The Mechanics of The Heckler & Koch NFA Conversions: Part I

Factory HK33E machine gun showing push-pin, swing down type of trigger group attachment.

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.