been used instead of the heat treatment process (es), but this could lead to localized heat distortion damage (unlikely) during the milling operation(s) if it is not done very carefully. Closely examine any bolts that show evidence of stress risers, hairline cracks, crystallization or burring.

The second method involved new manufacture of slotted bolts, either from machined forgings, bar stock or investment castings. Both forgings and bar stock are common, and perfectly acceptable as base materials for a machine gun bolt, with a slight nod being given to the forged bolts as they duplicate the factory original manufacturing method, while having intrinsically better metallurgical properties. Lastly, some new manufacture bolts were made from cast steel and then finish machined. Investment cast parts are acceptable, providing that the proper manufacturing methods were employed during casting, and were to industry standards. The single most common problem found with cast parts relates to improper cooling practices which allow finished dimensions to stray from spec. All cast Uzi bolts were not created equal, and there are unfortunately cast Uzi bolts that are known to be off-spec due to a materials specification change which substituted a different alloy than originally specified. This material had a slightly different contraction rate, which caused it to cool to a finish dimension that was larger than tolerance. There ended up being a small lot of these poorly cast slotted SMG bolts NFA registered, and they are quite obvious by the fitting marks that were required to get them to function.

All of these bolts of any manufacturing method, should be heat treated for long-term service, so check the pedigree of the bolt in question and verify manufacturing specs if at all possible. As regards reported, known problems with these registered slotted bolts, it boils down to two areas. First, for the reasons above, some bolts exhibit an excessive wear profile in the sear holding notches which can lead to eventual failure to properly hold on the sear fingers. Potential dangers from run-away firing aside, this is not that serious of a problem to repair, though it entails welding up the sear notches in a jig and then re-machining them to spec (followed by proper re-heat treatment!). Secondly, some of the improperly heat treated bolts have had their fixed firing pin wear down from extended use to the point where there is not enough pin protruding to reliably cause primer ignition. The only acceptable method of repair here is to bore out the old pin and install a replacement fixed pin which can be heat-shrunk into place. These repair pins are rare items, and few people working around MG’s have any experience with doing this, but since we are talking about a registered part, repair is the only legal way to go registered slotted bolts should, of and in themselves, be of no particular concern as regards wear and longevity in an SMG conversion, as they have the full potential to meet a normal factory original parts life and utility span, if they were correctly manufactured in the first place. As with all conversion parts though, once the general design was in the public domain many different sources made and registered them and all were not created equal, so a close physical examination is advised. And if a well-worn, or even damaged or improperly manufactured, registered bolt should be found, do not discount it out of hand, as it can be repaired and restored to operational status by a competent professional. The price to be paid should reflect the overall condition, but with registered NFA items getting more valuable with time, any item with a registration paper/tax stamp is worth a close examination prior to purchase, if at possible.

“As a general rule, ANY bolt gun is worth less than a registered receiver conversion of comparable condition.”

The final conversion type is the so-called registered receiver gun. As most people understand the meaning of the term, it is qualified by one underlying distinction, the removal of the blocking rail inside the receiver to allow utilization of a standard, solid, SMG open bolt. Though we noted that there are a select few registered receiver guns, as described above, that were converted by other methods, these are not generally perceived to be of the same general class, as they still have the blocking rail intact and therefore have not been converted to SMG description. However, due to their receiver status they most certainly could be completed to full SMG spec at the owner’s discretion. The owner who wishes to do so should check with ATF Technology Branch first before doing so – and get it in writing. But for most registered receiver guns, their attraction lies in the fact that they can be fully converted to a near clone of the factory SMG, in both function and appearance. Due to the registration status, with no legal barriers to altering the receiver itself, we are free to return the receiver to its normal SMG description.

This complete conversion of the gun back to its near factory SMG description is accomplished as follows, though individual guns may exhibit various stages of completeness of the full conversion. The complete conversion is begun by removal of the bolt blocking rail in preparation for installation of a factory original SMG open bolt. The IMI design for the mounting of the blocking rail incorporated two extension tabs near each end, that located the rail by mating with opposite slots in the receiver sidewall, and then were welded in place and had the welds ground smooth prior to finishing of the weapon’s exterior. When altering the receiver for the conversion to a factory type SMG bolt it became necessary to remove the rail completely, flush with the receiver sidewall, a job best accomplished with a vertical milling cut, as the mill could be controlled to safely remove the rail flush with the receiver sidewall without causing damage. The problem with this is that the last section of the rail was installed too far back in the receiver channel to allow milling it completely free, so some ingenuity was called for. Most conversions were done by carefully milling off the rail as much as possible rearward then using a small torquing force, or shear force, to break free the remaining tab joint. Done carefully, this resulted in no damage to the receiver sidewalls, though many people apparently used alternate methods than a mill, or rushed the job or applied too much torque/shear force as twisted/dented rear sidewalls are one of the most common flaws in conversions. These sidewall flaws can be straightened, but it is a difficult job and best done by an experienced professional.

The other required major alteration to the semi-auto upper receiver, to render it a clone of the factory produced SMG, was the conversion of the barrel mounting points. The barrel trunnion through-hole needs to be opened up to the SMG factory spec diameter, and properly converted guns will have had this done by line boring to the axis though more commonly this was simply placed into a vertical holding fixture and a drill press was used. This can result in misalignment of the mounted barrel and may be a root cause of feeding troubles, and failure to be able to properly tighten the barrel mounting collar. The rear barrel holding ring/feed ramp assembly will also have to have the entire ring structure milled off flush with the upper surface of the feed ramp, duplicating the SMG ramp contours, to avoid bolt face contact. Once these two operations were performed, an SMG barrel could be simply replaced.

The selector lever block in the top inside shelf of the trigger housing frame was also installed by the same locating tab method, as employed on the blocking rail in the upper receiver. The correct method to remove it was a simple milling operation, though other torquing/shearing operations were commonly utilized. If the block was not fully removed flush, or if any remnants of the weld are left, there may be interference with the elongated right-angle finger of the selector lever that prevents smooth operation of the disconnector function and subsequent improper semi-auto functioning. So, to do a proper conversion, one must completely remove the select lever block, as some improper conversions merely attempted to notch, or cut out, instead of fully removing the block, and a sticking or troublesome selector lever can usually be traced to this. An SMG selector lever is the ideal way to complete a fully correct conversion, and this can be simply exchanged, although the majority of conversions probably used modified semi levers (note: there are two different SMG selectors available – later production ones have the safety tang, early military ones do not.). The semi-auto select levers were modified by adding the third detent, and these detents may give trouble if not properly done. Check for correct depth and spacing as compared to the original two semi-auto only control positions if the selector will not engage the full-auto position smoothly and firmly. A correct job will have the third detent milled, as the factory did, and not hand cut or ground out.

As discussed above the remaining operation to the trigger housing was remarking of the housing exterior for the third selector detent position after it had been milled, cut, or stamped into the housing exterior. The exterior of the trigger housing is where it is easy to tell the care and professionalism, or lack thereof, that went into the conversion. The remarking of the housing for the third detent position can vary from either indistinguishable from factory, to sloppy and badly mismatched. A cold chisel and hand stamp, held by hand and eyeballed, was often the preferred method. The best conversions will have utilized a professional stamping fixture with a font-matching stamp to duplicate the lettering already existing in the other two positions, combined with welding up of the old fire position F, and restamping of the now required R for the semi-auto detent position. Some conversions may simply have had the entire semi-auto unit replaced with a surplus factory SMG type trigger housing, which should be unaltered except for the required modification to the mounting pin holes of the SMG trigger housing. If the original semi-auto sear was exchanged for an SMG sear, with the full width fingers, the sear clearance holes in the bottom of the upper receiver will need to be enlarged to SMG spec for proper clearance. The difference is slight, and many were not properly enlarged as required. For reliable operation with the SMG sear it is necessary to enlarge the holes.

“..Group Industries guns were offered from the factory in .45ACP and .22LR calibers in addition to the standard 9mm..”

The top cover modifications, required for the SMG open bolt installation as discussed above, will most likely have been accomplished by a simple replacement and exchange of the semi-auto original cover assembly for a surplus SMG, ratcheting type unit. This would be considered the most desirable method for this assembly, though modified semi-auto units are also common and perfectly acceptable, though less military looking.

The final aspect of the conversion is the installation, a drop in job now, of an original solid face, SMG style open bolt and its attendant recoil spring and buffer assembly. This bolt should already come equipped with a wide-lip SMG style extractor claw, but if not it should be exchanged for one.

With a complete exterior refinish, this is the ideal state of conversion in a complete conversion of the semi-auto carbine into as near as possible a clone of the factory produced, select fire, open bolt, SMG original, save for the mounting pin holes for the lower trigger housing and the semi-auto model A/B markings. These trigger housing pinholes and the receiver markings could have been altered if a registered receiver gun was remanufactured by a Class II manufacturer, but conversions using registered bolts cannot alter the receiver or its factory markings. As can be readily seen, since there exist such a wide range of possible conversions out there, respective values also vary widely. Most desirable would seem to be a fully converted registered receiver gun exhibiting all the alterations to render a near perfect copy of the factory SMG. These guns, circa early 1998 prices, are fairly priced in the mid to high $2000s for excellent condition guns with a known pedigree originally done by reputable sources, to the low $1200 range for a no-pedigree gun with mechanical or major aesthetic problems. Value increases with higher content of SMG style parts, or modifications to accept those standard parts. Registered bolts are fairly priced from $500-$1000 alone, depending on manufacturer and condition, and materials and methods used for manufacture. A complete bolt gun will depend upon the host semi-auto guns inherent value and whether any additional allowable alterations exist. These could command as a package anywhere from the low $1000s to the mid $2000s. As a general rule ANY bolt gun is worth less than a registered receiver conversion of comparable condition. Though, if you happen to run across one of the few very early conversions, those that fall under the heading of anomalies, such as those that contain a NFA registered selector lever, or are a registered receiver gun with a grandfathered un-registered slotted bolt, judgment will have to be used as to value based upon overall condition and the wear and tear on the registered part(s). Negotiations are in order, as there is simply no definitive value basis.

Hopefully now, with an understanding of the myriad definitions extant of the Conversion Uzi, a clear picture can be drawn of the complex nature of this particular class of civilian legal NFA firearm and its convoluted path to the marketplace and firing lines across the country. While there exists a large variance in this class of guns, it is also nice to know that they represent a solid value for the collectors money, while being easy to shoot and service, as spare parts and accessories are abundant and inexpensive.

Tom Hoel has done an outstanding job of his latest chosen assignment, classifying and technically describing the mechanical differences and roots of the various HK machine guns in the US Market. The work was so extensive that we are running it in a three part series. This is not a definitive work on “Who made what gun”, but a targeted technical work that should enable the prospective buyer to understand “How”, “Why” and perhaps more importantly; “What are the features that I want from the HK I am buying”. In this the final part, Tom continues into the mechanics of the registered HK “Sear” conversions – Dan

Second Choice: Conversion “Parts” and “Devices”…. Registered and Unregistered!!

As we have seen in Part II of this series, unless the receiver of the gun was restored into full machine gun configuration, the mechanical alterations needed to produce automatic fire were necessarily contained within the fire-control group and these modifications were usually performed in a similar manner, whether or not the conversion part or device was NFA registered or unregistered in and of itself. In the same fashion as the “Registered Receiver” categorizations, “Conversion Parts or Devices” can be divided up into two distinct sub-categories. Firstly, the mechanical conversion may be accomplished by use of a single conversion device, such as a conversion SEAR, or secondly, by alteration of the whole fire control group as a unitized assembly, such as a converted factory select-fire pack.

There is necessarily some cross-over and blurring of the lines of distinction in discussing these items as they all function absolutely identically, the only difference being with which basic components you started, and in which direction you preceded to arrive at the same mechanical destination. Conversion parts such as sears are necessarily part of conversion groups (devices), they are just discussed as a separate component part as opposed to the unitized assemblies of converted factory original groups, because of the needed mechanical alterations to install them into functional automatic-fire status in a semi-auto fire control group into which they are being adapted.

Registered Conversion Groups.

This category can be loosely described as either “Converted Semi-Automatic only fire control groups”, or “Converted factory original Selective Fire control groups” Each method has its’ advantages and limitations.

The equivalent to the “Full Registered Receiver Restoration” in terms of mechanical purity and factory feature replication would be what is known as a “converted Selective-Fire Trigger Group”. As the name implies, this is nothing more than a stock factory produced select-fire trigger group suitably modified to mount onto an unmodified clip-on style Title I receiver. These are almost always found to be an original metal grip framed “S-E-F” style housing, and an NFA registered item as a conversion DEVICE, in and of themselves, in order to not be in any legally grey area. Although as mentioned above a few may be found mounted on an NFA Registered, but unmodified, clip-on style receiver as an unregistered conversion device. All internal mechanical components are factory select-fire design and this greatly enhances the spare parts scenario, as there are no modifications done to any of the trigger group functional parts.

When we discuss this specific type of conversion device, we are considering only the actual mechanical pieces required to adapt the complete group as an assembly onto an unmodified Title I receiver mounting. This is accomplished by first adapting the factory select-fire sheet metal trigger pack frame to mount via a clip-on style. Then by adapting the outer frame grip housing to allow for this new mounting method by relieving the lower frontal area of the housing “box” to permit the receiver ledge to protrude through, and finally by providing a horizontal mounting point for the receiver ledge to hold fast to. This adaptation will also be seen when we examine the installation of newer style plastic grip frame housings to clip-on style receivers when employing registered “auto sears” in converted semi-auto trigger packs. Or in rarer cases, when an owner desires his converted semi-auto trigger pack utilizing an unregistered conversion part to be reinstalled in one of these newer housings. There are significant limitations to this in legal terms though as discussed above. The NFA registration reflects the status that the ENTIRE unit is the registered item as it requires extensive modifications to both the grip frame and the enclosed trigger pack to adapt them to fit and function on a clip-on style Title I receiver. These two individual components cannot ever be separated legally, as again, it is the combination which was originally registered as the “device”. What this means is that exchanging the metal “S-E-F” outer grip frame housing for a newer style plastic “S-E-F” grip frame housing is not allowed if the metal housing was part of a registered combination. Most of these units were marked on the metal housing exterior with the required identifying markings when registered.

The real intrinsic value of owning one of these converted, adapted, and NFA registered factory select-fire trigger groups is obvious in that it is, in a mechanical sense, it is absolutely genuine and identical to those trigger groups that came on factory machine guns, only the mounting method has been altered. The internal mechanical components are fully interchangeable, and more importantly, they are situated in the normal design positions providing the intended mechanical geometry of function. The designed in mechanical geometry remains fully unaltered which will retain the designed cyclic rate-of-fire, as on a factory original machine gun. This is a most significant point to be understood, as this mechanical geometry is not the same in conversions done by altering a semi-auto trigger pack to function in the selective fire mode using a “Conversion part”, or “registered auto sear”. This last statement directly leads us into the most commonly misunderstood arena of the whole “H&K Conversion” dilemma.

‘Sears’, ‘Trips’, and ‘Catches’: The Pandora’s Box of Conversion ‘Parts’

When HK GmbH set about to redesign the original select-fire control group to mechanically remove the automatic or cyclic fire mode, they chose to alter one of the original safety features built in to the design. Like the FN FAL mechanics, the “automatic sear” is also a form of safety sear designed to enhance the mechanical safety by preventing hammer release and firing pin contact before bolt lock-up in the final battery position. Removing the automatic sear removed one level of mechanical safety, as it is always functional whether or not the fire control selector is in the cyclic fire mode. Selection of cyclic fire mode merely allows the trigger to drop the “secondary” sear (also functioning as the disconnector) backward and down out of contact with the hammer engagement surface.

To adapt the currently produced parts with as little change as possible to a Semi-automatic only configuration was quite easily done from an Engineering standpoint. This adaptation was accomplished with a minimal redesign of the three elements of the fire control group. The main difference between the two fire control options lay in the two independent safety systems. Since these two independent functions were arranged as separate mechanical pieces, they only had to remove the automatic sear (‘catch’) and its’ attendant release lever from the fire control pack to eliminate the option of cyclic fire. With a small modification to prevent the selector lever from rotating all the way down into the cyclic fire mode position the trigger pack was now arranged to offer only two positions: “Safe” and “Fire”(semi-auto only). The secondary sear (‘disconnector’) remained as always, merely prevented from dropping down out the way of the hammer’s travel, thereby always forcing it to catch the hammer, “disconnecting” it from the trigger for normal semi-automatic operation. On HK GmbH produced semi-automatic guns, the primary sear notch on the hammer was also ground off to prevent easy re-conversions. On some of the Licensed produced HK91 clones the hammers are not fully adapted to semi-automatic-only operation. The selector levers are identical between select-fire and semi-auto-only versions.

Combined with the obvious redesign of the relevant fire control parts to eliminate the selective fire capability and restrict it to only semi-automatic fire, the actual sheet metal box frame that holds these components was modified into a semi-auto-only configuration. With the deletion of the automatic sear and its’ pivot pin position, the select fire trigger pack frame could be altered to mount on the newly designed clip-on receiver mounting ledge. This was accomplished by cutting a distinctive ‘notch’ in the lower frontal area of the sheet metal box, the area formerly occupied by the sear pivot pin holes. Importantly, one other major mechanical item was connected to this location. The trigger (and auto sear) return spring was mounted inside the automatic sear arms and was held in place by what is known as a ‘distance sleeve’, which is nothing more than a hollow tube contoured to hold the spring in position and it slips around the sear pivot pin. This single fact is the least understood problem of conversion packs using the clip-on mounting style.

Due to the deletion of the full-automatic sear position in the semi-auto pack, necessitated by the new front clip-on mounting ‘notch’, the pivot point for the trigger return spring had to be relocated to a more rearward position in the pack frame than in the selective fire frame. This relocation is the major mechanical distinction of the “registered/unregistered auto sear” conversion. The “conversion sear” is forced to use this new trigger return spring pivot pin location as the mounting point for the newly installed conversion auto sear which places the conversion sear in closer physical proximity to the sear contact bearing surface of the hammer. The trigger return spring used in semi-auto packs is also formed differently than the selective fire version. This relocated spring must also now function on the conversion sear in the same manner as the factory select-fire unit, but it is forced into a differing contact geometry and so acts accordingly. Whether NFA Registered or not, all these “conversion sears” mount and function the same! The only significant difference is found to be whether or not the original factory release lever is still employed in the conversion. In some designs of conversion “sears” the release lever is integrated into the sear design, so eliminating one extra part. And whether or not the conversion used a modified originally semi-auto only pack frame or an adapted, cut and blocked, originally select-fire pack frame, the “conversion” geometries of the newly installed mechanics are the same!

Conversion Sears? It’s all in the Timing!

As we have discussed above, the lower frontal area used to clip-on the front mounting ledge on a semi-automatic fire control pack is where the factory original select-fire, fire control pack mounts the pivot pin holes for the automatic release sear. Since the auto sear pivot pin is also where the trigger return spring is mounted, HK GmbH in its’ redesign to semi-automatic-only operation selected a new location for the trigger return spring. It was now riding on a pivot pin used only to mount the return spring and its’ redesigned semi-auto only distance sleeve. This new return spring location in the semi-auto only pack is the location that conversion sears are mounted in, the same effectual location as in a factory original select fire pack. NFA Conversion fire control packs, either original semi-auto or modified select-fire versions, employing either a registered or unregistered conversion sear, utilize that relocated trigger return spring pivot pin hole location. That relocated pivot pin position for the new conversion sear has a very important effect on mechanical functioning.

On original factory select-fire packs the auto sear pivot pin holes are located 7mm farther forward in relation to the trigger pin pivot hole, and 3mm lower than in conversion packs. This rearward relocation of the sear engagement to hammer contact surface in conversion packs has the net effect of causing the hammer to be held in a different vertical position than on a factory select-fire pack. With the hammer cocked and held by the two versions of auto sear (‘catch’), there is a large difference in allowable hammer travel upon auto sear release. On a converted pack the hammer rides 12mm higher than in a factory select-fire original pack. This difference in hammer travel to firing pin contact is equivalent to lock time, a decrease in lock time will cause an increase in cyclic rate! This is what is commonly known as the “timing” of the conversion, and proper timing is critical to more than just mimicking factory cyclic rates. Remember, the original factory design uses the auto sear also as an effectual safety sear, preventing inadvertent hammer release until final bolt battery lock up. Since the gun employs a delayed blow back action using roller locking, the only thing preventing a totally unlocked cartridge ignition is if the bolt rollers have entered their locking recesses. This is theoretically only possible if the locking piece can travel to full extension by forcing the rollers outward allowing the firing pin tip to protrude out of the bolt face. However, in certain situations the locking piece can forced under hammer spring pressure to contact the firing pin before complete extension of the bolt rollers, particularly in a gun with worn components such as the locking recesses in the barrel extension. As such it is critical that the auto sear release occurs only after complete extension of the bolt locking rollers into their locking recesses.

This timing is particularly effected by two things: firstly, the release lever camming trip surface on the underside of the bolt carrier, secondly by the hammer-sear contact geometry. The release lever camming timing is merely a matter of restoring the original factory bolt carrier underside contours, something that was usually done by most manufacturers of these conversions. Restoring the hammer-sear contact interface geometry is considerably more difficult to achieve. Due to the relocation of the new conversion auto sear, if an unaltered factory original hammer is employed the new conversion sear will hold the hammer in the higher position as described above. This is an induced reduction in lock time, and if combined with an improperly re-contoured bolt carrier tripping surface, it may cause early release of the hammer as opposed to factory specifications. If this occurs in a well worn gun with worn locking recesses the bolt rollers may not be fully extended when the hammer spring forces the locking piece forward to contact the firing pin. For this reason a properly converted hammer used in a “sear conversion” will have had the factory original sear notches cut on the front lower face of the hammers welded up and the re-cut to allow for the new geometry of the relocated conversion sear. Unfortunately, not all “sear conversions” have had this accomplished. While the safety reasons as explained above should be enough, the most common reason desired to have the hammer re-contoured is the restoration of the factory specified cyclic rate for the particular gun model in question. Even a minor increase in cyclic rate will cause a large increase in recoil forces and induced wear and tear, something to consider since these guns are often extremely expensive now. (Note: there have been observed slight variations in these actual geometries depending upon the individual conversion in question. These measurements are representative of industry accepted practices for alterations required to adapt these conversions, individual guns may exhibit minor variations that should not be considered a defect if the gun runs acceptably. An insignificant variation in cyclic rate is the usual net effect)

There are a few more minor modifications required to get these conversion sears to properly function. The original disconnector (“sear” in HK parlance) must be slightly relieved to allow it to properly function in the new mechanical arrangement. A “sear conversion” converted pack uses a factory original machine gun distance sleeve, as the conversion sears occupy essentially the same dimensions as the factory original ‘catch’, and so the longer semi-auto only distance sleeve cannot be used. The semi-auto only and factory select fire trigger (and auto sear) return springs are also different. The auto sear (or ‘catch’) release lever is almost always used in it’s factory original form, and location, the design of the NFA conversion sear accommodating this arrangement.

Conversion Sears: Equivalent, but often different

We have been discussing those NFA conversions that retain an unmodified Title I style receiver and attain select fire operation by utilizing a conversion “part” known almost universally as a “conversion sear” in nearly generic terms. There were only a relative few proprietary designs of these conversion sears, yet there are some important distinctions to be made.

As discussed, NFA conversion sears may be divided into two main categories, those that exist as a direct replacement for the factory designed original ‘catch’ and are designed to function in the exact same manner if in a slightly different geometry by using a factory type release lever. And secondly, there are those styles that are designed to replicate the factory mechanics by combining the functions of the factory ‘catch’ and the release lever in one conversion part. Again, depending upon the individual Class II Manufacturers’ or Qualified Individuals’ personal choice at the time of conversion, or the timing of the registration involved, these conversion parts may or may not be individually registered in the NFRTR as “machine guns” in and of themselves!! They do however all function identically and may exist in exactly the same physical form with some being NFA registered themselves, and some being installed onto gun receivers that are the registered item as described above.

The first kind of conversion sear is designed to directly replace the factory ‘catch’, or auto sear. Despite the method or materials of manufacture they are identical in all significant mechanical aspects. The factory ‘catches’ were initially designed to function with a curved end “finger” of the upper arm extension acting against the straight undersurface of the release lever by riding in a cammed arc in direct friction contact. As such they became known as “friction catches”. This, as opposed to an updated version which employed a ‘catch’ that was designed with a small roller wheel at the extreme end of the extension arm, acting against a curved underside of a redesigned release lever arm, these are known as “roller catches”. All NFA conversion sears of the direct replacement type were made in the “friction catch” style!!

Direct replacement type conversion sears were made of steel and by casting, stamping, or machining of bar stock depending upon individual choice or manufacturing ability at the time. Quality and durability vary, and though all are serviceable there are differences. Several physical styles of these “direct replacement” sears were created. Some are dimensioned and shaped very differently than the factory friction ‘catch’ they are designed to replace, and as such they may require proprietary changes to the other trigger group parts that are slightly different than what would be considered “industry standard” as described above.

The “most desirable” types are constantly being argued, but if replication of factory design, dimensioning, and materials are considered there is one clear winner: the stamped and formed conversion sears are hands down closest to a factory friction ‘catch’, and are regarded as such. Despite construction type and materials the single most important factor in durability of any of these conversion sears is proper heat treatment. It is known that some of the machined or cast sears were not heat treated correctly, if at all, and they will exhibit excessive wear profiles. Most of these are known to be installed as unregistered conversion sears residing in conversion packs mounted to receivers that are themselves NFA registered. Fortunately, any of these incorrectly heat treated sears can be re-heat treated by a competent and knowledgeable Gunsmith. Additionally, the upper arm extension finger can exhibit excessive wear, galling, or cracking. The only other problem to watch for is that some of the cast sears have been found that have improperly located pivot pin holes, and these off-axis pin holes can be worn out of round over time. This too can be fixed rather easily. Those stamped and formed conversion sears produced in a tandem manufacturing effort by Qualified Mfg, S&H Arms and Fleming Firearms, and the cast sears by S&H Arms are regarded as the most likely to offer effectual service and lasting value.

When moving into the realm of the combined ‘catch’ with release lever type of “conversion sear” we find a much broader interpretation of what constitutes a “conversion part” as mechanically and legally defined. These are generally speaking found to be of the earliest era of NFA conversions and exist in physical forms and dimensions that are generally totally different than a factory part. As described earlier, many Class II Manufacturers were forced to modify the existing semi-auto-only parts that came in the to-be-converted gun because of the total lack of available factory select fire parts then available on the open market. Accordingly many of these converted semi-auto parts resemble Modern Art masterpieces!! At least one competent Class II manufacturer, Jonathan Arthur Ceiner, Inc., essentially replicated the factory combined ‘catch’ and release lever as found employed in the HK21E/21A1 LMG into a proprietary design that is exceptionally well done. It is generally found though, that most of these combined function conversion parts are far from an ideal set-up. It is impossible to offer much serious advice separate from the general advice given above as there are just too many varying styles installed by manufacturers with widely varying mechanical competencies. Except as stated above for the singular exception for Ceiner conversion sears, these combined function sears are regarded as being the lowest value in terms of desirability.

Through the Glass, Darkly.

With these NFA conversions employing a gun receiver with the Title I style clip-on style trigger group attachment point there remains a very tempting trap awaiting those whom would knowingly or unknowingly slip into it. Why this is may not be readily apparent, and even then there remains some stubbornness in accepting the legalities of the current situation regarding this. The desire to update their gun with a newer style plastic “Ambidextrous” or “2-shot/3-shot Burst” style trigger group remains almost universally strong among current owners, however there needs to be issued one final warning regarding doing this.

The ONLY LEGAL WAY to install a newer style “Ambidextrous” or “2-shot/3-shot Burst” style trigger group is if the original, registered or unregistered, conversion sear (or “catch”) can be mated to function in the “Ambidextrous” or “Burst” trigger pack mechanics. This means you must discard the factory “sear (‘catch’)” that came in the new pack and affect installation of the original registered or unregistered conversion “sear” that was utilized in the original fire-control pack. Some few proprietary “sear” designs can be adapted to fit and made to properly function, most can’t.

As detailed earlier, the conversion pack frame itself has to have been altered to fit the clip-on semi-auto receiver, while having the new style plastic housing adapted to fit the clip-on receiver, and then “blocked” to prevent re-installation of an original select-fire trigger pack. Now often enough this is done with the newer style plastic S-E-F housings which are essentially identical in function to the older style metal S-E-F versions, and this poses no legal problems by doing so. Because most of the “direct replacement” style conversion sears are modeled after the friction sear found in the factory original metal or plastic housing style S-E-F trigger groups, adapting just the plastic housing presents no mechanical problems.

The legal and mechanical problems arise when we are trying to adapt a newer style “Ambidextrous” or “2-shot/3-shot Burst” style trigger group. And there is a very good reason not to simply go ahead and have such an adaptation performed by simply discarding the originally installed conversion pack and clipping on a newly adapted one. The mechanics of the Ambidextrous or burst trigger groups are VERY different than the standard S-E-F type of fire control pack, with many of the component parts totally redesigned to properly function with the burst counter clockwork mechanism installed. Ambidextrous style trigger groups only offering selective, but not burst, fire modes are also arranged identically and so suffer this same problem. H&K GmbH actually offered two very distinct styles of “burst” mechanisms. The first version externally resembled the S-E-F type of housing and located the burst clockwork mechanism in the rear lower portion of the pack frame. This is significant as it left the front ‘catch’ position identical to the S-E-F fire control pack!! This means that this style burst pack, and only this style, can be readily adapted to function with those conversion sears resembling externally the factory friction catch!! These very rare fire control packs command an extreme premium in today’s market as they can be easily adapted to function with the most common versions of conversion sears, and the market for using them to install registered conversion sears primarily, is very strong. The second style of burst pack located the clockwork mechanism more toward the frontal region of the pack frame and this forced a complete redesign of the internal components. These new style burst packs cannot be easily or readily converted to use an NFA conversion sear!! Many styles of proprietary NFA conversion sears simply cannot be adapted to function without major mechanical redesign. Due to this fact, there is a large temptation to avert the hassles in doing so by simply adapting the entire trigger group and exchanging it on a “Registered Receiver” guns’ receiver that formerly used an unregistered conversion sear. Attempting to do so leaves one extremely vulnerable in a legal sense?.

One must remember that when doing semi-auto conversions was a lawful enterprise, there were not that many shops actually performing these jobs, and the vast majority of guns converted were done by a relative few. It became quickly known which shops preferred to do which manner of conversions, and by what methods they commonly converted these guns, so a savvy ATF Field Agent can easily tell if a particular gun should have come with a particular type trigger housing, or fire control pack, etc. Also, the easy availability of the “new style” Ambidextrous/Burst trigger groups today was not the case even a few short years ago, so the likelihood of one having been originally installed is not too great. BE FOREWARNED!! And be fully prepared to PROVE that your “Navy Group”, or “Burst” trigger group now residing on your clip-on style “Registered Receiver” conversion gun contains, and has been adapted to function with, the original unregistered conversion part that the gun came with……….and yes, it HAS happened.

“Losing” the original fire control pack, and its’ original unregistered conversion part, for a newer Ambidextrous/Burst style trigger group is a good way to lose the whole expensive gun……and possibly more.

So?..Now you know!

Hopefully now, there is a clearer understanding of just how complex the topic of NFA Conversions on H&K style weapons is! It is a fact that since these guns represent one of most modern and desirable types of Civilian legal machine guns available, many enterprising Class II manufacturers sought to do conversions on these guns. Unfortunately, all of these attempts were not created equally. Therefore, it is paramount to assess the basic conversion type in question before a fair market value can be assigned in this quickly escalating market place of recent years.

Photos for this three part series were provided by James O. Bardewll, Esq. and Dr. Phillip H. Dater, M.D.

By Thomas Hoel

Tom Hoel has done an outstanding job of his latest chosen assignment, classifying and technically describing the mechanical differences and roots of the various HK machine guns in the US Market. The work was so extensive that we are running it in a three part series. This is not a definitive work on “Who made what gun”, but a targeted technical work that should enable the prospective buyer to understand “How”, “Why” and perhaps more importantly; “What are the features that I want from the HK I am buying”. – Dan

With the Civilian NFA Weapons collecting market in a continual stage of flux, one thing remains constant. Those firearms of the general type almost universally known as “H&K” models remain THE most favored and desirable amongst those collectors of firearms of the “modern”, post World War II era.

Their almost mythic popularity and desirability are not without pitfalls however, as unless one is well versed in what is available in terms of the mechanics and cosmetics of the available versions, it can come as a rude awakening to discover what one has acquired is not necessarily what one expected! The fact that these guns, as a class, are some of the priciest civilian legal semi-automatics makes it incumbent upon the owner, or individual considering ownership, to grasp the many significant factors that determine relative value of the vastly different incarnations of these guns. With very few exceptions, those “H&K” type machine guns available for civilian purchase as fully transferable Title II NFA weapons all began life as Title I semi-automatics. This is the basis for the most confusion, as there were many different routes taken to convert the guns into functioning semi-autos. These converted semi-auto guns are often surrounded by confusion and misunderstanding, and it is that problem that this article will address in detail.

Even the common identifying term of “H&K” is a misnomer since an examination of the history and pedigree of this general firearm design will reveal that the salient features of its’ design were well established during the late WWII years in the work of the famous Mauser Werkes firm on the evolutionary prototype SturmGewehr-45. Only after a period of considerable movement around Continental Europe post-war did this basic design eventually find a permanent home with the reconstituted pre-War Mauser factory in the guise of the then new West German firm of Heckler & Koch GmbH, and final conversion to the then new NATO 7.62mm caliber. This occurred, after having been initially designed and experimentally first produced in post-war Spain by the Government entity CETME during the period 1949-1956, with the engineering guidance of Ex-Mauser Werkes employees who had fled Post-War Germany and settled in Spain. After a hurried initial adoption of the new rifle in 1957 by both the Spanish and West German militaries, the Spanish and West German Governments co-operated for a time on the final development and perfection of the basic operating system until it became clear that there existed differing requirements for a new service rifle in each country. Development had begun in Spain not only because the prominent ex-Mauser Werkes engineers were presently residing there, but also because West Germany had initially adopted the also then new FN FAL rifle as the Gewehr-1, the “G1” FAL. Only when it became quite clear that FN-Herstal was not going to allow a production licensing agreement to take place, allowing for West German production as opposed to direct contract purchase from FN, (…owing to a lingering bitterness over NAZI Germany’s roughshod run over Belgium) did the West German Government begin in earnest a search for a suitable equivalent rifle design that could be domestically produced. The differing specifications extant in Spain and West Germany by 1963 caused termination of the second joint-development agreement and ultimately resulted in the production of two similar, but different versions of the perfected original CETME 7.62mm NATO caliber Modelo “C” rifle design. The CETME produced versions issued to the Spanish military were in a slightly different form than the now ubiquitous German Army standard Gewehr 3, or “G3” as it became universally known. Parts and fittings were not fully interchangeable, though fundamentally similar.

That Heckler & Koch has become synonymous with the particular mechanical design type is due more to that firms’ aggressive marketing and promotional strategies, than any original conception of the design parameters. Although since the pre-War Mauser Werkes firm resurfaced as H&K GmbH there were no doubt a multitude of significant individuals still on hand from the time of its’ wartime conception, and the return of the engineering team to West Germany from Spain/CETME no doubt cemented HK GmbH as the preeminent production facility of this design weapon. It goes without saying too that H&K GmbH has considerably mined the potential for expansion within the basic design in the ensuing years, producing a vast family of mechanically similar weapons. Clearly this potential is owed in large part to the soundness of the original concept which allowed an inherent capacity for expansion into differing calibers and applications.

So for purposes of discussion, when we talk about an “H&K” type firearm we are using that term in the general sense, an even more important distinction when it is known that there have been more than a dozen Countries which have produced guns to the “H&K” military pattern(s) since the mid-Fifties, with a few even producing guns in the semi-automatic configuration for intended sale to Law Enforcement or Civilian markets worldwide where such sales were lawful. “H&K” type guns made by H&K GmbH licensees to the pattern of the semi-automatic versions made and marketed by the West German firm in the U.S.A.. have also been imported here in substantial numbers over the years. Imports of the Semi-auto versions were clones of the HK91 .308 caliber rifle and came from Greece (Springfield Armory SAR-3/SAR 8) and Portugal (FMP XG3S; various importers). H & K GmbH had the sole honor of manufacturing, and offering for import through their U.S. based sales and marketing arm, HK U.S.A., Inc., civilian legal sporting guns in the .233 Remington and 9mm Para calibers with their models HK43, HK93, HK94, and their “pistol” version gun, the SP89.

Foreign Licensee production and importation of semi-automatic sporting arms was limited to the .308 caliber class weapons, the “HK91” clones. No COM-BLOC 7.62x39mm caliber sporting weapons were ever imported into the U.S.A. by anyone, though H&K did catalog a few versions of their guns in that caliber for military sales. (NOTE: as this article was being prepared it appeared as though a new U.S. based concern, Special Weapons LLC, has obtained ATF approval to begin manufacture and sale of a “U.S. made” version of the HK91 & HK94 type semi-auto Sporting rifles using receivers manufactured in the U.S.A., with other major components obtained from foreign Government Licensee producers of HK type weapons, together with enough U.S. manufactured minor components to qualify the guns as being “Made in the U.S.A..” according to the new Import restrictions currently in effect. The information available seems to indicate that these new made receivers will be close copies of the original design, though not identical in all respects)

“I want an HK machine gun!”

“I want an H&K machine gun, but it has to be fully transferable……..What are my choices?” Exactly two. In order for any H&K type machine gun to be classed as fully transferable to individuals it must have either been imported prior to the enactment of the GCA ’68, or else “manufactured” here by either a licensed Class II Manufacturer or an individual filing an ATF Form 1 making application, prior to the passage of the McClure-Volkmer Firearms Owners Protection Act of 1986.

What this means in reality terms is that almost all of the fully transferable H&K type machine guns available for purchase by individuals are those guns converted from semi-automatic versions originally imported as Title I guns, since only a very, very few of the factory original machine guns were ever imported prior to GCA ’68. And the vast majority of those Pre-68 factory guns being imported were imported either in an attempt at attracting Law Enforcement and/or Government sales, or for sales directly to those entities, which means they may never have moved into the arena of normal commerce. There were also an extremely limited number of H&K weapons imported into this country Pre-68 by domestic Firearms companies who sponsored the designs under their own name and model designations in U.S. trials, and would have built and supported them had the contracts ever been awarded; the guns from the firm of Harrington & Richardson are most well known, and these guns eventually made it into many private collections when the H&R Museum collection was sold off. It is known, also, that a certain small number of private individuals did in fact personally import these factory original guns when it was still legal to do so, so they represent a legitimate source of fully transferable factory guns, though their numbers are unfortunately extremely small and their collectors’ value is understandably extremely high. So if we wish to discount these select few fully transferable factory original machine guns on grounds of pricing or rarity we are left with the vastly more common NFA registered conversions of their semi-automatic cousins, which should indicate our choices are narrowed considerably, but unfortunately that is not the case!

What causes the confusion about these “conversion guns” is that they can exist in a myriad of differing mechanical and cosmetic configurations, and that within a specific sub-type of conversion method there are even differences. All conversions are NOT the same, or equal! Within this framework we will be examining the differing mechanical methods used to perform the conversion of a semi-automatic weapon into a select-fire near copy of the factory machine gun. Previous works have examined the vast catalogs of offerings from those who actually performed the conversions on these guns when it was a legal enterprise; this treatment will be limited to an examination of the mechanical means by which these conversions reached a means to an end of obtaining an H&K type machine gun available for individual transfer and ownership. Since this is a specific treatment of the conversions of those semi-automatic guns originally imported as Title I Sporters, by nature we will only be considering those guns known commonly as HK91(HK41), HK93(HK43), HK94, and SP89 types.

There were other special versions of the basic rifles imported and sold on a very limited basis by HK U.S.A. Versions of the .308 NATO caliber HK91 known as the HK911, SR-9, SR-9T & SR-9TC were imported, as were a dramatically small number of a special “Marksmans’” version of the HK94 9mm rifle. Due to import restriction factors of when they were imported, these guns were ordinarily priced considerably higher than their standard counterparts. They were also configured in vastly different styles than the standard rifles, so it is extremely doubtful if any of these guns were ever used as the host gun for a select-fire conversion, although with the increasing rarity and escalating prices of “pre-Ban” guns it is quite possible they may one day be used as the host weapon for a conversion project using a NFA registered sear or trigger pack. (Due to extreme cost and mechanical differences we will not be examining the PSG-1 Precision Marksmans’ Rifle, although it too was imported as a semi-automatic Civilian legal Rifle.)

As far as the non-West German origin imported Sporter rifles go, there may have been different designations or appellations given to the various clones of the “HK91” type rifle by non-H&K GmbH producers, but the guns are mechanically the same and will be considered on a parallel basis to the West German produced HK91.

The Receiver: Where It all Begins.

Machine gun or Semi-automatic, ALL H&K type receivers are the same! Lets take a long look at THAT statement because the receiver of these conversion guns is often the point of the most contention when it comes to understanding, and defining, a particular conversion method or its’ official legal status under the Regulations of the National Firearms Act. And sometimes more importantly, interpretations of those Regulations by BATF Technology Branch.

To understand the above statement in the context of a converted semi-auto Sporter gun into a mechanically viable select-fire version, one needs to understand how the receiver component is actually manufactured. All H&K type receivers are made by high pressure sheet metal press-forming under a mandrel… commonly called “stamping”. This finished formed ‘blank’, formed in the shape of both sides of the receiver laid out flat, is then carefully folded and seam-welded into the common finished form or ‘receiver body’. Part of the forming process is the punching or indexing of the various holes in the receiver required for assembly, or alignment of the sub-components or assemblies of the completed weapon. Up through this point ALL H&K type receivers, within a common caliber grouping, are the same during the manufacturing process.

One of these sub-assemblies is the entire fire-control group, which contains the “fire-control trigger pack” mounted inside the trigger grip housing, and which together are commonly identified as the “trigger group” on H&K type guns. (For purposes of this discussion, we need to standardize some terminology regarding the fire control components as even the factory official nomenclature can be confusing at times. The fire control group as mentioned above is composed of two distinct sub-assemblies: the outer grip frame housing, which can be either of full sheet metal construction or in later versions of a hard plastic molded over an inner metal frame, and the internal sheet metal trigger pack which contains and provides a secure mounting for the individual mechanical component pieces which provide the fire control functions.) These fire control groups can come in many different styles as will be examined, but in basic terms they were produced in either of three major categories. Originally with a “slab-sided” full sheet metal exterior housing with a separate contoured palm grip piece offering the standard “S-E-F” (Safe-Semi-auto-Full-auto) fire control selections, later a cast metallic housing with a molded in ergonomic finger/palm grip With the introduction of the full plastic style ergonomic featured housings, a wide range of fire control selection offerings including groups with standard “S-E-F”, 2 or 3 shot burst features either alone or with full-automatic selection, and the latest groups offer a second external configuration of full plastic housing resorting back to a ‘smooth’ grip contour while introducing an ambidextrous control selector feature to the full range of fire modes.

On most factory original machine-gun versions the fire-control group is mounted by, and pivots on in the front, a horizontally sliding assembly pin front and rear. (Some notable factory original exceptions to this trigger group mounting rule are machine guns such as the HK21E and G41, which for specific and intended design purposes, are equipped with a non-pivot pin type front trigger group attaching mount. A contrary exception to the general rule is the MSG90 Selected Marksmans’ Rifle; though the MSG90 is intended as a semi-automatic weapon it is designed with a front pivot-pin mounting attachment for the semi-automatic fire control group, as such, BATF has classified this particular weapon as being legally classed as a “Machine Gun” even though it is incapable of automatic fire in its’ intended form.) This front assembly/pivot pin is aligned and held by a set of indexed holes located on a rounded protrusion of the receiver bottom at the rear of the magazine housing upper area. It is mated with a permanently affixed (usually welded, though press fitting is sometimes used) fitted bushing on the inside of the two pressed steel sides of the receiver stamping for the assembly/pivot pin to pass through and bear against. By this method the trigger housing can be held fast in the proper position on the receiver, and also pivot downward for quick inspection, disassembly or cleaning. This arrangement is commonly known as the “push-pin trigger group assembly”, or “swing-down trigger housing” type. It is the original and most common method of attachment of the trigger housing on factory machine-guns. This feature alone, the holes and mated bushing for the front assembly/pivot pin, has been determined to be the sole defining feature by BATF Technology Branch as to what constitutes in a legally defining sense a “Machine Gun” receiver for an H&K type weapon under the National Firearms Act.

A “semi-automatic” receiver intended for a Title I semi-automatic Sporter rifle is taken from the same assembly line as its’ machine gun cousins. It is only after the receiver is completely formed does any distinction arise. The ONLY difference in the receiver of a semi-automatic gun is in its’ method of attaching the trigger housing, or “trigger group”. Within a selected caliber grouping of weapons there are NO internal receiver differences between, say, a select fire G3 and the semi-auto-only HK91!! This is significant.

On semi-automatic-only receivers, in order to pass the scrutiny of U.S. import restrictions intended to prevent easy or rapid conversion to semi-automatic fire mode, H&K had to devise a suitable method of preventing attachment of the standard factory ‘push-pin, swing-down’ type trigger group/trigger housing used on factory original machine guns. It also had to be able to correctly hold alignment for the trigger pack mechanical components on the receiver. Further, it had to be able to be developed from the basic receiver component already being produced for the machine gun production line, as to re-tool for completely different stamping dies would be economically unfeasible for the limited market represented by Civilian (or Law Enforcement) sales in areas where such markets existed.

This redesign was accomplished by altering the forward point of attachment of the trigger housing group, from a bushing lined set of holes through the lower rear of the magazine well combined with a transverse mounted push-through retaining pin, to a transverse oriented ledge at the lower rear of the magazine well fitting into a rectangular cut out in the lower front of the semi-automatic only trigger housing (and a suitably modified fire control pack). This rectangular cut out in effect ‘clips on’ the ledge, or lip of the rear of the magazine well. This ledge, or ‘lip’ is actually a separate U-shaped machined piece which is welded around the location of the normal pivot holes and bushing, and in some versions is attached after the normal pivot pin holes are punched into the receiver stamping, leaving the holes clearly visible from the inside, though the pin bushing is not present, and the holes themselves are blocked with a welded in bar. Guns produced in Greece by Hellenic Arms S.A. (Springfield SAR3/8) are known to be made this way. Due to this, on semi-auto trigger frame housings the front semi-circular “ears”, which contain the pivot pin holes on machine gun versions, are removed leaving a squared-off shape to the front of the trigger housing which does not fully contour into the lower portions of the receiver, leaving a cosmetic mis-match.

The other significant difference that this causes functionally between the factory machine gun receiver and the semi-auto-only receiver, is that since the front pivot-pin holes and bushing are not available, the standard machine gun style “flapper type” magazine release cannot be installed in the usual manner. This “flapper” is actually an additional mag release point which pivots on the push pin bushing in the machine gun style receiver, and by contacting a wedge shaped contour on the actual magazine release shaft, causes the shaft to move laterally releasing the mag catch in the same fashion as pushing the catch itself would in the normal manner. As such, all semi-auto-only receivers are produced with only the actual magazine release catch button, though most operators find the “flapper” style release to be more desirable and convenient. Many enterprising Class II manufacturers have designed a modification of the factory flapper style lever that passes BATF Technology Branch scrutiny, allowing a functional flapper release to be installed on semi-auto receiver without modifications in the legal sense.

The removal of this U-shaped ledge piece, or its remaining intact, is the MAJOR DEFINING DIFFERENCE IN VALUE of those guns converted into registered machine guns from semi-auto Sporter beginnings. This is simply because with the “clip-on” lower feature removed, the gun reverts back into its’ full machine gun receiver configuration and can be equipped with a normal swing-down, push-pin type trigger housing (and fire control pack), the same as a factory delivered machine gun came equipped as originally designed. In terms of operating system function, if you can attach a swing down trigger group, there is no remaining mechanical distinction between a semi-auto gun so converted and a true, factory original, machine gun. A semi-auto gun converted in this fashion can use any and all factory original push-pin type replacement trigger groups, and spare parts for such items are plentiful and relatively inexpensive….something that is not necessarily true if the gun was converted by other methods.

All other conversion methods employed were done in an attempt to circumvent this fact that the ‘clip-on’ ledge has been left intact on the individual gun in question. The fact that the receiver of the gun in question may in and of itself be the NFA registered item, as opposed to employing a “machine gun conversion part or device”, is of no consequence in this. It is solely based upon how the gun was mechanically converted ORIGINALLY AT THE TIME OF CONVERSION to a functioning machine gun (and its’ attendant registration status), that determines its’ respective monetary value as a transferable NFA weapon.

Registered Receiver Conversion? Maybe…….maybe not!

This is where a pivotal distinction needs to be made before we are able to go further. As has been alluded to above, NFA converted guns can be most basically categorized two ways. First, there are those guns that have been converted not just to replicate the functionality of a factory selective fire product, but actually physically altered to duplicate the factory machine gun in mechanical function, appearance, and physical dimensions by removing all the defining features of the semi-auto iteration and replacing with the features identical to a factory produced machine gun, in effect reverse-engineering the semi-auto design changes.

Secondly, there are those guns that have been cleverly altered to effectually replicate the selective fire capabilities of a factory produced machine gun, either by employing an NFA Registered conversion part (such as a “Registered HK Auto Sear”), or a legally installed unregistered conversion part or device, while retaining the legally defining features that identify the receiver of the gun as a semi-automatic Title I firearm by definition.

As a general rule these types of conversions retain the single legally defining feature of the clip-on trigger housing attachment, even if the receiver itself was NFA registered as the “machine gun” in the case of those conversions employing an unregistered conversion part. So if the receiver itself was registered as the NFA controlled item, but not mechanically converted at the time of conversion to utilize a swing down lower, the mechanical alterations to achieve semi-automatic fire capability must have been done to other components of the weapon in question.

Such is the case that the mechanical differences between a semi-auto-only capable rifle and a fully automatic capable rifle can be contained completely within the fire control elements located in the trigger group. It is common to employ the phraseology “conversion part” for a single NFA registered piece such as a “registered HK Auto Sear” when used as the basis of a lawful conversion. And the phraseology of “conversion device” is used in describing an assembly converted as a unitized item utilized as the basis for lawful conversion, such as the adapted factory select fire trigger packs and grip frames. So when we speak of an “unregistered conversion part or device”, or a “NFA registered conversion part or device”, we are relating the fact that within the fire control trigger pack, certain parts not present in the semi-auto-only capable version can be introduced to replicate the functionality of the factory original selective fire version, all without mechanically altering the actual receiver of the gun. This is the case whether we are discussing a “Registered Receiver” conversion where the actual receiver of the gun retains all the defining semi-auto features, yet is NFA registered itself, or if a “Registered Conversion Part” is utilized to convert an existing Title I gun. In both of these legally disparate cases, the receiver of the gun remains unaltered mechanically. All mechanical changes necessary to affect conversion into a functioning selective fire weapon (Other than adding a trip shelf to the bolt carrier) are carried out on the fire control group alone.

Whether registered or unregistered themselves as conversion parts, the introduction of these additional parts into a semi-auto fire control pack is designed to restore the three missing functions present in the selective fire version. First, since these guns fire from the closed bolt and are hammer fired, there must be a mechanical means to prevent hammer fall until the bolt is locked into battery. Secondly, there must be a means to time that hammer fall correctly. And lastly, there must be a means to select the mode of fire desired. In H&K style guns these three requirements are met by the addition of two parts not normally present in the semi-automatic-only fire control pack.

The first and most important of these “missing” parts is what is known in official H&K nomenclature as a “Catch”, but is more commonly understood as an “automatic sear”; this part serves to hold the hammer from falling until mechanically released at the proper time. It is also present to prevent inadvertent or premature hammer fall, in short a secondary safety system. H&K nomenclature specifies that the part performing the disconnector function is known officially as the “sear”, because it functions in both capacities when it is operating independent of the “automatic catch”. The “automatic catch” and “sear” are totally independent mechanical systems, operating in conjunction only with the selector, or “change lever” in factory parlance. With the selector lever in the cyclic fire mode, these two independent systems operate in parallel to allow cyclic fire operation. The automatic catch ALWAYS functions, only when the selector lever is set on cyclic mode does it allow the trigger to lower the sear out of position to prevent the disconnect function. This means the only mechanical system acting on the hammer then is the automatic catch, functioning as the primary sear in this mode. This is why it has come to be commonly known as the “automatic sear”. (To avoid further confusion we will use the term “sear or automatic sear” when specifying the factory term of “catch”, and disconnector in place of the factory term “sear”. This is due to the universal application of these terms currently in the Civilian NFA community, not to instill confusion with factory nomenclature!!)

The only other required additional “missing” part is what is called the “release lever”. This is a vertically mounted swinging arm located on the forward right side of the trigger pack frame. At its’ mid-section it is in contact with the extension arm of the “catch”(auto sear). Its’ upper end is located in the path of travel of the right underside of the bolt carrier which activates its’ forward movement upon final dwell of the bolt carrier after the bolt has moved into battery, causing release of the catch and hammer fall.

In terms of component parts, these additional two parts are all that distinguishes the two versions of the basic fire control pack, although the actual physical specifications of certain parts vary slightly between the two versions. With these simple additions to the standard semi-auto trigger pack, and minor modifications to a few other small areas discussed below, a functional selective fire conversion can be achieved. The fire control selector switch is identical in both semi-auto and selective fire variants of the guns, and is merely prevented from moving into the full-automatic selected position in the semi-auto trigger groups by a mechanical detent formed into the exterior of the trigger housing.

In real life terms, these so called “conversion parts” usually took the form of a proprietary design of the “catch”, or “auto sear”, although there were a number of variations on that theme. Besides several variations of “auto sears”, such other items will be encountered as combination release lever/auto sears, or combined auto sear and pivot. In those cases, only the “catch” was legally defined to be the actual “conversion part”; the release lever and required modifications to the trigger frame itself to affect proper installation were not considered in a legal sense. In most installations a factory release lever was utilized without modifications.

When moving into the realm of “NFA registered conversion parts or devices”, it was mechanically almost the very same items and choices, the single legally defined conversion part was simply marked and registered as the “machine gun”, it being the only controlled part. When exploring NFA registered conversion items though another category is sometimes encountered, that being the registered conversion trigger pack itself, as a complete entity. This was the actual stamped metal trigger frame of a stock factory produced MG trigger group complete with all unaltered factory MG parts, suitably modified along with its’ companion trigger housing to fit the clip-on attachment of a semi-auto receiver, marked and registered as a complete unit, and were known to have been NFA registered as a “conversion device”. While rare, these units are highly desirable as will be shown.

(Just to muddy the waters a bit, there are known a very few “Title I” guns that have had the receiver trigger housing attachment point originally converted to a push-pin, “swing-down” style while employing a Registered “Auto Sear” as the NFA registered item, leaving the now highly modified Title I receiver in a questionable legal status. These use a modified semi-auto trigger pack the same as most “Registered Sear” conversions, and ATF is surely not happy with these guns for sneaking through, and their legal status is at best murky.

Additionally, some of the very first “HK91” style semi-automatic rifles imported were actually made with a receiver that was identical to a factory MG, that is it used a push-pin type trigger housing attachment to attach a SEMI-AUTO capable only trigger mechanism, which was nothing more than a select fire trigger pack modified to prevent automatic fire. These guns were even identified on the receiver as “G3”, and were imported by Santa Fe Div., Golden State Arms Corp. A select few were snapped up by enterprising Class II’s and some enlightened individuals and converted to transferable machine guns by registering the actual receiver and simply pinning on a standard factory MG trigger group. Such a gun would have doubly interesting value to a collector!)

It can be readily seen then that the most desirable method of conversion was just to fully restore the receiver of a semi-automatic gun back into a machine gun receiver able to use factory original machine gun, push-pin attachment type, swing down trigger groups by removing the ‘clip-on’ lower attachment block and preparing the forward attachment point to accept a pivot-pin attachment. This was most usually accomplished by milling off the ‘clip-on’ ledge, and then align drilling the two holes in the lower rear of the receiver, followed by welding in the pivot-pin bushing. With these modifications accomplished a factory swing down trigger group could be attached in the normal manner, and that combined with installing a factory original bolt carrier (or a modification to the semi-auto version) completed the mechanical alterations needed to produce selective fire. If this method was chosen, at the same time a factory machine gun type “flapper” style magazine release could also be installed in the correct manner, mimicking fully the factory machine guns.

So why weren’t all these guns being converted this way?? Two simple reasons. First, with the surplus parts market the way it is today many people are unaware that things weren’t always as they are now. The availability of foreign machine gun parts and/or parts kits in the quantities they are seen now is a fairly recent phenomenon, due to a positive change in the relevant laws. The ready availability of these parts today from factory produced machine guns makes it much easier to contemplate doing a near exact copy of the factory made gun as many of the needed parts could be simply exchanged. This situation however was definitely not the case during the time frame that most of these semi-automatic guns were being converted and registered as Civilian transferable machine guns. The most common reason that so many Class II manufacturers performed conversions using heavily modified semi-auto parts is that they had no choice! The registering of the semi-auto configured receiver as the ‘machine gun’ was just the most commonly accepted sequence, after which one went to work on the semi-auto trigger group parts to affect the needed mechanical changes to render automatic fire. Then a further realization, due to a late ATF ruling, that certain dedicated parts used to convert a semi-auto gun to automatic fire mode were also now being legally classed as “machine gun conversion parts” in and of themselves prompted a rush to register these parts alone as “machine guns” in large quantities. It became far more economic in terms of time and manufacturing ability to concentrate on producing these so called “conversion parts” then it did to take the time to locate, obtain, and mechanically convert complete guns. Such was the genesis of the common “Registered H&K Auto Sear”. Thus a time line developed whereby before this ATF ruling most NFA conversions had the actual receiver of the gun registered as the “machine gun”, whether or not the receiver had actually been fully restored into a machine gun receiver, and after the ruling most manufacturers resorted to registering the conversion part, or device, or “kit”. It has become synonymous within the industry to recognize the term “Conversion part” as exclusively meaning a conversion auto sear (the “catch” in HK terminology), whether it is in and of itself NFA registered or not. This is due to the BATF determination and legal classification of this part as a “machine gun” as described above. As will be seen, the other required “parts” for a select fire conversion are not discussed separately in a legal definition.

Ranking right up there with cleaning weapons after firing them, linking ammunition prior to going out shooting a belt fed machine gun has got to be one of the least desirable aspects of the hobby for the Civilian MG shooter/owner. Historically the only away around the tedious task of hand linking ammo (..when you could) was to obtain a Military surplus belting machine or linker. Even these were not always an easy or fast method to link up large quantities of ammo, despite their design origins, and they were never inexpensive, when they were obtainable at all.

Fortunately for all of us with this problem, Doug Melton, of the D. H. Melton Co. (1221 East Del Rio Drive, Tempe, Arizona 85282 (602)967-6218), grabbed the Brass Ring and ran with it. His new creation, The Link-Master(reg. tm), is THE simple, well-engineered, and well-manufactured answer we have been anxiously waiting for!! With his latest creation the job of loading large quantities of ammunition into either disintegrating or non-disintegrating metallic link belts prior to going to the Range has been reduced to a simple and efficient task, while the built in alignment feature guarantees that each linked round has been properly located at the correct shoulder depth position for each type of link or belt being used.

Knowing that crank type, hopper or strip feed, loading machines are sometimes temperamental to get to operate reliably, Doug opted to go with the more conventional throw-lever design as typified by the USGI M7 linker issued as standard equipment for the manual linking/de-linking of .50 caliber ammo in the field. In fact, Doug realized that the basic design of the M7 linker was so good that it only had one failing; it was designed to load only one caliber, .50 BMG. Rather than try and make a down sized M7 linker like others were offering (usually modeled after the old GI issue .30-’06 linker made for the 1919 series of MG’s) that could only load one caliber on one type of link, Doug opted to use the issue M7 as a universal base unit by slightly modifying it and then designed a series of 20 rd capacity “die-plates” that could be mounted to the M7. This allowed a virtually unlimited number of combinations of ammo calibers and link/belt types that could be loaded/un-loaded, all while still allowing the base M7 to still be able to load/un-load .50 BMG caliber ammo. Changing calibers or link/belt types only requires that the die-plate for the desired caliber/link combination be mounted to the slightly modified base M7 and loading may commence. Changing die-plates takes less than 1 minute. Each die-plate is engineered and manufactured to function flawlessly in the selected caliber/link type specified.

The standard issue M7 is only altered by having three mounting holes placed in the original throw lever to accommodate the new pusher bar used with the smaller caliber ammunition die-plates to adapt it to the new set up. The individual die-plates are made from forged and machined, heat treated, alloy aluminum bases and have hardened steel pin posts to hold the links in position. The new throw bar is also made from the same aluminum alloy, and is universal for all calibers other than .50 BMG. Doug currently offers ready-to-go die plates for .308 caliber ammo in disintegrating link styles for Israeli .308 BMG links and USGI/NATO M13 (“M60”) links, in .30-’06 and 8mm calibers for USGI BMG links, in .223 Rem. caliber for NATO M27 SAW links(25rd capacity), and for non-disintegrating belts in caliber .308 using German MG34/42 or HK21 belts (the pitch of these type belts is close enough to not cause any problems during linking). He will be offering many more styles of die-plates as time goes on, he can also make any custom order die-plate if he is provided with a handful of the required links to work from. He will also shortly be making available special die-plates to load USGI (.30-’06 caliber), or Israeli (caliber .308) fabric belts for the Browning 1919 series of machine guns.

We tested the Link-Master(tm) in .308 caliber with both IMI .308 BMG links and US M13 “M60” links. Both die-plates ran flawlessly through over 800 rounds of linking, and once the procedure for quick changing the die-plates was learned we could switch link styles as fast as the hands could move. There was some small surface roughness from the final machining operations but after a few dozen rounds these were all burnished away and cartridges could be linked as fast as our helper could drop 20 new links and cartridges into their slots and then throw the lever bar. (To help in the smooth sliding of the cartridges, we lubricated the top surface of the individual die-plates with MIL-COMM TW25B synthetic flouropolymer spay lubricant which impregnates the metal pores with a TEFLON(tm) coating.) Timing the operation we could easily load 5 20rd sections into complete 100rd belts in less than 4 minutes, which is fast by any standard!

The Link-Master(tm) is currently available two ways. Doug has obtained a quantity of new, never issued USGI M7 linkers which he is offering already modified and set up to go with the die-plate of your choice included, with optional die-plates available at extra cost, or he will accept your M7 linker and perform the necessary work to adapt it and then return it with the desired die-plate included (again, optional die-plate sets available). Remember, your original M7 linker will still function normally with .50 caliber BMG ammo, so if you do not need to have two linkers to still load .50 BMG ammo, just have your M7 linker modified and then order all the other caliber die-plates you need! The price of the Link-Master(reg. tm) and several optional die-plates is still often less than a Military surplus single caliber/link type crank loader. For the shooter with multiple guns in different caliber/link types this is the ONLY way to go!

The pricing structure is as follows:

Complete Link-Master
w/1 die-plate $185.00

Extra caliber/link
style die plates $95.00

Deduct $20.00 for supplying
your own M7 linker to be modified.

Shipping and Handling
charges are variable $8.00-13.00.

D.H. Melton Co., Inc.
1221 E. Del Rio Drive
Tempe, AZ 85282

Few firearms in the civilian NFA world have garnered such wide ranging attraction, and criticism as the short barreled version of the ubiquitous Stoner/Colt military standard- the “Shorty M16”. This “Shorty” label can variously be applied, and be construed to describe, virtually ANY version of the standard M16 rifle with a barrel length LESS than that originally intended by Eugene Stoner in his benchmark design. Despite real-world reasons to the contrary, NFA gun owners have had a virtual love affair with the shorter barreled versions of the design. While trying incessantly to chop the barrel on this weapon down to ridiculous proportions, they still expect the original design, forgiving as it may seem, to perform and function as well and reliably as the standard 20” barreled rifle. The firestorm of reliability complaints can usually be traced to a practical misunderstanding, or unfamiliarity, with the design concepts of the base design as envisioned by Mr. Stoner. As such, this article will attempt to bring to the forefront the practical and mechanical aspects of the synergistic effects of altering the barrel length on the overall reliability and functionality of the design.

In the world-wide catalog of short-barreled designs employing small caliber, small case cartridges, such as NATO 5.56mm M855/M193, there is basically a three way division of mechanical operating systems employed by the various firearm designs: the first and most basic mechanically, is some form of blowback direct acting type action as typified by such guns as the FAMAS, CETME Modello LC, or the numerous H&K incarnations. These rely upon the direct backward force generated against the bolt face in propelling the projectile down the barrel to function the self-loading mechanism of the specific design. Due to this direct action energy transfer, it readily becomes apparent that barrel length, per se, has no relevance upon functional reliability, as the backward impulse imparted to the bolt face by the forward projectile movement remains, in practical applications, nearly constant down to all realistically employable barrel lengths. An HK53K is a good example of this in action; with a barrel length little more than 4”, it functions reliably in the full-automatic mode.

The next type of basic design, and the most commonly encountered, is a gas operated mechanism operating through some form of piston/action rod movement acting upon a bolt carrier to transfer the energy of the expanding propellant gases to work the self-loading action. Guns in this type group include such designs as the Galil SAR/Glilon, Beretta SCS70/90, SIG 551, or Zastava M85. Though they may seem similar in design concept to the M16s’ direct gas system they are fundamentally different for one very basic reason; all gas operated, moving piston type designs allow the face of the gas piston to rest in close physical proximity to the actual gas port on the barrel from which the high energy operating gas stream is tapped. This close proximate relation allows the high energy gas stream to almost immediately, in time relation, begin transferring its energy into mechanical movement of the operating mechanism by impinging upon the face of the gas piston and start it moving the operating parts rearward. The short distance between the gas port orifice and the face of the gas piston, as a function of time, is the critical relation which allows reliable functioning.

The distance between the gas port on the barrel, and the muzzle, is the critical factor on ANY gas operated design because the duration of TIME between projectile passage of the gas port (when high energy gas can begin to act upon the operating mechanism), and projectile exit at the muzzle (when gas pressure instantly drops to ambient atmospheric levels), is the only point in the action cycle that operating energy is available to function the mechanism. Thus, it becomes clear that the most important of design criteria is to allow for a sufficient exposure to the high energy gas stream, in terms of TIME, to consistently transfer energy from the propellant gases to the operating mechanism through the gas system. In almost all piston actuated gas operated designs the size of the gas port hole is determined by the MINIMUM amount of gas (energy) needed to start the piston backward and instill sufficient residual momentum to overcome the mass of the operating parts, plus the recoil spring energy, there-by allowing for the self loading cycle to repeat itself. The port size is then optimized for this minimum level, plus a sufficient reserve to compensate for adverse conditions, or variations in ammunition. The significant design advantage of this type system is that once the piston/bolt carrier is started moving backward at the required velocity, any excess gas can be bled off through a venting system, so it is self compensating for differing ammunition or environmental conditions. By sizing the gas port hole larger than absolutely necessary for normal operation, an operating reserve is built in, yet because of the venting or gas bleed off, there is no worry of excess energy not being able to be dissipated. It becomes apparent then that there are two separate factors, in terms of energy, to be considered. First, it is necessary to allow for the gas stream to instill SUFFICIENT energy to cycle the action through the recoil phase, and secondly, it is necessary to prevent EXCESS energy over and above that required for reliable extraction/recoil spring compression from being instilled as residual momentum into the operating parts. Thus it is that RELIABLE ultra-short barreled, gas-operated designs, like the AKSU-74 or Micro-Galil, invariably utilize a gas piston type action, with a self compensating cylinder provided with overboard bleed vents. Such designs may also typically utilize a muzzle mounted expansion chamber type device to provide for more reliable functioning by providing a fractional time delay in the absolute pressure decay of the high energy gas stream prior to the instantaneous drop to ambient pressure after muzzle exit. These devices, in effect, lengthen the effective distance, and thus add time, between the gas port and the muzzle. It is an absolute misnomer to call these devices “sound moderators”, or believe they exist for any reason other than to improve functional reliability. (Use of these devices may significantly increase the velocity of the bolt carrier, and it would be prudent to adjust gas port size accordingly).

The third type of operating system is, in reality, a simplification of the piston actuated, gas operated system. This is commonly known as a ‘direct gas’ type of operating system because the energy of the expanding propellant gases is transferred directly to the bolt/bolt carrier without the intervening action of a piston or action rod. This is the heart of the M16 design. By simplifying the mechanical end of the design by eliminating a need to employ a separate gas piston or action rod, a significant amount of weight and complexity was eliminated, unfortunately though, so was a significant amount of forgiveness in terms of operational reliability when deviating from the original rifle specifications. The only current production, short barrel, designs to employ this type of operating system are the numerous “M16” variants, and the Daewoo K1. So now with a bit of the mechanical differences explained between various short barreled automatic weapons, some with a reputation for reliable function, some with a not so good reputation, we are ready to delve into the specific functional problems encountered when trying to extract reliability from the M16 design when utilizing other than the original barrel length of 20” (and the usage of the original fixed stock).

The M16 ‘Shorty’ problem is no mystery to solve. Many people have erroneously attempted to correct mis-behaving guns by altering what they perceive is a “timing” issue related to barrel length. That is to say, they believe that a gun with a short barrel should employ an altered point of release of the hammer by the automatic sear during the final dwell movement of the bolt carrier during the counter recoil stroke to compensate for problems encountered when utilizing a shorter barrel. Timing IS NOT the problem. If a certain receiver will work with a 20” Barrel, it will work just fine with ANY barrel length, theoretically (there may be other inherent problems with a particular receiver, but that is beyond the scope of this discussion). The problem with Mr. Stoner’s gun, when altering barrel lengths, is that the gun and ammo were designed as a SYSTEM, with everything balanced in harmony: gas generation pressures and volume from the cartridge, gas port size, gas tube length, cylinder and piston displacement (bolt carrier and bolt), recoil counter balance energies, etc. Stoner designed his cartridge and gun as a paired system to function in harmony with a 20” barrel, period. The problems arise when solutions are attempted that do not take into account the synergistic effects on the entire operating system when altering a single component or variable in the entire equation. The direct gas type operating system needs to be understood completely before an effective solution can be achieved. While the below discussion is also relevant to the semi-automatic mode of fire too, it will be seen that functional reliability problems often encountered with short barrel M16’s are mostly peculiar to the cyclic mode of fire due to the high speed of the operating components when operating automatically resulting in, most commonly, “light primer strikes” though “short recoil” may evidence itself in the semi-auto mode too.

The M16 operating system (Gas system), to function reliably in the recoil portion of the cyclic operating mode, needs two things from the energy contained in the burning powder charge: 1. Sufficient energy to begin functioning the reciprocating parts and overcome the counter recoil energies contained in case extraction and the recoil spring/buffer mass and, 2. Sufficient exposure to those minimum energies in the time spectrum to overcome these counter recoil forces, complete the extraction cycle, and instill sufficient residual energy in the operating system, as retained potential energy, to fully compress the recoil spring/buffer throughout its’ travel limits. This covers the first ‘half’ of the operating cycle, up to the point of counter recoil. The other potential area for problems is in the portion of the operating cycle that encompasses the counter recoil travel of the operating components during re-chambering and final bolt carrier dwell movement following bolt lock-up.

The two most common problems with these “shorter” barrels ( i.e., barrels with LESS distance forward of the gas port than the standard 20” barrel), are “short recoil” (failure to extract fully and/or re-chamber a fresh round), & “light primer strikes” (bolt CARRIER rebound during counter recoil). These are really very simple to understand in the context of time exposure. The problem with these “Shorties” is NOT, I repeat NOT ‘timing’, it is TIME. There is a direct correlation between the location of the gas port and the remaining length of barrel before the muzzle exit point. THIS IS ALL THAT MATTERS, the length of barrel FORWARD of the gas port. This is TIME. The design substitution of a long gas tube to pipe gas stream energies directly back into the bolt carrier “cylinder”, instead of using a gas piston located in close proximity to the gas port, has the practical mechanical effect of inducing a time delay into the operating system between when the high energy gas stream exits the barrel at the gas port, and when it can actually start to force the moving components rearward. This design “feature”, combined with the extremely short distance from the gas port to the muzzle combines to produce the majority of the functional reliability issues experienced with this particular short barrel weapons system.

Assuming you have not altered the diameter of the gas port hole, if you swap an 11.5” barrel, on an otherwise properly functioning receiver, you may FIRST experience short recoil; this is clearly indicated by the fact the high energy gas stream delivered to the operating system sufficient energy to BEGIN the extraction/recoil cycle but then stopped; it did not instill enough residual energy to overcome the counter recoil forces and movement stopped when the pressure it was exposed to instantly dropped to zero as the bullet exited the muzzle. Had it continued to be exposed to this high energy stream for just a little more TIME, you would have no “short recoil” type functioning problems at all. TIME is the key to building up in the operating system that residual energy, over and above that required to begin extraction, to complete the recoil cycle. Three barrels that ALL work reliably are the 20”, the 16”, and the 14.5” M4; notice anything between them? They ALL have the same or greater (in the case of the 16”) amount of tube BEYOND the gas port to the muzzle. Ever wonder why Colt came up with the 14.5” barrel anyway (from the Colt Model 653)???? It is the least barrel length, while utilizing the other standard carbine components, that held the minimum distance from the gas port to muzzle exit point the same as the standard rifle barrel. Colts’ never did achieve TOTALLY RELIABLE operation of a “Shorty” gun until they went to the 14.5” barrel. Do not believe what you may have heard about the late 1960’s XM177/E1/E2 muzzle device being used as a “sound moderator”; that device was, pure and simple, a pressure accumulator designed to delay the high pressure decay in the operating system for a fraction of a second longer than a plain 11.5” barrel, enough to get “reliable” operation, by instilling just a bit more residual energy to the operating system before the pressure dropped instantaneously at the muzzle exit point.

Mr. Stoners’ genius was evident in his design of the direct gas system using the bolt and carrier as the gas cylinder and piston, instead of using a large separate piston and rod set-up like other designs; this works great AS LONG AS THE OPERATING SYSTEM IS EXPOSED TO THE REQUIRED MINIMUM ENERGY LEVELS FOR ENOUGH TIME. The long gas tube used eats up time; look again at other gas operated “Shorty” designs, they all have a piston which is physically located very close to the gas port; gas stream energy can immediately begin to transfer energy to the piston as soon as it leaves the barrel, for all intents and purposes. Shorty Galils work fine, shorty M16s’ have trouble. Colts’ attempted to solve the problem with the redesign of the operating system to include a gas piston arrangement in their experimental Model 703, work which later showed up in the Taiwanese version of the M16A1, their Type 65 rifle. The civilian ‘Rhino Conversion’ for the M16 also capitalized on that work when that company added, in reverse engineering to Mr. Stoner, a gas piston operating system to improve operating reliability, the same as the new ZM-Weapons Co. is doing today.

For most people struggling with the problem of short recoil, there seems to be almost a universal belief that the answer lies in increasing the size of the gas port hole in the barrel. Opening up the gas port hole will only add more VOLUME of gas to the system; since, in the practical application, pressure remains nearly the same you will in all likelihood achieve the next problem commonly encountered, i.e., “light primer strikes” causing failure to fire and stoppages after two or three shots. Light primer hits are caused by ONE thing (assuming your gun is mechanically OK to start with); excess counter recoil energy is not being dissipated upon completion of chambering and the bolt CARRIER rebounds off the rear of the barrel extension and the bolt itself literally starts to unlock again, at which point the hammer, traveling only under the spring energy imparted to it from the comparatively weak hammer spring, hits the backward traveling bolt carrier with insufficient energy to send it forward again, relock the bolt head, AND strike the primer anvil with enough energy to cause ignition of the cartridge. Under these circumstances, the hammer has acted basically as an anti-bounce deadener. The reason the gun will fire correctly for MORE than one round, but will usually stop after two or three, is that the face of the bolt carrier and the rear of the barrel extension experience a cyclic increase in what is known as the “coefficient of restitution”. This physical phenomenon is best described as a decrease in the ability of the impacting metal surfaces to return to their original “shape” or “height” following repeated impacts, thereby robbing them of their ability to absorb, or “deaden”, the forces of impact in the exact same capacity following the initial impact. In practical terms, after the first shot the impacting surfaces are unable to provide a sufficient cushion to prevent excessive rearward movement of the bolt carrier until a point is reached to where the energy of the hammer spring is overcome and firing ceases.

If the gas port hole diameter has been increased in an attempt to cure short recoil, you have now most likely ADDED to your list of problems. By observation of your gun following such a ‘fix’, the operating system obviously is imparted with enough energy to go through the extraction and recoil phase but it now cannot dissipate the EXCESS residual energy that the additional gas volume has now imparted to it by drilling out the gas port hole (giving an effective increase in gas flow per unit time). Remember, Stoner designed every element of the rifle system to work in harmony; any excess gas volume, above what he balanced the gas cylinder (bolt carrier) for by specifying the standard diameter of the gas overboard vent holes in the side of the bolt carrier to a specific size, will impact the system’s performance.

The time and distance the cylinder (Bolt carrier) travels while moving to the rear and unlocking the bolt, relates to the point where the remaining gas charge is dumped through the ports. The retained gas in the system continues to ADD energy during recoil, energy which cannot be dissipated by the spring energy of the recoil spring and buffer mass. This increased energy, having been transferred to the recoil spring/buffer assembly, will manifest itself as INCREASED driving force during the counter recoil cycle (read ‘cyclic rate’). This is more energy than the carrier/bolt/barrel extension were designed to absorb during re-chambering and so the carrier ‘bounces back’ after chambering and bolt lock up.

The solution to fixing “shorty” problems then is simple, but most people don’t want to believe it. You must return the TIME element in the operating system (Gas system) to the designed level of the 20” barrel standard, while maintaining the original energy levels designed for in the recoil system. The primary reason that excess counter recoil energy is induced into the operating parts is that the gas port hole has been over ported in an attempt to cure short-recoil, but what causes the gun to display a tendency to seemingly NEED that extra ‘power’ in the first place? This is the real START of the problem(s)! The base cause for this then can almost always be traced to ONE element in the system, the barrel, or more correctly the chamber of the barrel. With rare exception, the barrels available from commercial sources are produced with chamber dimensions of SAAMI specification to the .223 Remington caliber, with differing minimum and maximum tolerances around that general specification. Commercial .223 Remington IS different from 5.56mm NATO chamber dimensions, and Colts’ proprietary chamber dimensions are different still. More importantly to our case at hand, if there exists a variance in tolerance in these commercial barrels it usually goes toward the “tight” side in a vain attempt to advertise the barrel in question as being more “accurate”. The problem is that in a MACHINE GUN you DO NOT desire absolute accuracy as a first criterion in designing barrels; it is more important to design in any attributes that would lead toward a higher degree of functional reliability. It should thus be obvious now that what we seek in a chamber of a MACHINE GUN barrel is a dimensioning that allows RELIABLE chambering and extraction, using the LEAST system energy possible. This is the heart of many “troublesome” short barrels installed on civilian M16’s, and why they won’t function reliably.

Almost without exception, the commercial producers of “M16” barrels are selling the exact same barrel, with the exact same chamber dimensions, as those they sell for use in semi-auto AR15’s and clones. Because SOME individual barrels MAY have a chamber dimension that is closer to NATO spec, they may run fine, but most will not. And to compound the problem, most commercial barrels are now sold as being Chromium plated in the chamber area which renders them impossible to alter the chamber dimensioning (be VERY careful about so called “polishing out” a chrome chamber; it generally CAN NOT be done in a uniform manner!). And so, because the chamber dimension is not optimized for high speed full automatic cyclic function, we immediately introduce into the equation a factor of high required extraction energy levels; this is the reason most people want to open up the gas port diameter. It should be noted now that the standard size of .063” for a gas port in a 20” barrel is a dimension optimized for all the preceding reasons, and that because of the reduced TIME element of the short barrels’ gas port-to-muzzle distance, this size is often increased as a compromise to the time factor by adding additional gas volume-per-unit-time, as described above. Since the chamber dimensions of commercial barrels are known to vary widely, it is generally impossible to state with certainty an optimized gas port hole diameter for a specific barrel length such as 10” or 11.5”. The next compounding problem is that a gas port hole selected to give reliable function without exhibiting ‘short recoil’ tendencies may already have reached an energy level in the operating system that will induce ‘light primer strikes’ from excess energy levels in the counter recoil stroke. There is a very fine line between curing ‘short recoil’ and inducing ‘light primer strike’. The best guarantee that a short barrel will function reliably in the cyclic mode is to start with a barrel that was INTENDED from the point of manufacture to be used in a machine gun, as the chambering dimensions will already have been optimized for this intended usage. It is far better to obtain a barrel of ANY length that is a MIL-SPEC M16/A1/A2 new/replacement/surplus part, and then alter the length to suit and re-mount the front sight/gas block assembly in the proper position, than it is to gamble on a ready made “shorty” barrel of unknown pedigree. This DOES NOT have to be a “Colt” barrel; any barrel that can be positively identified as being MIL-SPEC manufactured as an original part or a replacement or spare will do, such as those made past and present, by H & R, GM-Hydramatic, FNMI, ‘Bushmaster-Quality Parts’ (M4 type), or DIEMACO. The trick is to get a “machine gun” barrel from the outset, as then your chance of developing a cascading series of problems while trying to “fix” a problematic gun will greatly diminish.

Since most people use a “CAR”, or telescoping, stock with the short barrels our problems are compounded again. To begin with, the telescoping “CAR” stock assembly, by its’ physical dimensions, has a reduced capacity to absorb recoil energies compared to the original full size fixed stock/recoil spring/buffer assembly. This already puts us at a disadvantage! Remember, any increase in recoil spring RATE, in an attempt to absorb excess recoil energy, will cause an increase in counter recoil velocity leading to light primer hits. (Conversely, there is an incorrect belief that cutting or shortening the CAR recoil spring is the correct answer; while it MAY work for a PARTICULAR combination of barrel/receiver, you are seriously compromising the longevity of your expensive weapon by tremendously increasing operating stresses!! Do NOT cut springs, unless you don’t care for your weapon!!). The most logical place to cure the problem(s) is in the recoil assembly, but that requires some expanded understanding of its’ capacities and inherent weaknesses. Adding mass to the recoil assembly in the form of a different buffer is not a good solution as this only causes an increase in the striking energy of the bolt carrier on the barrel extension, and the added mass may not be sufficient to discourage rebounding. What IS desired is some form of energy ABSORBING function contained within the recoil assembly, or barring that, some form of anti-bounce device to deaden the eventual bolt carrier rebound energy before hammer release. Testing by trial and error for a PARTICULAR barrel/stock assembly may result in satisfactory performance, but do not be tempted to alter the recoil absorbing abilities of the normal recoil spring/buffer assembly, and it is also unwise to significantly increase the mass of the buffer due to increased stresses developed. If you must have a short barrel and can’t make a CAR stock assembly function reliably with your particular barrel used, simply switch to a standard fixed, full length stock/recoil assembly. The increased length of recoil travel and buffer mass increase will almost always give reliable function with a short barrel, without undue stress and strain being imposed on the gun. For those that must use a CAR stock assembly, the most practical thing to do is to obtain, as an “off the shelf” component, one of the “Hydraulic” buffer units currently available. These are a mechanical adaptation of a simple hydraulic cylinder in which the front end of the buffer acts as the piston, acting against a fluid medium, to compress an internal spring, thereby absorbing a great deal of recoil energy without transferring it to the recoil spring. This simple action does much to prevent excess counter recoil energy and its’ attendant problems. By installing this singular component, it will allow the gas port hole to be opened up for reliable function without any further ‘short recoil’ tendencies, while eliminating any problems with ‘light primer strike’. There are other possibilities to solving this delicate balancing act between curing ‘short recoil’ and inducing ‘light primer strike’, but they all require a LOT of advanced weapon modification. Such things to try would be re-balancing the gas venting equilibrium by resizing the vent holes in the bolt carrier side; adapting an expansion chamber type device into the gas tube itself or the front sight/gas block; incorporating on the rear section of the bolt carrier a sliding mass anti-bounce weight device; incorporating on the relieved front face of the bolt carrier a section containing an impact deadening material, such as phenolic composition plastic; or using a set of variable increasing rate telescoping recoil springs.

With the advent of the M16A2-M4 Carbine, Colts’ has for the first time acknowledged that the gun comprises a DEDICATED operating system, utilizing components designed to operate in harmony as a short barreled weapon. The basic system comprises the barrel with Colts’ proprietary chamber dimensions, a modified angle on the cartridge feed ramps, and a modified recoil spring operating in unison with the so called “Hydraulic” buffer unit, or more currently, a one ounce heavier non-hydraulic buffer called the “Heavy buffer”. This Heavy Buffer is marked on the face with an “H”. What is significant is that ALL these features are designed to help alleviate the above mentioned problems, while retaining the ability to posses a usable reserve of available operating energy for adverse conditions, without sacrificing functional reliability in the cyclic operating mode. By employing the hydraulic or heavy buffer unit they can “over port” the barrel gas port to give a valuable reserve of energy for reliable extraction and then dampen it out on the recoil stroke to prevent excess energy build up in the counter recoil stroke. The hydraulic buffer unit is no more than a mechanical means of returning the time element, and thus reliability, of the standard rifle barrel gas port-to-muzzle relationship by inducing a mechanical increase in the amount of recoil energy absorption prior to the end of the recoil stroke. And as the operating parts reach their final dwell movement of the counter recoil stroke, the hydraulic buffer can act as an anti-bounce deadener by allowing any rebound energy to be absorbed in the movement of the hydraulic piston.

For most civilian shooters, it would be most practical to follow Colts’ lead and simply substitute the hydraulic or heavy buffer, after determining the correct gas port hole size for their particular barrel being used. With a better understanding now of the synergistic effects of the change to a short barrel, the civilian M16 owner should be able to assemble a reliable version of the “Shorty M16”, or cure his particular guns’ ill manners!!

The M1 begets the T4; Origins of the familiar M2

When the Light Rifle Program was introduced, one of the initial specifications was for the selected design to have an automatic fire capability. This specification was quickly dropped in the haste to get the selection process underway. It was initially found that among the entries in the Light Rifle Trials difficulties were encountered in handling many of the guns in full?automatic fire using the new cartridge that had been selected. By the time the successful entrant, submitted nearly too late for consideration, from the Winchester Repeating Arms Company, had passed the Trials and been selected and placed into the hands of the Ordnance Board for standardization and conversion to series production, many were still pushing for the full automatic feature to be included. With the winds of War blowing ever stronger in the Fall of 1941 the Military Liason Commander to the program, Colonel Rene Studler of the Ordnance Board, vetoed this feature as being a hold up to starting immediate production as it would have necessitated a period of engineering study and redesign he deemed to be too long to manage, so it was to be standardized as only a semi?automatic design. As it turns out in the end, the final configuration of the full automatic conversion was so quickly developed that it would have necessitated almost no slow down to the initial manufacturing set up program!

Among its other unique qualities the M1/M2(M3) carbine is the only US standard weapon in which the designs for the receiver between the semi?automatic and the full?automatic versions (where there exists such a distinction) are identical, an all important fact which allowed the design to be easily and readily altered to offer a selective fire capability from its original semi?auto only issuance (and a fact that causes ATF, in modern times, to absolutely HATE this gun!). The first Wartime call for an addition of the full automatic feature to be redesigned into the M1 carbine came from the Pacific Theater of Operations (PTO) following horrific encounters with the Japanese Army in close range jungle combat in which US soldiers had fared badly. The resultant analysis of these encounters convinced many that the OPTION of selective full?automatic fire would enhance the Carbines’ battle effectiveness in close range or confined scenarios. (This was to be the continuing theme song of the Carbines’ converted status: that the selective fire option was indeed an OPTION and NOT to be thought of as the sole means of usage and fire discipline.) In early 1944 then, the Ordnance Department set forth a requirement for the development of a suitably engineered redesign of the standard semi?automatic components to allow a conversion to selective fire that was able to be implemented into the normal production lines of the already being produced M1 variant. The most desirable aspect of any conversion, from the Ordnance Boards’ point of view, would be to allow uninterrupted production while incorporating the new components in the production process. Along with the Army’s Springfield Armory the two original prime contractors to the Carbine Program, Winchester Repeating Arms and General Motors Corp.? Inland Division, were the only developers of practical conversion methods that did not adversely hamper the production programs then underway, as such only their proprietary methods were ever seriously considered.

During the spring of 1944 Inland had been developing a conversion in?house under the authority of two Inland Engineers, Paul H. Hamisch and Frederick W. Sampson, that was essentially a redesign of the existing M1 Carbine fire control system with the addition of the necessary components to allow a selection of fire control modes. They managed to achieve a satisfactory conversion, while only minimally altering the standardized production design, with the addition of few new parts and some simple modifications to existing ones. This was their submission that was designated, when assembled onto a normal M1 Carbine, the T4 and that met the approval of the Board for all aspects of design requirements and production processes. For firing trials as a Service Test Type at Aberdeen Proving Grounds in July, 1944, Inland supplied 4 T4 carbines built from production M1s to the Army. Following these trials, in early September, Col. Studler recommended that the T4 design be tested for standardization, and to facilitate this a contract was let to Inland for the production of approximately 500 T4 carbines. On September 14, 1944, at the conclusion of extensive Service Test Trials by various units of the Army, the Inland T4 was recommended “..without reservation” for standardization as the “U.S. Carbine, Caliber .30, M2”

Winchester had not been so quick to jump on the bandwagon when the Board called for a conversion. They had already demonstrated at Aberdeen Proving ground in March of 1944 a proprietary design, called the Winchester “Number 3 Mechanism”, which they felt would show its superiority over any others. It has been suggested that Winchester had been expecting, unofficially, an approval of their design for the conversion request, and so had not exerted themselves to the Board.

when the Research & Design request?for?Proposal was let to all the Prime Contractors in May 1944. It is a disputed historical conjecture that the Winchester conversion offered some theoretical mechanical advantages over the Inland submission, chief among them that the Winchester Number 3 Mechanism combined into the normal safety lever the additional aspect of a fire control selector, while the Inland design required a separate selector lever. Both the Inland and Winchester conversion used a remarkably similar lay?out and concept, deriving their selectable fire feature from a simple release lever operated by the operating slide during its return stroke, and acting on a simple hammer catch to time the release of the hammer to occur after final bolt lock?up. The Number 3 Mechanism also involved 3 fewer new or modified parts than did the Inland design, a fact that Winchester was unsuccessful in convincing the Board would be a major benefit to production considerations. All the while Winchester was being forced to change over their production set?up to the Inland designed M2 they continued without success to persuade the Army to reconsider their submission based on its obvious merits. Finally they conceded that their delay in introducing their submission had prevented it from being considered side?by?side with the Inland entry and on March 6, 1945, demonstrated one last time their conversion officially to the Ordnance Board who, predictably, stated that no matter the design merits it was a moot point as the M2 had already been in production as a standard design. Winchester knew they had lost and commenced full scale production of the M2 Carbine in early May, 1945. Despite losing out on their proprietary conversion Winchester was designated as the only other Prime Contractor, along with Inland, to be allowed to produce complete M2 Carbines under the auspices of the original Carbine Contracts.

It had come down to a function of opportune timing that the Inland design was chosen for standardization from their “Carbine, Caliber .30, T4”, their submission for testing. The T4 then, was standardized as the “Carbine, Caliber .30, M2” on October 26, 1944. The Hamisch?Sampson conversion was known by two different designations in official Military nomenclature: as a complete from?the?factory gun it was the “Carbine, M2”, as a conversion set of parts, designed to be field installed in existing M1 Carbines, it was known as the “Conversion kit, T17”. T17 kits were produced sparingly toward the end of the War alongside complete guns, to augment the demand from the Pacific Theater, but it was not until the Carbine rebuild programs came into full swing post?war that the T17 kit found its true destiny.

United States Patent #2,465,487

Engineers Paul H. Hamisch and Frederick W. Sampson were granted a US patent protection, United States patent number 2,465,487 on March 29, 1949 for their wartime work on the M1 Carbine conversion program at Inland. Their conversion kit was a basic redesign of the original operating mechanism utilizing as many of the original parts as possible, simply modified where feasible, combined with the addition of a few new parts, resulting in a mechanically simple and totally reliable new mechanism that retained all the Carbines’ desirable features while adding the added protection of full?automatic fire capability.

The actual conversion can be broken down into two distinct elements: original M1 mechanism parts that were suitably modified to accommodate the selective fire feature, and totally new parts added to the mechanism, required for the additional phase of the operating cycle when operating fully automatically.

Using the operating mechanics of the M1 carbine as a basis, it was found possible to slightly alter 4 elements as follows:

1. TRIGGER HOUSING? the standard M1 trigger housing was adapted by milling 2 additional features into the housing, a vertical clearance cut in the right hand side of the rear magazine support guide, and a small double half? circular cut in the left side of this same support face to form a retaining and dismounting surface for the selector lever spring. ( figure 2)

2. OPERATING SLIDE? by milling a semi?circular cut on the right side of the slide box at the juncture of the slide arm to provide an actuating surface for the added disconnector lever to run against and force the disconnector lever downward, necessary to actuate the disconnector block for tripping the hammer in full?automatic firing mode. (figure 3)

3. HAMMER ? by milling a clearance cut in the right hand side to accommodate the added disconnector block assembly; the hammer pivot pin now also serves to hold the disconnector block assembly in alignment against the (modified) sear. (figure 3)

4. STOCK? the stock is modified by milling two shallow surface cuts, one on the right hand side to allow the added disconnector lever to swing freely, the other on the left hand side to allow free movement of the added selector lever.

An additional two parts common to the standard M1 were required to be design altered and cannot be modified or remanufactured from base M1 parts:

1. SEAR? the M2 sear is design modified by adding a vertical ledge surface forward of the pivot pin hole to the top side to act as a contact surface for the added disconnector block to act on and allow hammer disengagement when in the full?automatic fire mode. (figure 3)

2. MAGAZINE CATCH? concurrent with the M2 carbine development request came the request to design a higher cartridge capacity magazine to take advantage of the M2’s increased firepower potential which culminated in the adoption of the familiar curved, 30?round capacity, M2 magazine. Due to the increased hanging weight of the 30?round magazine it was found necessary to augment the two lateral projections of the M1 mag catch by adding a third, finger shaped, projection to the left side of the catch arm to engage with an added third nib on the side of the new design magazine. Employed without this modified mag catch the 30?round magazine will consistently disengage from the M1 catch and drop from the gun when loaded full. Most M1’s were Arsenal updated to incorporate the M2 mag catch Post?War to allow interchangeable use of the original 15?round, or now standardized 30?round, mags. (figure 3)

The preceding six named parts may be interchangeably used in the M1 or M2 carbine as desired (although use of an M2 type hammer in an M1 will require a spacer between the right side of the trigger frame and hammer); they will give no functional change to the semi?auto only operation of an M1 carbine, and were routinely added to M1/M1A1 carbines that were Arsenal reconditioned in Post?War programs. By 1946 all of these parts were only being manufactured (or re?manufactured from wartime manufacture spare parts stocks) in the now standardized M2 configuration as only the M2 carbine was the standard version of the gun officially, with M1/M1A1 now listed as Limited Standard. Originally manufactured M1 parts may be found with any of the original Prime contractors, or their sub?contractors, markings existing in the M2 configuration, and were mixed indiscriminately with Springfield Arsenal new manufactured M2 parts in the rebuilt guns.

The second major division in the M2 conversion set was the addition of seven, actually issued as six, totally brand new parts (see figure 4) that were required to be added to the operating mechanism to allow selective fire as follows:

1. DISCONNECTOR BLOCK, SPRING AND PLUNGER? when operating in the full?automatic mode the trigger is lowered out of engagement, and only the sear is holding the hammer cocked. When actuated by the disconnector lever from the final dwell of the operating slide the disconnector block is pivoted upward from the front on its axis on the hammer pin and presses downward on the top surface of the M2 sear releasing the hammer to fire the cartridge primer. The top of the disconnector block contains a drilled hole for the placement of the disconnector spring and plunger, which act against the bottom surface of the receiver to apply a constant downward pressure to force the front of the disconnector block down in contact with the rear projection of the disconnector lever for reliability of operation. It also raises the rear finger off of the top sear contact surface to improve trigger pull characteristics.

2. DISCONNECTOR LEVER AND PIVOT PIN? as the connecting lever to convert mechanical movement of the operating slide to the vertical movement required of the disconnector block, the disconnector lever rides in the vertical camming surface of the modified operating slide undisturbed until the final dwell of the slide following bolt locking and then the front nose projection of the lever is forced downward by the milled camming cut of the slide causing the lever to pivot on it’s pivot pin raising the disconnector block to trip the hammer. The disconnector pivot pin is riveted (in all but the earliest parts) to the lever, and in the M2 carbine the disconnector pivot pin replaces the front trigger housing attachment pin. The pivot pin is attached to the lever at an eccentric and rotates under influence of the selector lever to either raise or lower the lever into or out of contact with the disconnector block, thereby achieving a selection of operating modes of fire.

3. SELECTOR LEVER AND SPRING? a selector lever is added to the left side of the receiver area and in the front is connected to the disconnector pivot pin by a notched cutout in the selector mating with a notched cut on the pin. The selector lever rotates about this front mating notch on the pivot pin and is held in either of the two selectable positions by the tension of the selector lever spring, which is a half?moon shaped piece of spring steel pointed at the selector lever end and engaging in a hole in the rear of the selector and held in place on the trigger housing by the spring retaining hole described above. The spring is mounted or dismounted by sliding up or down through the dismounting notch cut in the trigger housing above the retaining notch. The selector lever is held only to the pivot pin and does not (SHOULD NOT!) contact either the trigger housing, receiver, or stock for reliable functioning.

The above salient features are the same for either original USGI carbines or after?market commercial clones. MOST mechanical operating parts will interchange between military and commercial production M2’s, although as discussed later, certain commercial carbines did depart from utilizing some of the traditional military parts as a manufacturing necessity as the surplus military parts scene changed.

A note of Caution: from a purely historical standpoint, the “second type” or “round bolt”, which has commonly, but erroneously, been come to be known as the “M2 bolt” was in fact NOT a result of the development of the M2 carbine, at all. It was, in fact, a result of studies by Inland designed to increase the reliability of the M1 carbine by providing an increased surface bearing area between the bolt and the receiver, thereby improving bolt ride during the operating movement of the bolt; it also had the secondary benefit to reduce manufacturing time of the M1 bolt by leaving off the final milling cut to the top surface of the bolt, and was an accepted production modification by the time the M2 came about. Notwithstanding historical errors, UNDER NO CIRCUMSTANCES should an M2 be operated WITHOUT a round bolt in place; it was quickly discovered by Inland that the flat bolt was subject to catastrophic failure under cyclic fire conditions due to harmonic torsional flexing and it is EXTREMELY RISKY to use this bolt in a machinegun version of the Carbine!! All original experimental Inland T4’s came with round bolts, as did all wartime production contract M2 guns, and the round bolt was replaced as needed in Arsenal rebuilt or converted M1’s.

Early factory produced M2’s and Arsenal Modified M1 guns often utilized a modification of the M1 stock, although it was found that in doing so, the milling cuts weakened the side rails of the stock and cracks and fractures were common. To alleviate this, the well known “Pot?Belly” style M2 stock was developed and issued. The Pot?Belly stock was identical to the M1 except that it contained more wood in the areas of the side rails to strengthen them against cracking. After WWII ended, all GI stock production was changed over to the Pot?Belly configuration, and as stocks needed replacing these were issued in the rebuilding programs as needed. There were numerous investigations into stocks made of alternate materials and manufacturing methods as fractured stocks were all too common place with the carbine during its’ issue lifetime; stocks made from laminated wood, plastic and even aluminum were all tested, but in the end they remained issued from but two selections of wood, standard Walnut and later Birch as the walnut shortages of Post?War times caused the search for an effective replacement for the normal walnut stock material. Both were used interchangeably and were listed as substituted standard issue. Most “new surplus” M2 stocks available today are of Birch wood and carry a lighter coloration than walnut. Both seem to give acceptable service although the Birch stocks seem to be more variable in their fit and finish.

So you want an M2 Carbine.Okay, which Flavor ??

So what IS an “M2 Carbine”? Well for the modern collector/shooter of NFA Title II weapons the “M2 Carbine” represents a bewildering array of choices and definitions. Defined PRECISELY, it means an original WWII production carbine, produced under the Hamisch?Sampson/Inland conversion design, by either Inland or Winchester, and factory marked M2. Keep in mind that many of the truly original Winchester WWII production M2 carbines were NOT marked “M2”, they were over stamped 2?over?1, only very late production guns were truly fully stamped “M2”; contrast that to the fact that almost ALL true original Inland Div. production M2’s WERE stamped “M2”!! There were no other military contracted M2 Carbines ever produced for the US Government, as from the factory guns. If you will allow a somewhat broader definition of “M2 Carbine”, to include any M1carbine that has been converted to fire in the selective fire mode, or Post?War clone production, the selections grow immensely.

One must start with then, the original conversions that inspired the standardized M2, the Inland T4, which was produced to the extent of approximately 600 guns marked “T4”, which for all intents and purposes ARE M2’s. These guns would command a major premium in the collector markets as such. (There are many known private holdings of T4’s, they are rare and expensive, but out there. Most came from Inland directly after the War. ) Additionally, from the early part of the M2 saga, there are the One?off conversions done by Winchester and Springfield Armory during the initial conversion projects; several Winchester variations are held in the Cody Museum Collection. Springfield Armory examples/prototypes are held by the Smithsonian. There were also several post?war attempts by various people at bettering the Inland design. Granted, these all represent a rare line?up of M1 conversions, but they do exist, and from time to time some have come to the collectors market, although they are not M2’s they do occupy a significant place in the M2 Carbine story and may have interest to some collectors.

The next most obvious example is the MASSIVE number of M1 Carbines which were run through the various rebuilding programs that the Military held between the end of WWII and the Korean Conflict, and after the Korean Conflict ended, up to the Vietnam era. Not all M1’s were rebuilt as M2 configuration, but a LARGE number were. It is then, entirely possible to have a genuine USGI M2 carbine with a receiver marked as having come from ANY of the original Prime Contractors employed for WWII production. This is important knowledge when grading/pricing a gun you are looking at buying. It gets confusing!

When you stray from original Military production guns you arrive at the myriad number of post?war clones , production by such familiar makers as Iver Johnsons, Universal, and Plainfield Machine Co., all of whom made and marketed “M2” versions in several unique variations, such as the Enforcer Pistol, or Chrome plated M2’s, to the Civilian and Law Enforcement markets.

Finally, in terms of complete guns, there were the Presentation and Commemorative guns that came from both the original WWII production programs and later post?war civilian offerings. Some of the absolute most valuable M2’s are those few guns that Winchester Repeating Arms Co. presented to certain special members of management and to a few Military Liason Officers following their completion of the War Contracts; these guns, or certain numbers of them, are known to be M2’s and were registered to the presentee under the NFA ‘34.

The last source of NFA Title II guns are those multitude of “T17” kits that were registered as true conversion kits, in and of themselves, over the years until 1986, and were sold as an inexpensive way to convert an already held M1 to an M2. The vast majority of these kits were simply assembled from commonly available M2 spare parts, marked as required by Law, and then sold as a kit. There ARE however a select small number of Military Issue, and so Marked and Packaged, “Conversion Kit, T17”, which were acquired by various channels and then marked and registered too (It would be interesting if someone who owned one these original marked and packaged Military issue, but NFA registered, kits could persuade ATF Technology Branch to have them declared to be Curios & Relics eligible, in and of themselves.). The generic conversion kits are perhaps the most in need of a careful understanding when you go to purchase a gun that has been converted using a kit, or a bare kit itself. They were not always manufactured with the most complete understanding of the gun’s operating principal and as such, some bear watching out for!