The feed tray assembly is a simple device that serves to hold the linked belt in position while in the feed mechanism, guide the linked ammunition to the cartridge stripping position, and then guide the discarded links out of the gun. The tray itself is a unitized part manufactured from stamped, formed, and welded sheet metal. Into this fabrication are fitted a cartridge holding pawl, and on the M60D variant (and not used with ground gun variants) two belt feed guide spools, or rollers (roller, linear-rotary, PN7269333/NSN3120-00-608-5300) which help to align the incoming linked belt from the fixed ammunition guide chute. In place of these guide rollers on ground guns this location is provided to attach a fixed feed guide plate known as the “bandoleer hanger” (PN 8448414/NSN 1005-00-403-9507).

For ground guns, the bandoleer is the common ammunition-carrying container. These bandoleers are disposable and made (in USGI issue) from lightweight cotton fabric with a cardboard insert to provide a measure of form and stability to the linked belt within. They are designed to slip over the “bandoleer hanger” and support the full linked belt for ease of feeding into the gun; they also provide a measure of protection for the belted ammunition to keep it free from contamination. Introduced later was the all plastic (molded polyethylene), snap-on, “100-rd Assault Box.” This black plastic box is intended to be a sturdier form of disposable, containerized, ammunition issue packaging. It is issued as a single-use disposable package that snaps onto the same location as the cotton bandoleer, but is designed to withstand significantly more abusive service, including amphibious operations, without failure or the deterioration that was common with standard cotton bandoleers. It has the added advantage of a sliding-lock type of plastic cover that is generally proof against environmental contamination, and which can be positioned to keep “belt flapping” to a bare minimum as the belt is drawn into the feed cover during operation. (Due to the angle the feed mechanism is forced to pull a loose hanging belt into the feed tray at, the section of the belt hanging directly below the feed tray entrance will tend to snap upward and forward in jerking movements as it is fed into the gun. (Minus a proper feed chute as used in vehicle installations, this is an unavoidable nuisance.) Belt flapping of a loose belt can not only lead to possible jamming of the feed mechanism, but also will noticeably mar and scuff the exterior surfaces of the top cover assembly. Use of a suitable belt holding container not only prevents these problems, but also leads to the least strain being imposed upon the actual feed mechanism parts as the belt is drawn more horizontally into the top cover. It is when the gun is fired with an unsupported, loose hanging belt, that the belt feeding pawl(s) are exposed to the most strain, and wear. It is recommended that for maximum service life the gun be fired only with an attached belt holding container at the hanger location, or from an ammunition can when used in a cradle mount. While the plastic 100-rd Assault box is the best of the issue containers, there are several purely commercial equivalents available. Most are fabricated from heavy-duty synthetic fabrics, such as Cordura™, and are in effect a premium quality constructed version of the issue bandoleer. These soft-side containers are probably more durable over time than the plastic boxes, and they are not premium priced accessories as they currently retail for only slightly more than the surplus assault boxes.

Even with the wide availability of belt holding containers, some operators still choose to run loose belts; if this is the preferred method of operation than, if at all possible, try to arrange for Assistant Gunner to support and help guide the belt horizontally into the gun as it is firing. If an A-Gunner is unavailable, when firing from the prone (bipod supported) or seated (tripod supported) position, the gunner should reach forward and support the free belt section with his left forearm as support and guidance. This situation is perhaps the primary justification for using the hinged buttplate.

The belt feed pawl fingers are case-hardened (“nitrided”, or nitride vapor-bath-hardened) and can be chipped, or cracked, if struck with force. They are extremely durable on the surface of the “fingers” to resist wear from link abrasions, yet brittle along certain lines of stress. In order to avoid this, one should never grab and yank the belt hard in normal use, the only exception to this being if needed to stop a run-away gun. Also, though tempting, additional lubricants should not be used on the feed pawl, as this may lead to failure to feed. The pawl should be clean and dry. Never polish the pawl fingers beyond normal wear of use, for the same reason. With these caveats, failure to feed can usually be traced to improper spring tension in either the feed pawl helical tension spring, or one or both of the cartridge depressor (cartridge guides, front and rear) compression springs. These springs must be at full rating in order to tightly grasp the belt. In rare cases, the pawl itself may have suffered a broken pawl finger. If the belt advances, but then slips backward or falls out, then either the belt holding pawl spring is worn or broken, or the pawl itself is too worn to hold the belt stationary. Feed jams, when all else seems normal, may be the result of a bent or distorted link exit chute, which will cause ejecting links to clog the chute. And one simple, and damaging, operator behavior can cause all of these problems.

The Top-Cover Blues

One of the most problematical areas of the basic M60 design was always the manner in which the bolt feed cam actuator drove the feed cam in the feed cover mechanism. The problem, is that neither the cam actuator roller assembly, nor the feed cam itself in the top cover, are provided with any vertical displacement when they are out of engagement relative to each other. And they are always out of alignment except when the bolt is fully retracted and held cocked against the sear. And unless they are aligned and engaged, attempting to move the top-cover can easily damage the components. The only time the top cover on an original M60 can be safely opened or closed is with the bolt locked back. Unfortunately, most operators close the top cover all too often with the bolt forward, which will invariably dent, bend or damage the feed cam (feed arm), or the roller on the feed cam actuator assembly on the bolt. This design deficiency was resolved with the E3/E4 update that employed a spring-loaded feed cam that allowed sufficient vertical displacement for the cover to close safely and latch with bolt forward. With this new cam, when the cover is closed with the bolt forward, it will spring the feed cam track into alignment with the cam actuator roller on the bolt body when the gun is cocked normally. While it is possible to close the cover with the E3 feed cam update installed, it is still a good idea to always open and close the top cover only with the bolt fully locked back as it programs good habits. It is recommended that all variants of the M60 be modified with new E3/E4 feed cover mechanism to help eliminate otherwise costly potential damages.

The last area to consider in the feed system, is the feed tray and its relationship to the operating system components. The feed tray is designed to lay flat on the top of the receiver, with a small clearance for the stripping lug of the bolt head to pas through the slot on its bottom face. The bolt must be allowed sufficient forward movement to allow for full chambering of the cartridge head; the feed cam actuator at this full forward dwell will be located directly at the rear support cross member of the feed tray. It is a very close tolerance fit, and if the tolerances milled into the bolt guide rails are a bit off, the cam actuator roller will impact the rear support of the feed tray, denting or bending it. Due to the combination of military and commercial parts used in commercial receivers, this situation is common. This will only happen when the bolt closes on an empty chamber, like when a belt is run dry during firing, or is allowed to close under driving spring tension upon trigger release any other time. If your feed tray shows evidence of this damage, the sheet metal can probably be forced back into shape one time, but changes in operating philosophy should be immediately instituted to prevent repeat or further damages. The only way to prevent this when firing, is to stop firing before the belt runs dry. Learn to watch the belt entering the top cover entrance, and when the last cartridge is seen at the entrance mouth, release the trigger. This will keep 1-3 unfired rounds still in the feed tray, and the bolt will be safely locked back. This single, simple, procedure will also avoid the most serious of the bolt head and barrel locking cam damages as they occur when these components are allowed to close without the cushioning effect of a cartridge case in between for “headspace.” (Remember always, safe handling practices, as the gun is still loaded. Engage the safety, unlatch and raise the top cover and clear the remaining short belt section.)

The final mechanism to consider is the top cover latch assembly. The latch and corresponding receiver bridge that it latches into, will last considerably longer if the top cover is not slammed down to latch. The best method to employ is to hold the latching lever fully open, lower the top cover tight against the stop, and release the lever to hold it. Reverse the procedure to open the top cover. The receiver bridge is only a sheet metal stamping, and if the latch recess becomes damaged or distorted, a new bridge will need to riveted into the receiver assembly.

This concludes the basic series for civilian owner-operators for the M60 GPMG system of weapons. Hopefully now, through this series of discussion, you will be better able to use and maintain the valuable and rare investment that these privately owned M60 machineguns represent.

Recommended Manuals and Spares

The following military manuals should be utilized in correctly operating and maintaining the M60 series of weapons:

• 9-1005-224-10 Operator’s Manual M60 Machinegun (older booklet style Field Operational manual for the M60 ground gun)
• FM 23-67 Machinegun, 7.62 MM, M60 (an excellent Field Operational manual for the M60 ground guns; includes gunnery instructions)
• TM 9-1005-224-24(or -23&P) Technical Manual, Unit and Direct Support Maintenance Manual (covers all variants except the new M60E3, and includes M2/M122 tripod)
• TM 9-1005-224-24P Technical Manual, Organizational, Direct Support and General Support Maintenance Manual Repair Parts and Special Tools Listing

The following basic spare component parts are highly recommended for maintaining the M60 series of weapons. This constitutes only a most basic list of the most commonly required parts. Obviously, with M60 parts getting scarcer and more expensive, a prudent owner will acquire all the spare parts he can locate.

• pin, bolt plug retaining PN 7792920
• extractor, cartridge PN 7790907
• plunger, extractor, PN 269083
• pin, firing PN 11010376
• sear, PN 7269209
• spring, sear, helical compression PN 7269211
• washer, key, gas cylinder PN 7269035
• pawl assembly, feed cam PN 7269120

In addition, you will likely be well served by obtaining at least one spare bolt body, and one spare operating rod assembly. Several spare driving springs are also recommended.

The last and final installment of this series will depart from the current format slightly as it will describe detailed repair methods for commonly seen problem areas using techniques and information that are more appropriate to the civilian realm than the official methods and recommendations shown in the original USGI repair and service manuals. This departure is a consequence of the fact that certain methodologies and techniques used by the US Military infrastructure are almost now impossible to obtain or perform due to the dwindling lack of support for this weapon system.

By Thomas T. Hoel, historical and technical editing by Dan Shea, Photos by Dan Shea

The operating system group is considered to be the heart of the M60 machinegun’s basic design, and it is here that the potential for the most common and expensive wear and damage within the whole weapon can occur. The operating system group consists of the operating rod, bolt group and the recoil/counter recoil parts. If problems with these parts, or improper operation of this group, are allowed to occur they can also greatly affect several other attendant systems within the weapon, primarily the barrel and the feed system and it’s components.

The kinetic motion of the weapon centers on the reciprocating movement of the operating rod, and the rotational action of the bolt assembly as it is driven forward and backward by the operating rod throughout the operational cycle. The bolt and operating rod are involved in a seemingly very simplistic relationship. But it is the inherently violent momentum of their interdependent movements that result in a host of wear-related problems that must be dealt with for continued reliable and safe operation.

Both the bolt and the operating rod are manufactured from high-grade ordnance steels, close-tolerance machined, stress-relieved and heat-treated to provide exceptional strength-to-weight ratios and resistance to wear and failure. Despite this, both parts will evidence clearly visible wear and deformation almost from the first time they are put into use. While it’s always unsettling for a new operator to discover these wear patterns, this is a natural occurrence and while it cannot be avoided, it can most certainly be mitigated. However, the fact that the gun quite normally “wears-in” new parts in this group, does not mean these parts should be ignored. If left unattended, these areas of galling and peening can produce dangerous stress concentrations, possibly leading to premature failure. As a consequence, all of the military manuals covering both operational field use and normal maintenance delve into this topic in great detail.

There are known locations within this system that generate wear. Together, the cam cut on the bolt body underside for the operating rod, the bolt locking lugs, and the cam ways in the barrel extension are designed to force the bolt body to rotate, both in locking and unlocking actions. The points of contact along these two camming surfaces are subject to greatly localized stresses, and will seek to spread this stress over a greater surface area. This action is what produces the visible areas of galling or peening visible on the bolt lugs, barrel extension cam ways (rare), and operating rod yoke (or yoke tower). This phenomenon was recognized early on, and several steps were taken to help alleviate the problem. The most noticeable modification was the addition of a roller bearing on the operating rod yoke, though more subtle changes were made in the angular diversion of the cam paths used, and a generalized increase in the contact areas of the individual components. If the formation, even if small in surface area, of galled or peened edges is observed, immediate corrective action is required to prevent continuing damage. Any evidence of galling or peened edges must be removed to restore the normal passage of lines of stress. There are three main locations within the system where this type of wear is observed.

First, examine the yoke tower of the operating rod along its frontal face, receiver guide ways, and the firing pin bearing channel. These surfaces essentially serve as forward and rearward travel limit stops for the bolt body at the end of its camming slot travel.

Conversely, the large mating surface area of the bolt camming groove located along the bolt body underside effectively spreads out any stresses within the bolt body there, and wearing of the sharp edges on this camming cut is unusual. Normally, a smoothly polished interior travel surface will be seen, with occasional evidence of slight surface deformation(s). In accordance with service publications such as TM 9-1005-224-23&P (or -24), visible surface burrs, gouges, or galling on the yoke tower and attendant firing pin bearing channel and receiver guide ways do not render the operating rod unserviceable, provided the damage is corrected by returning the surface(s) to a smoothly contoured area.

Experience has shown that on the yoke, the rear edges of the firing pin channel will evidence the most burring, directly above the firing pin roller bearing. (Note that the right-hand top edge, as viewed from the rear, is manufactured with a forward raked cut to the top edge; this is normal and reduces wear at this corner)

Use of the proper method for disassembly the bolt body from the operating rod yoke will greatly reduce operator induced wear during maintenance at the firing pin channel location, a major cause of premature wear here.

To correctly disassemble, invert the operating rod and bolt group, grasping the inverted top of the bolt body in the left palm, rear end facing away. While grasping the shank of the operating rod forward of the sear notch, apply pressure rearward against your left palm compressing the firing pin spring. Apply this pressure straight back until the yoke just starts to rotate…do not allow yoke to rotate. While holding the yoke firmly against rotation and firing pin spring tension, lift the forward end of the operating rod gently upward, carefully allowing the front edge of the firing pin channel to come up out of the bolt camming slot, and then slowly ease the firing pin forward to clear until it stops. Do not allow firing pin to snap forward. Done properly, neither the yoke tower or bolt body will slip out under pressure of the firing pin spring, which often times will cause gouges or burrs to form if allowed to happen. Reassembly is performed by reversal of these steps.

The second area to observe is the bolt head, and its locking lugs (and cartridge feed lug). These areas, in conjunction with the cam ways in the barrel extension, are the most stressed in the group as they bear the full intensity of the firing pressures of the cartridge. These areas encounter the brunt of the horizontal and rotational shock during bolt locking and unlocking actions as contact along these points is what starts and stops the violent rotation, and subsequent linear movement, of the bolt assembly in each direction of travel. These lugs will usually show more signs of wear than the corresponding cam ways in the barrel extension, and will be where the most attention must be directed in terms of preventative maintenance actions taken by careful removal of burrs or peening. Even more so than with the operating rod yoke, extreme care must be taken here when correcting any wear to make absolutely sure no lug contours are changed or excess material removed as safety of the locking action can be compromised. The top locking lug also forms the cartridge feed lug (cartridge stripping lug), and as such has significantly less material to start with than the lower locking lug. The top lug should always be given the closest scrutiny for any possibility that the incurred damages might not be removable by normal corrective action(s), without altering the underlying strength of the lug. The common wear pattern observed on the lugs is for the frontal surfaces to evince galling or slight burring around the sharp edges of the lug(s) where they travel in the cam ways of the barrel extension. Again, in accordance with service publications such as TM 9-1005-224-23&P (or -24) these minor deformations of the frontal surfaces of a particular lug are not cause for rejection of the bolt body as long as these areas are returned to a smooth contour.

The most severe damage that can occur to either the bolt lugs or barrel extension cam ways is in the form of cracking or chipping. Heavy gouging or deep indentations formed in this area will almost always lead to incipient cracking, and evidence of this type of damage is cause for an immediate inspection for integrity and subsequent rejection of the component. The equivalent problem may be observed along the exterior edges of the barrel extension cam ways. If any evidence of cracks, incipient cracking, or chipping, is determined to exist on the surfaces of the cam ways, a full inspection of the barrel extension (cam ways) is also immediately indicated. The only safe method for proper determination of any cracking suspected in the above areas is to subject the area to a fluorescent dye-penetrant test (with equivalency to MIL-I-25135), or MagnaFlux(r). (Virtually all aircraft service and repair shops will be able to offer these inspection procedures). If such an inspection procedure is unavailable, the only safe recourse is to replace the suspect part.

For all other cases of correctable repair, the proper remedial action is to carefully restore to a smooth surface the sharp edges of any galled, burred, or peened area by stoning with a fine grit polishing stone, without altering the basic underlying contour of the area. When using this technique great care must be taken not to alter or remove any of the underlying material, but merely to smooth out and remove the tiny displacements. (Never use a coarse grit stone or highly abrasive compound. 800 grit is an ideal basic) A final polishing of the affected area is recommended. Once these initial wear-in displacements are corrected, additional stoning is usually not required. However, if any of these components are changed, were-in problems may again arise. This may also occur if the firing pin or operating rod roller bearing are replaced. In rare instances, the spools of the firing pin may exhibit wear, which is, in turn, indicative of excessive wear of the firing pin channel edges. A damaged yoke roller bearing will cause excessive wear patterns on the inside surface of the bolt camming cut in the bolt body. It is therefore prudent to monitor these areas as well for additional signs of galling, burring, or peening, if any of the component parts of the operating system are exchanged.

The operating rod both retains the bolt and helps to cause it to rotate by a camming action as it is driven back and forth. The bolt is also supported and guided in its reciprocating movement by the feed-cam actuating roller, which is supported by the two bolt guide rails that form an integral part of the receiver. The gas tube, lower receiver operating rod supporting rail, and the bolt guide rails are the primary vertical support members for the operating rod and bolt assembly. Smooth and unbinding free movement along these paths of support is critical for correct functioning of the bolt and operating rod. It is always recommended to inspect these areas first, if wear begins to appear on either the operating rod tower or the bolt lugs. A simple lack of proper lubrication can cause excess drag along these supporting areas, which may lead to sluggish operation of the weapon in general, or failure even to fire a chambered cartridge. Lubrication of the operating group and its supporting components is critically important.

For virtually the entire military operational use of the M60, there have only been two normally approved MIL-SPEC lubricants, LSA and CLP. LSA (Lubricating oil, Semi-fluid, Automatic weapons, MIL-L-46000B) is a medium weight lubricating fluid, actually an emulsified mixture of multi-grade lubricants designed to provide effective lubrication protection over a wide range of atmospheric and temperature extremes. LSA was approved for field use in virtually all small arms in inventory and is a substantially effective general weapons lubricant when used in areas appropriate for its composition and formulation. Due to its chemical composition, LSA stocks that have been allowed to sit idle for any length of time will drop out of emulsion; to restore the lubricant to its full specification the mixture must be vigorously shaken, by mechanical means preferably. (To restore small quantities, individual containers can be heated in boiling water for a short period, or placed in a microwave oven set on Low for several seconds, and then shaken by hand.)

Standard M60 GPMG gas cylinder. This one is particularly nasty and came from a very old barrel that hadn’t been cleaned since the Stratford Hollow shoots, left in the back of a travel box. Note the scale and carbon build up, that made it necessary to drive the piston from the system with a rod and hammer. This is a prime example of a piston that would not slide when the barrel was tilted. It is not, however, beyond saving. Some good old fashioned cleaning solvent and scrubbing, and it will be usable again.

CLP (Cleaner, Lubricant, Preservative, MIL-L-CLP) was adopted to address what were considered to be shortcomings when maintaining weapons with the previous standard combination of LSA and RBC (rifle bore cleaner, MIL-C-372B or C). CLP is a complex formulation of cleaners (powder solvents), synthetic and natural lubricants, and anti-rusting and anti-corrosive (anti-acidic) compounds. While touted as a miracle formulation, CLP in general and widespread use has proven to be less than satisfactory for any of its intended uses. Its primary value as a weapons lubricant is dependent upon micro-grannular deposition of DuPont™ Teflon® (a fluoropolymerized lubricant) particles carried in suspension. When proper deposition of these minute particles occurs in the grain structure of a treated metal surface, the resulting lubrication properties are phenomenal. The main problem has been that successful Teflon® deposition rarely occurs and the remaining petroleum-based carrier fluid is not an effective lubricant, leaving much of the weapon unprotected from accelerated wear. It is mandatory that bulk concentrations of CLP be thoroughly agitated before use to efficiently distribute the Teflon® particulates in uniform suspension. For maximum Teflon(r) adhesion and deposition, the area(s) to be treated must be cleaned down to the bare metal surface with no trace of prior lubricants or preservatives remaining. Additionally, CLP does not achieve its’ maximum potential lubricity without numerous, long-term, continued applications.

Both LSA and CLP have their places and can be used effectively, although LSA is a far better choice for general lubrication of small parts if thorough cleaning and complete re-lubrication cannot be accomplished after every shooting session.

Neither LSA nor CLP are the best lubricants for several locations on the M60. On small parts, with small surface contact areas and low operating contact pressures, they are sufficient, but for large contact areas under heavy pressures another lubricant type is preferred for civilian use of the weapon.

Just as was found with long term experience with the .30 M1 Garand, .30 M1 Carbine, and M14 rifles, the use of any lubricant other than a heavy weight, high viscosity, grease for the bolt and operating rod contact areas led to immediate and disastrous problems. These critical parts often “froze” when used with lightweight lubricating fluids, even in ideal atmospheric conditions. The similarity of the operating components of these weapons to the operating system in the M60 suggest the same lubricant choice in certain highly stressed areas such as the operating rod yoke, bolt camming cut surfaces, bolt locking lugs, barrel extension cam ways, bolt and operating rod guide rail slots, feed cam actuator roller, and feed cam lever. The benefit of using such a high viscosity, extreme pressure, type of lubricant on these areas is that even under extreme operating conditions the lubricant will tend to stay where it is needed, continuing to provide vital lubricity.

The military standard for such applications is the unsurprisingly named MIL-SPEC lubricant, Rifle Grease (Grease, Rifle, MIL-G-46003 (ORD) Amend. 2). GI Rifle Grease is a high-grade, high-viscosity, temperature-stabilized, all-petroleum base lubricant with exceptional qualities for extreme pressure use and is formulated with anti-rust and anti-oxidant properties. While no longer a MIL standard lubricant, it is widely available as surplus stock. If GI Rifle Grease cannot be found in sufficient quantity for cost-effective use, it is also acceptable to use a more modern substitute. These products should conform to NLGI No. 2 multi-purpose applications, and will usually be found as a lithium-12 hydroxide base, molybdenum disulfide bearing, and extreme-pressure grease, such as generic automotive “wheel bearing grease.”

Liberal, but careful, application of this type of high-viscosity lubricant will provide long-term benefits significantly greater than if the weapon is maintained solely with CLP, LSA or other lightweight lubricants. Synthetic weapons lubricants such as MilCom TW-25B, and other such purpose-designed lubricants, are excellent for general use on the components of the M60, for those weapons in military use. However, civilians are not bound to consider all the same operational considerations as a military user, and use of the high-viscosity greases are more beneficial on certain components.

The last area of the operating system we need to consider is that part of the operating rod group that interacts with the fire control mechanism and counter recoil components.

The firing mechanism of the M60 is a relatively basic and straight forward design employing a simple spring loaded, single form sear which acts in combination with another simple machined cut on the underside of the operating rod. The sear is pulled into or out of engagement with the sear-cut on the operating rod by mechanical action with the movement of the trigger and its spring, and the sear spring. At least, that was the intention of the design. In use, however, the firing mechanism has two idiosyncrasies that must be understood and examined. The following applies to any variant of the basic design, and although with the M60D aircraft weapon variant the trigger is replaced with a linkage and trigger bars, the actions involved are exactly the same as in the ground guns.

The sear engagement surface cut on the operating rod underside is neither overly large, nor particularly aggressive, in its engagement with the sear. This has led to a reasonably accurate branding of the weapon as being prone to experiencing “run-away” operation. While the inherent design does indeed play into it, there are a few things the operator can do to prevent the situation from happening. As a relationship of the way the sear is forced to engage and hold the operating rod to cease firing, the operating rod sear engagement cut is placed under considerable impact loading which can over time cause the face angle to distort, lessening the available engagement contact area. This may continue to the point that the mere impact of the operating rod hitting the sear nose will cause the face on the rod to simply jump over the sear nose allowing the gun to fire additional rounds. The successful engagement of the sear with the rod face is dependant upon the nearly perpendicular mating of the two surfaces, any proclivity for either face to lessen that angle of contact through damaged contours, may be enough to prevent the trigger mechanism from holding fast the operating rod. While a certain amount of this distortion is unavoidable, the greater part of this wear is unnecessary. The most important thing any operator can do to prevent wear of these surfaces is to control the trigger mechanism in such a way as to prevent the two surfaces from experiencing “casual contact” as the gun operates. “Casual contact” occurs when the operating rod sear notch comes into partial contact at any point with the sear nose. This will inevitably cause frictional wear and low-level impact loading, forming galling or peening of the top edges of these surfaces, and greatly increasing the tendency for a run-away. For the sear nose and operating rod to function with the least amount of contact wear, they must never be allowed to contact each other, except when placed into sudden, full and complete engagement. Too often operators will not fully activate the trigger, allowing a small portion of the sear nose to continue projecting into the path of operating rod as it cycles. The gun will fire in this condition, but accelerated wear of the firing control parts is guaranteed also. The give-away is to view the underside of the operating rod and see if there is a polished track along its bottom edge. There should only be evidence of contact at the sear engagement notch, and nowhere else. If a polished track is present, the sear nose, via the trigger action, was not being pulled down out of the way. Again the correct remedial action is to restore these surface faces and remove the damage, without altering the underlying angular relationships of the base contours. Past a certain wear point, this may not be possible and replacement of the component may be required.

On the other end of the firing control spectrum is another problem that may result in a run-away gun. The sear must rise forcibly into the path of the operating rod in order to engage and hold the rod to stop firing. The sear is under spring pressure to provide upward vertical travel of the sear nose. In order for the trigger pressure to be within acceptable limits (trigger pull should be tested to be within a 6.0 lbs minimum, 11.5 lbs maximum), this spring cannot be overly powerful. When a run-away gun is encountered, the first item to check is the sear spring. Examine the sear spring to be sure it is not broken, or damaged. Additionally, serious problems can arise if the sear spring has taken a set over time. It can progress to the point where the operating rod is able to over-ride the sear nose if it is not being pushed far enough upward. Sear springs are an often-overlooked maintenance item, but should be routinely replaced with a progressive maintenance policy every 3,000 rounds at least. It is also acceptable to replace the stock spring with a custom spring with a higher spring rating. The compression length must remain the same to prevent jamming of the mechanism. Proper trigger manipulation combined with a higher rate custom sear spring will greatly increase the useful life span of sears and operating rods as they will encourage proper, full face surface, engagement of the two parts.

Finally, the recoil spring should be inspected. There are two variants of recoil springs currently available, and each has its own advantages and disadvantages. The gun was originally fitted with a multi-strand spring to combat the well-known tendency for single wire coil springs to potentially lose some of their strength rating due to spring wrap. The use of a multi-strand spring is exceedingly effective in the M60. However, with use it can become damaged as the spring itself is in nearly full contact with the inside surface of the operating rod tube and over time this will cause wear to occur in the form of flat spots There is nothing that can be done to totally prevent this from happening, it is inherent in the design of the recoil system. In order to allow the spring to deliver its greatest possible useful life, the coils should be checked often for flat spots, and the spring wire itself should be rotated to a different indexing each time the gun is disassembled so that wear is spread around the full exterior of the wire rope coils as much as possible. It a flat spot has completely worn through one strand, or is close to doing so, the spring should be replaced. The newest single strand coil spring introduced with the E3/E4 variants is intended to help eliminate the flat spotting associated with the wire-rope type spring. While the new spring design is also quite efficient, the single strand spring also requires the use of its own dedicated guide rod as the coil wire diameter is different than the older version spring. The new style spring can also be prone to wear, and is more susceptible to spring-wrap damages if strained. If this occurs, it can jam inside the operating rod spring tube, and become difficult to disassemble.

Although either version of main recoil spring is serviceable in the civilian realm, the surplus older style wire-rope springs are much less expensive and no new guide rod is needed. For use of either type recoil assembly, the spring guide rod should be smooth and free of any nicks or burrs that may catch a wire strand. Polishing of the spring guide rod to eliminate as much friction as possible is beneficial. Also, liberal use of lubricant inside the operating rod tube, the spring, and along the spring guide rod is recommended to enhance spring life, as long as this excess lubricant is thoroughly cleaned after each shooting session to preclude accumulation of abrasive material.

The recoil buffer unit and its retention yoke must also be inspected. The buffers in use since the mid-1960s and currently still issued are sealed hydraulic spring type, and essentially are maintenance-free except for periodic inspection for damage, including leaks. If the buffer starts to fail, often the first sign will be marked increase in pounding felt at the shoulder stock by the operator, The buffer retainer yoke will also begin to deform in a backward direction. If left unattended, the yoke may cause damages to the receiver channel and guide rails, along with the butt stock eventually. If the rear face of the butt stock appears dented or deformed outward, immediately inspect the buffer assembly and buffer retainer yoke. If any obvious signs of fluid leakage from the buffer body are present, replace the entire buffer unit.

The buttstock itself is supplied as a manufactured assembly, and cannot for all practical purposes, ever be disassembled or repaired beyond minor touchup of the finish. Individual spare parts are simply unavailable for this assembly, unless from a salvaged unit that has been broken down.

This brings us to the last and final area of consideration on the M60, which we will cover in Part 5 next month. It is an area that results in the most common problems and the costliest repairs for the civilian owner, yet virtually all of these problems are avoidable.

By Thomas T. Hoel, historical and technical editing by Dan Shea, Photos by Dan Shea

Now that we have explored the basic receiver assembly in detail, let us turn our attention to protecting and assuring its structural integrity and overall longevity.

One of the greatest areas for potential problems with an M60 is centered on the receiver assembly, and its component parts. Because the receiver is an NFA-controlled item, and the general problem of receiver damages requiring expensive and difficult repairs, the civilian owner should become quite familiar with common inspection procedures, and learn to avoid common, potentially damaging operational actions. (The greatest source of damage to the trunnion, and its attached components, is from operator abuse or carelessness.)

The barrel trunnion does not have to contain any direct stresses during the firing cycle, but careless or abusive handling can still damage it. As first fielded in military service, the basic M60 receiver was assembled by riveted fasteners primarily, with press fitting and a small plug-weld to secure the gas tube (extremely early military production trunnions, extending into the T161 series, utilized a threaded and pinned fitting of the gas tube, but this was quickly amended in the production line and replaced by a press-fit and plug-welded joint). After evidence of receiver separations in military use came to light, a different, more secure type of riveted fastener was tried as a cure, though with limited success. The approved solution was the combination of enhanced grip rivets and a series of welded seams to join the major subcomponents to the barrel trunnion, tying the individual subcomponents into a more solid, unitized, assembly. The welding operations are performed to join the barrel trunnion at its interface with the sheet metal channel underside surface, the gas tube (op-rod tube) at the trunnion socket, and the front side stubs of the milled bolt rails to their respective mating slots on the barrel trunnion. Once unitized by welding, the receiver assembly thus became an extremely strong and solid unit and this operation has virtually eliminated concerns over receiver separations.

One of the very most important things for the civilian owner-operator to check for is whether or not a specific commercially manufactured receiver has had this welding reinforcement done. A commercial receiver that is simply riveted and press fit together is very likely to offer a vastly reduced service life span and it is highly recommended that the proper welding reinforcement repairs be made before any further use of the gun.

The specific welding operations are outlined in many of the Military Depot Level M60 maintenance manuals. (Dept. of the Army) TM 9-1005-224-23&P (revision May 1998 or later is preferred), is the best general repair reference manual commonly available and is highly recommended for reference, even if the gun owner does not anticipate undertaking any repair actions on his own. (The more commonly seen TM 9-1005-224-24 is the condensed version minus certain equipment lists, and is a very good substitute for general information) This TM, in which receiver inspection and remedial procedures are outlined, should be consulted to determine if a specific receiver has had the recommended reinforcements accomplished.

For the conscientious owner, one of the best pieces of peripheral maintenance equipment to own is the standard M60-series receiver gauge set (NSN 5220-00-921-5005); this standard gauge set is used to gauge receiver subcomponent alignment and serviceability by establishing angular displacement and stretch variances in all three planes.

While it is primarily used to determine receiver serviceability limits, it is also the required and specified tool to use to provide correct alignment of the receiver subcomponents when initially placing receiver reinforcing weldments by properly positioning and holding the various subcomponents that attach to the primary receiver component, the trunnion, during primary tack-welding operations. Attempting to weld the subcomponents to the trunnion without this critical alignment holding tool will invariably result in the receiver actually being assembled and held together in an un-aligned, or even twisted, configuration. If this has happened critical wear surfaces between the receiver rails, op-rod, bolt roller, or other stressed parts will almost surely develop leading to excessive or detrimental wear patterns being established that will severely limit the useful lifespan of these components as they are forced to wear together. (A gun that exhibits a difficult or hard to retract cocking motion should immediately be checked for proper receiver alignment with the standard gauge set, followed by careful examination of the reciprocating parts involved, inspecting for unusual wear patterns or locations.)

Even if the receiver in question has been weld-reinforced, it is necessary to periodically perform a visual inspection of the welded joints for integrity, cracking, or other damage. As the standard gauge set is the only proper method to gauge receiver subcomponent alignment and serviceability by establishing angular displacement and stretch variances in all three planes as indicated above, periodic usage of the gauge is highly recommended, too. Since the welded receiver is now a much stronger unitized assembly, which is not free to “give” at the many points as an un-welded receiver will, any damaged weld joints may concentrate forces along small highly stressed areas, if not corrected. This is most probable at the sheet-metal channel joint, and a damaged weld seam along the bottom of the trunnion may cause long cracks to propagate well into the sheet-metal bottom of the receiver. Though easily repaired, a deep rend in the channel may eventually lead to more serious problems with the operating-rod and bolt.

Aside from the above, the most commonly damaged area(s) of the receiver (trunnion) is the barrel socket, followed by the gas tube at the gas cylinder interface aperture. Both of these areas sustain damages from incorrect or abusive treatment of the barrel assembly where it mates into the receiver assembly. These damages can be avoided in almost every case. The barrel assembly is extremely heavy for the amount of longitudinal support it receives within the trunnion, and all the weight is leveraged on the far fulcrum point of the bipod yoke, at the very end of the barrel itself. The entire mass of the barrel assembly is supported only by the rearmost 2½ inches of the barrel extension. (An additional 1/4-inch of barrel assembly interface occurs at the circular lip of the gas cylinder nut when properly mated into the gas tube, but this in no way should be regarded as a true supporting interface.) Unless the tolerances of the mating interface is extremely tight and without measurable play, the continued movement of the barrel extension within the trunnion socket will eventually cause the socket tolerances to open up and expand the free play run-out. The barrel latch itself is really not designed or intended to hold the barrel extension tightly, simply to keep it from slipping out. The end result is that over time a trunnion socket that has become worn will not hold the barrel extension tight enough to hold a zero setting, the barrel will shift with every movement of the gun, sometimes to the point of causing jamming, as the bolt lugs will not properly rotate to lock up in the locking cam ways of the barrel extension. It is virtually guaranteed that this wear will occur within the trunnion itself, as the barrel extension is far more hardened to inhibit wear. If the trunnion socket becomes excessively worn in this manner, or even driven out-of-round, expensive receiver repairs are the only option. Generally the only option available, aside from total replacement of the trunnion itself, is to align-bore the socket and sleeve it with a shimming sleeve of compatible alloy, suitably machined. Even if shrunk-fit or brazed into position, it may not be a permanent repair and may again loosen with heavy use. (WARNING: It has been noted on some civilian guns that sometimes an unfortunate owner faced with this problem has tried to solve it with an easier fix by applying a shimming sleeve to the barrel extension outer surface. This is ill advised, as sleeving the barrel extension in this fashion only allows a bare 1¾ inches of clear outer surface to sleeve without interfering with the locking cam ways, and even if done carefully the bolt may still not be locking smoothly within its proper rotational alignment, causing undo strain and wear on the locking lugs of the bolt as well as increased wear on the barrel extension locking cam ways. This damage may not be immediately noticeable. Even then, trying to retain a shimming sleeve on the barrel extension surface may not be possible with complete safety, as any high temperature fixative methods may damage the critical heat treatment of the extension, upon which a large measure of it’s inherent strength depends.) All of this potential barrel assembly-to-receiver damage can be avoided almost completely, with two simple procedures.

Without arguing or belaboring the point of the design of the original model M60 bipod design and placement, it suffices to say that it’s location on the barrel assembly can lead to problems as concerns longevity of the weapon’s other operating assemblies. These specific problems began to be addressed way back with first military Product Improvement Program variant, the M60E1, and the civilian owner-operator now should be just as concerned as was the military. And while the recent military adoption of the PIP’ed M60E3/E4 variants has given opportunity to address the original bipod situation to some degree, the following comments are valid to large degree even if a particular civilian M60 has been retrofit upgraded with the addition of the E3/E4 assemblies. The first problem is a simple one in that even though the gun was designed to be fired off the bipod in the prone position, to do so truly places a tremendous strain on the barrel trunnion socket.

Compound this arrangement with the nearly universal tendency for operators to place heavy downward pressure on the gun when firing in an attempt to stabilize it and the induced moment-arm leverage strain can produce damaging results in a relatively short order of a few thousand rounds. Even if the E1/E3/E4 modifications have been performed to get the bipod support off the barrel and onto the gas tube, the use of the gun fired from the bipod position can lead to damages of the trunnion mating joint at the gas tube-trunnion interface, especially if the previously mentioned welding reinforcements have not been accomplished as may be the case with multitudes of commercially manufactured receivers. Gas tubes used on guns have been retrofit upgraded with the addition of the E3/E4 bipod assembly will experience additional strain along the tube itself, and longitudinal stress cracks are well known to develop along the tube body, particularly at the (relatively newly required) hole that has been drilled to accommodate the new-style fore-grip used on these configurations. The E3/E4 style modifications are at best a compromise solution to one set of problems, and bring to the gun a potential for a new set. Heavy users of a gun configured as an E3/E4 variant are wise to procure a spare gas tube, and be prepared to periodically weld-repair their current one.

The use of any bipod-supported firing position, with any M60 variant, may look appealing, but be forewarned that continued use of the weapon from that deployment can become a headache when the inevitable excess wear occurs. For the civilian owner-operator then, for maximum service lifespan and use of any M60 variant, the use of the self-contained bipod is NOT recommended.

A better arrangement, and one that will lead to the longest life of the weapon, is to fire the M60 from one of the receiver supported mounts where the receiver is fully supported and the barrel places no unnecessary stresses into the trunnion barrel socket, or gas tube (or gas tube-trunnion interface). There are several military designed mounts for the M60 series, including tripod mounts, vehicle mounted pedestals of various descriptions, and even a dedicated M60 cradle mount (the M142 cradle assembly [PN 10900945] which incorporates a built-on 200-rd ammunition box hanger) for use with pedestal bases or vehicle sockets. Any of these are far better than the integral bipod.

The most commonly encountered of these mounts is the US standard M2 tripod, already in service for decades with the .30 cal. Browning series of machineguns, and useable with minimal adaptation to the M60 GPMG. The newer designation for the current manufacture M2 tripods is “Mount, Tripod, Machinegun, 7.62mm, M122 (PN 7790723/NSN 1005-00-710-5599)”, or more simply, the M122 tripod. For all practical purposes, the only real difference between the World-War-II-vintage M2 tripod and the re-designated M122 is the date of manufacture. Both M2 and M122 require dedicated pintle adapters for use with the M60 (or specific M60 pintles), along with an M60-use specific T&E mechanism adapter bar; the tripods themselves are identical in all other ways. There are also two distinct designs and vintages of tripod pintle adapter/pintles available, although one version is clearly a better design. Both will require the use of a dedicated M60 T&E mechanism adapter “H”-leg (PN 7792991/NSN 1005-00-772-0194) to tie the M60 receiver into the older Browning machinegun T&E mechanism.

The earliest version of pintle adapter (PN 779284) was indeed a true adapter as its function was to adapt the M60 to use a standard Browning-style tripod pintle. This is usually referred to as the ‘pintle adapter platform’, or early-issue ‘pintle-platform assembly’. It has two distinct advantages over the late-issue M60-dedicated pintle, commonly known as the ‘Gooseneck pintle’, as when using the pintle-platform adapter the receiver is tightly held along the greatest surface area of the underside of the weapon, retained by the pintle mount pin and locking into an additional mounting pin on the gas tube. It is this two-point retention that gives the pintle-platform adapter its exceptional rigidity and stability. Since it is designed to interface with a standard M2 pintle, it may be used anywhere the older Browning pintle can be mounted, such as the ubiquitous M24 or M25 “Jeep” or vehicle pedestals. The M60 pintle-platform adapter will simply slide into the yoke of the M24/M25 pedestal frame. Mounted thusly, and with the pedestal bolted to a suitably heavy legwork frame base, or vehicle, the M60 remains absolutely rock solid when firing. Currently, the early issue pintle-platform adapter is cheap and plentiful.

The (current) late-issue M60-dedicated pintle (PN 11010408/NSN 1005-00-945-9756), commonly known as the Gooseneck pintle, was an attempt to render the M60, when used in the “heavy” machinegun role, a more manageable package by reduction of weight and simplification of design. It does not require use of the old Browning-style pintle, as it is a single unit, which combines the tripod pintle function with a single-point receiver, mount pin latch. Unfortunately, though weight was shaved, utility and stability were sacrificed. The only real advantage over the early-issue platform style was that due to it’s single-point mounting to the receiver, the gun could be traversed and elevated by hand (frees swung) on an advancing target more quickly and with less physical force, though even when tied to the T&E mechanism stability when firing was considerably reduced as the gun was simply not as rigidly mounted as before. It was a trade-off that found many supporters, but keeping stresses off the gun and instead transmitting them into the mounting, it is not nearly as good a choice. It also caused more strain to be put onto the T&E mechanism and adapter leg, and it is commonly noted that the lugs of the T&E adapter mounting plate under the receiver channel will show signs of wear and deformation more rapidly when used with the late-style pintle; loosening of the two attachment rivets is fairly common. These late-style ‘Gooseneck’ pintles are currently also quite available, though asking prices vary widely, and they are certainly not rare or hard to find.

However, even when mounted onto a tripod, pedestal yoke, or vehicle cradle mount there is still more that can be done to reduce barrel-assembly-induced strain on the trunnion socket and gas tube. The M60E1 variant pointed the way, and was confirmed in the later E3/E4 variants. The high levels of induced strain are a combination of the small surface contact area of the barrel extension in the trunnion socket, combined with the very heavy weight of the barrel assembly itself. While there is nothing to be done about the barrel mounting and retention design situation, one can reduce the operational weight of the entire barrel assembly by following the lead of the M60E1 program. The current E3/E4 variants are the result of two different objectives. The first was to reduce overall weight of the machine gun, the second objective was to provide operational improvement in several previously identified areas of concern. This resulted in the combined effects of relieving the barrel assembly of its bipod unit, greatly reducing the actual mass of the tube by reducing the diameter and length, and a thorough redesign of the gas system. As such, either of the E3/E4 variant barrel assemblies is efficient at reducing stresses on the trunnion, but this comes at an operational penalty that seems particularly relevant to many civilian users. The reduced mass of these redesigned barrel units greatly reduces their ability to fire in a sustained fashion, something all too many civilian users seem to disregard. If proper fire discipline can be maintained as intended with the E3/E4 barrel groups, they are the ideal choice so far as limiting receiver wear is concerned due to their greatly reduced weight, though they are considerably more expensive than the standard M60 barrel group, even if purchased used.

For most civilian recreational shooting, a better option can be created from a slight modification of an original standard “heavy” M60 barrel group to optimize it for use with a fixed receiver mounting, such as a tripod or cradle mounting. As it is, the original “heavy” profile barrel is probably the best choice for the civilian recreational shooter as it is designed to handle a far greater volume of sustained fire and can tolerate considerably more abuse. A simple modification to remove the complete bipod group and original flash hider will result in a dedicated barrel assembly that is in the best possible configuration for use with the rigid receiver mounting options discussed above. To complete the conversion, the use of the newer M60E3 “short” birdcage style flash hider will provide a highly efficient, and cosmetically pleasing package. This is an efficient and cost-effective method to duplicate most of the advantages found in the M60E3/E4 barrel groups, while retaining the full barrel profile. Handling in this configuration also improves markedly, as the reduction in weight at the end of the barrel is significant, greatly narrowing the advantage enjoyed by the newer barrel versions.

In the next part following we will begin a detailed discussion of the peripheral assemblies that complete the weapon, with an added emphasis in a discussion of general service recommendations particularly appropriate for the civilian recreational shooter.

By Thomas T. Hoel, historical and technical editing by Dan Shea, Photos by Dan Shea

There always seems to be a certain few military firearms that bear an unusual and universal interest for collectors and students of military weaponry. The US M60 General Purpose Machinegun has always occupied just such a position within the civilian NFA weapons owning population. Although there also seems to be little ability to compromise on feelings toward the design, as it is either highly praised or deeply maligned. While in the past literally volumes have been written on the military adoption and use of this interesting weapon, the scope of this discussion is strictly to aid the civilian owner-operator of the M60 GPMG as a guide in obtaining maximum enjoyment, usefulness, and longevity of his personally owned machinegun.
Additionally, specific recommendations discussed herein for operations strictly in the civilian realm will be presented as the civilian owner-operator has certain freedoms, and also certain limitations, that the original military end-users may not. The M60 has a number of well-known characteristic wear and failure profiles for certain component parts and assemblies. Anything the civilian owner-operator can do to decrease operating stresses, and attendant wear and tear on component parts or assemblies, will serve to prevent unnecessary or premature wear and failure of these parts, adding tremendously to the life expectancy, serviceability, and enjoyment of the weapon as a whole. This should be the primary concern with such the large investment these privately owned machineguns represent.

While certain distinct characteristics of the M60 design may not at all pose a serious or dangerous matter for the civilian owner in a recreational shooting environment if or when they result in a failure to function, they bear discussion to further the serviceability and enjoyment of personally owned examples. This discussion is divided into two distinct sections. The first, discusses in detail the receiver, its sub-components and construction, along with recommended practices for the care and preservation of the receiver as an assembly. The second part will discuss the various and sundry peripheral assemblies that complete the weapon, with the same emphasis in the discussion provided as appropriate.

Many of the long established military use and service protocols exist for reasons specific to the military/combat environment, or specific governmental exigencies. These include repair or service protocols where the actual military end-user is not to be concerned with the costs of repair, even if the supply of spare parts was unlimited. Since the civilian owner-operator is not bound by such exigencies, he should exploit any and all maintenance and operating schemes that benefit preserving of the serviceability of the weapon over an extended period of time and use, along with minimizing actual financial outlays for avoidable repairs parts or services. As such, certain of these recommendations are intended for use only in the civilian realm, and do not represent advice or recommendations for operational use, or maintenance functionality, of the weapon in any military or law enforcement scenarios.

The Heart of the Matter

Those M60 machineguns that are available to the civilian collector of NFA weapons can be described as being of two basic origins. The first important distinction, and the one which relates most strongly to their value as collectibles only, concerns the manufacturing origins.

The top rung on the collectability ladder is those guns that were originally manufactured by a genuine military contract producer. Although their numbers are quite small there are, remarkably, a certain number of M60’s that are of true military production-line origin and pedigree. Due to these contract producers being private companies, they were able to offer the exact same weapons they were producing under military contracts additionally to both domestic law enforcement entities, or for export to approved Friendly Nations clients. Not all guns originally destined for either of those last two markets ended up there, mainly due to some quirks in salesmanship domestically, and thus a few of those guns have been able to enter the civilian market and placed into the NFRTR. The salient feature of these specific guns is that they can all be described as being manufactured and assembled to full military specification, in both materials and workmanship, in all component parts and assemblies. While this distinction often has no bearing on serviceability for sport shooting purposes, it can have a definite in their value. The second group of guns is that in which the receiver was manufactured by any method, by anyone, other than the original military contract producers. This is the most critical distinction in the pedigree (and value) of civilian available M60 guns, and bears careful discussion.

Receiver Basics

The “receiver” of the M60 machinegun on all of its several variant models is actually an assembly of several component parts, permanently or semi-permanently joined to render the complete functional assembly. BATF Technology Branch has recently authored a reply to an inquiry letter asking for a definition of what legally constitutes an M60 machinegun “receiver.” By rendering this recent official opinion they introduced considerable confusion into what was previously the legal definition under previous Technology Branch Administrations. The “current” (as of 11/2001) BATF Technology Branch definition of a “M60 machinegun receiver” is: “…the frame or receiver of the M60 machinegun consists of a bottom plate(sic), two side plates(sic), the bridge, and a trunnion. The top cover, while attaching to the receiver, is not considered a part of the receiver.”

This definition is significant as it fails to consider two of the most structurally important receiver component pieces, namely the receiver’s sheet metal channel and the gas tube. Both parts are structural members that are permanently joined to form the receiver structure, and in the case of the sheet metal channel, to which the above mentioned five component parts must be joined together.

The “receiver” of the M60 machinegun has traditionally been defined to be made up from eight permanently assembled individual component parts, as follows:

• Barrel trunnion
• Gas tube (or Operating Rod tube)
• Stamped sheet-metal “lower receiver channel”
• Left and right side, milled, bolt bearing/guide rails
• Lower, milled, operating rod guide channel (similar to the bolt guide rails)
• Stamped rear receiver “ridge” (top cover latch)
• Mounting plate, T&E mechanism adapter.

(There may be one possible additional component part not counted above. Some very early military trunnions utilized a separate, riveted-in, cartridge feed ramp. This was soon replaced in the manufacturing process by a simple machined ramp feature in the basic trunnion forging. Virtually all, new-manufacture, commercially produced trunnions have the machined in feed ramp feature.)

These above eight component parts are assembled, by either riveted and/or welded joints, into a unitized structure that cannot be disassembled to a lower individual state without destructive separation. This is a significant definition, as it relates to what BATF Technology Branch is using now to determine the legal definition of a “M60 machinegun receiver.” By electing to render this current definition, BATF Technology Branch has ostensibly recognized the true component nature of the M60 machinegun receiver, thereby presumably allowing a duly registered weapon to have any of its five legally defined component receiver parts replaced if necessary due to damage. (While it is of course prudent to get a personal reply to any proposed repair actions inquiry direct from Technology Branch, this “opinion” has been rendered verbally to others in the past) The relevant statement from the recent opinion states: “…removal of one or more of these (component) parts would not change the classification of the item as a machinegun.”

This statement, making no mention of the other two common, and structurally required, component parts, can be assumed to indicate that they do not comprise a part of a “machinegun receiver” in the current opinion of ATF and are of no consequence in its definition. However, this now introduces considerable confusion into individual guns being compliant with the markings requirements (Title 26 U.S.C., Chapter 53, section 5861(g)), as there now exists a discrepancy between their (recent) definition of the “receiver” and where certain older, Pre-5/19/1986, registered machineguns bear their legally required serial number(s), which by the recent definition, can be located on a part that is not now considered to be the “machinegun receiver, or a component part of a machinegun receiver.”

This utterly bizarre and unexplainable situation becomes highly relevant when one observes that the legally required manufacturers’ identification markings and required serial number(s) have been observed to have been applied to at least three of the major “receiver” components when examining individual commercially manufactured guns, which by the recent definition can be located on a part that is not now considered to be the “machinegun receiver, or a component part of a machinegun receiver.” The most common location to place these required markings have been the top surface of the barrel trunnion, the large flat surfaces of the sheet metal channel, or the bottom surface of the gas tube, with only the trunnion now being legally considered a component part of a machinegun receiver. (It should be noted that all of those guns that were originally manufactured by a genuine military contract producer bear their markings on the top surface of the barrel trunnion.)

In addition to these above described permanently assembled component parts, a functional M60 receiver is usually considered to also comprise the following semi-permanently installed components or assemblies, (Though these additional parts are required for a functional weapon, they do not comprise the legally defined receiver.):

• Rear sight assembly (any variant model specific assembly)
• Carrying handle assembly (M60 and M60D only)
• Tripod pintle retaining latch pin
• Cocking handle assembly (cocking handle and cover plate)
• Barrel retaining latch mechanism

It is important to understand that in the sense that though the “receiver” of the commercially produced M60 machinegun may bear markings of a Class 2 Manufacturer, only a select few parts are not original military contract origin parts. Usually in most cases, the only component part(s) known to be actually made by these civilian concerns was the barrel trunnion itself, though in some cases additionally the sheet metal channel, and/or the bolt and operating guide rail sections were of new, commercial fabrication. The remaining component parts or assemblies used to complete the weapon were simply military contract spares, commonly available on the surplus market. This was a simple necessity from practical reasons as since the barrel trunnion was the most massive and sturdiest component in the receiver design, its failure or damage in service was not seen as a common occurance, hence a supply of ex-military origin spare-part trunnions was virtually non-existent. Unlike most other common spares or replacement parts of the gun, barrel trunnions as spares simply never really reached the surplus markets, so they had to be fabricated on the civilian side. The other reason is that even in the military environment, the barrel trunnion was considered to be the (tightly) controlled part of the weapon (“the receiver”) as it bore the manufacturers markings and serial number. The remainder of the receiver components could generally be obtained, so that from that point on the “manufacture of a civilian M60 was nothing more than proper assembly of the component parts and assemblies needed to render whatever version of the gun you were trying to build. In general terms then, the civilian M60 machinegun is built up from all MIL-SPEC parts and assemblies, with the exception of certain receiver parts. How this was accomplished was not always the same, either from a mechanical standpoint, aesthetic, or “quality” perspective.

Trunnion Variations

Excluding the few guns that came from the true military contract producers, the manufacture of the barrel trunnion is where the greatest difference in civilian M60’s comes into play. The original MIL-SPEC calls for the trunnion to be manufactured from an ordnance steel hammered-pressure forging, machined to final dimensions and tolerances. This is ideal if the economics of a Governmental contract with a large manufacturing concern allow for it, but for the civilian market, such expenses for what was always a extremely limited production item, were simply impossible. Fortunately though, the design of the M60 places no direct firing stresses on the trunnion itself, as the rotating bolt locking into the barrel extension contains these; and these two component parts are MIL-SPEC items in every commercial M60 known.

Due to this fortunate circumstance alternate methods of manufacture for the trunnion were conceivable, and fully within the limits of complete safety. As the M60 barrel trunnion only receives indirect stresses of firing, and since all other highly stressed component parts are MIL-SPEC items, the civilian trunnions could be manufactured by more cost efficient methods. And no matter how the basic trunnion blank was fabricated, all M60 trunnions required machining to final dimensions and tolerances.

All forms of manufacture for the trunnion blanks, be they forgings, castings, or fully machined, require slightly differing methods of arriving at the actual blank form shape as each method must be designed to accommodate the intended metallurgical profile employed. In other words, a blank sized and shaped to be formed from a pressure forging will not be directly the same as one intended to be cast, as the different metallurgies require that different physical areas of the trunnion be formed correctly for the method used. This results in slightly differing exterior shapes and sizes of otherwise “identical” parts that have been manufactured by different methods.

The most commonly employed method for the manufacture of civilian M60 trunnions was by investment casting the receiver blank form, though machining from full barstock or billet was also done. It may be argued that, in theoretical terms at least, a firearm receiver made from a machined billet of solid steel may offer certain theoretical advantages in the metallurgy of grain structure and orientation over a cast-steel version. However, in terms of the commercial M60 trunnions machined out of barstock or billet steel, this perceived advantage is of no consequence. The widespread application of CNC machining tools now being used even in the small-production shops today, was not the case when these receivers were machined and registered close to twenty years ago. At that time, without having access to the extremely expensive pressure forging methods employed in military production line manufacture, investment casting of the trunnion blank was the most reliable method of duplicating the compound curvatures and other hard-to-duplicate exterior forms of the originally designed MIL-SPEC forging. As a consequence, fully machined billet trunnions bear a distinctively more “heavy,” or “squared-off” appearance totally negating the size-to-weight ratio advantages of the strong, but lightweight, forging of the mil-spec original, or even the casting.

Commercial investment-cast steel trunnion guns are every bit as serviceable as their military cousins, and are equivalent in life-limit and durability if the private owner takes certain common sense precautions. Importantly, these castings also virtually duplicate the exact exterior physical contours of the forged original design, and can actually be very hard to tell apart from a MIL-SPEC forging if the telltale surface features of a casting are not known. These cast trunnions are particularly close to the forged MIL-SPEC originals as the basic casting alloy used was an excellent firearms-grade of steel, and the casting process employed produced a nearly exact unfinished blank that required nearly the same machining steps as the MIL-SPEC forging to complete. When finished, these cast trunnions displayed the greatest degree of uniformity of all commercially produced M60 trunnions. They readily accepted standard military component parts and assemblies used to complete the receiver assembly, and mil-spec parts interchangeability is virtually certain.

With those commercial M60 trunnions machined out of barstock or billet steel though, certain Class 2 manufacturers were content with only replicating the functionality of the original military spec forged trunnion. Due to the difficulties in replicating the hard-to-machine contours and compound curvatures of the original forged-blank design, exterior contours were often left in a rougher and not-exact state of duplication on these billet-steel trunnions. Fully machined trunnions are easily identifiable by their more “squared-off” contours, and while detracting from the aesthetics a bit, remain fully serviceable and inherently strong. Fully machined trunnions are readily identifiable due to their telltale machining marks along all major surfaces, and the less pronounced curvatures around the top of the barrel channel and rear sight mounting base. Interchangeability of MIL-SPEC component parts and assemblies on these trunnions can vary considerably depending upon the tolerances of the final machining; many of these fully machined billet trunnions exhibit indications of hand-fitting of individual parts during final assembly. A thorough inspection of an individual receiver for evidence of hand-fitting of parts or assemblies is highly advised, as later fitting of any replacement parts may be considerably more involved than with another type of trunnion based receiver.

In the many decades the gun has been in military service, literally thousands have been removed from service and DEMIL-ed, and then surplused out as scrap value. It is known that over time various methods for the actual DEMIL procedure of ex-military firearms were used, generally becoming more fully destructive over the years as concerns mounted as to usability of even component parts remaining. It is further known that a few M60 ‘s were DEMIL-ed in a more “friendly fashion” leaving open the possibility of salvaging an un-cut trunnion itself, from which a new “receiver” could be fabricated quite easily. It is therefore possible to have a commercial receiver fabricated using an original, un-cut, MIL-SPEC trunnion still bearing military production markings, and while having no collector’s value, it will be desirable receiver. The major problem here is just how the trunnion was originally joined to the other component pieces of the receiver, if only rivets had to be drilled off the DEMIL-ed pieces, it may render a perfect trunnion, but if there was any welding of the other component pieces this can complicate matters. For a DEMIL-ed receiver to have escaped the mandated reinforcing weldments, it had to have been DEMIL-ed very early on, or come from an out-of-the-way locale in the inventory. (While likely exceedingly rare, there a few known to exist in civilian hands, of this exact origin.)

The last variation in available “commercial” trunnions, undeniably the least desirable, are those individual trunnions that have been re-manufactured from original military production guns that have been DEMIL-ed. Commonly known as “re-welded” trunnions or receivers. Usually these re-manufactured commercial production receivers/guns were built up with all new receiver components assembled onto an original military production forged trunnion that has been re-manufactured into a homogeneous component by welding existing-but-damaged pieces back together. Despite any cosmetic touches rendered during final finish to make it appear “new,” original forged trunnions that have undergone welding should be carefully and thoroughly examined. When originally manufactured under military contract production, these forgings were final heat treated to produce casehardened surface hardness of 57-58+ Rockwell C scale. Case hardening is not very thick as it is intended to reduce surface wear primarily, and drops off quickly to a core value in the high 20’s Rockwell C scale. When rejoining such pieces by welding, followed by seam grinding, the basic welding process along these long and deep seams will likely produce micro-granular fracturing and general stress concentrations that can possibly extend to structural cracking and failure in/near the weldments, or along lines of concentrated stress. (This is far different type of welding than the reinforcement welds described above which penetrate only to a very shallow depth, and with minimal linear progression.) To be clear then, a re-manufactured trunnion that has undergone welding is always suspect for cracking and stress failure along the re-joined seam(s). Though generally serviceable as shooters, these re-manufactured receivers warrant close inspection before purchase (a MagnaFlux®, or fluorescent dye-penetrant type, inspection is recommended), and continuously thereafter in service at regular intervals.

In Part II we will begin discussion of the various component parts and their assembly and interface to the basic receiver assembly, along with a continuing look at factors that contribute to the structural integrity of the receiver.

Even in modern times, the shortest repeater type actions rarely lowered the barrel length below approximately 12.0”, due to the need to accommodate the factory action system. The resulting barrel length was simply a function of where the need to preserve the action components stopped. A Remington 870 for instance, can be cut to 12.0” and still utilize the normal factory pump-action, and remains far handier than the stock length barreled gun. Even with a pistol-gripped stock installed, at that barrel length it is hardly “concealable” in the normal sense of concealed carry of a handgun type weapon. Break-action shotguns, either single or double barrel versions, can be cut to bare minimalist dimensions, and with careful working of the pistol grip stock shape can easily approach the dimensions of a large handgun….the highly modified gun carried by Antonio Banderras in the movie ‘Desperado’ is a classic example. But in the end you still are only going to have 2 shots, at a maximum. If only a repeating-type action could be adapted to preserve it’s inherent larger ammunition capacity…….

The Serbu Super-Shorty AOW grew out of a specific request by a friend of Mark’s, David Prall, a true AOW lover with a fetish for bizarre and obscure AOW’s. Dave had requested Mark to build him “… a really short repeat action AOW”. Once challenged Mark eagerly went to work on the problem. For Mark Serbu, President of Serbu Firearms, Inc. and chief designer, this was more than just a joking matter. His new ‘Super-Shorty’ AOW is an amazing piece of engineering minimalism. It is also a most remarkable firearm. Since this was to be transferable as an AOW status weapon, fully transferable on a NFA $5.00 Tax Stamp, Mark had to choose a factory shotgun host weapon that had come from the factory originally equipped with a pistol grip stock. Both gas-operated and recoil-operated actions were eliminated for specific mechanical reasons of desiring the utmost in simplicity, and the fact that none of these host guns are currently offered from the factory without a full stock. As such, they could only be transferred as a NFA registered short-barreled shotgun, not an AOW, despite any shortening modifications done. So a pump action became the natural choice remaining, and the current number of factory guns coming this way can be counted with a couple fingers.

So sticking with the other original goal of making the conversion economically attractive as well, Mark selected the Mossberg Model 88 Maverick home defense shotgun in 12-gauge as the host weapon for the conversion. This is an excellent weapon with a time-proven action and well manufactured from high quality materials. The receiver is made from aircraft grade aluminum alloys with a steel breech mechanism. The barrel is made from ordnance grade steels. The weapon’s safety button is located ahead of the trigger guard, in easy reach of the trigger finger.

Mark’s conversion entails remanufacturing the entire front end of the host shotgun. The receiver and internal action parts are stock, with the exception of the action slide bars which Mark remanufactures into the customized configuration by shortening and welding them onto the new cocking handle mount. Serbu Firearms has a full-capability manufacturing shop completely equipped with CNC manufacturing centers, which Mark and his employees use to produce most of the conversion parts in-house. All conversion parts, whether made in-house or contracted, are manufactured on CNC machines using materials which are appropriate for their purpose. The cocking slide is made from 1018 steel, as is the new custom barrel lug. The new vertical cocking handle and mounting lug are made from 4130 ordnance grade steels. The cocking handle itself is beautifully CNC checkered and attaches to the mounting lug via a pull-down type spring-loaded catch. It pivots down and locks into the extended position with a very firm and satisfying ‘click’. All conversion parts are joined by TIG welding, and the welding job looks extremely professional and cosmetically pleasing. The new barrel lug is welded to the underside of the barrel and is attached to the shortened magazine tube by a 5/16” allen screw, providing a very strong attachment point for the vigorous cocking actions this gun will likely endure. The original barrel is shortened to 6.5”, extending fractionally ahead of the new barrel lug. When the conversion is complete and all parts forward of the receiver are completely assembled and welded, the entire unit is bead blasted then manganese phosphate parkerized to a pleasing flat, dark grey finish. This is a MIL-SPEC finish and should endure quite well for years. The Parkerizing blends well with the black anodized factory finish of the receiver. As a finishing touch, Mark laser engraves the catchy Serbu Firearms logo on the receiver side. When converted the AOW measures only 16.5” overall. The altered magazine holds two shells, either 2 3/4”, or 3.0”. This capacity combined with one shell in the chamber gives the Super-Shorty the capability of discharging 3 attitude-adjusting surprises as fast as the action can be pumped!

The host Maverick is shipped from Mossberg with a normal full pistol grip stock. While other AOW shotguns may employ a more horizontal Birds’ Head grip, this grip is intended to work in conjunction with the new vertical slide operating grip that Mark installs as an integral part of the conversion. We had initially wondered whether this nearly vertical factory pistol grip was ideal for control reasons, as other short BBL AOW’s in 12 gauge had become much more pleasant to shoot when the Birds’ head type grip was installed. To our surprise, when using the two handed shooting stance that is intended with this weapon, the combination of the two vertical grips lends a great amount of control to this weapon. It also causes a most natural shooting stance as the hands are configured in a strong gripping position, which also lends to the natural motion of cycling the slide mechanism between shots. The Deluxe version of the Super-Shorty ships with a silver-soldered-on stub front sight, though this is mostly cosmetic; in our mind by it’s very nature this is a point-and-shoot weapon intended for instinctive point aiming.

With the forward hand gripping the slide handle only about 13.5” in front of the rear hand (when cocked), it is quite easy to master a natural cocking stroke by simply drawing both hands together after each shot, then extending them until they stop. There is no need to divert the eyes from the aiming point when doing this, it is an instinctive action to close the hands together, and then draw them open when they are so closely positioned. And since this is a point-and-shoot weapon anyway, having the eye-to-hand coordination so closely aligned enhances the instinctive pointing nature. Bringing this gun into action from under cover, and to bear on a target at the intended engagement ranges is even more natural than using a handgun. For most people, the slide cocking hand is usually controlled by the master eye, and swings instinctively where the eye leads it on target; follow up shots are merely the result of quick hand motion while the master eye keeps the end of the gun aligned without conscious thought. With the cocking handle extended, the gun is only 7.5” tall. The Super-Shorty AOW may be fired with the vertical cocking handle in the fully folded position. Also, the action may be cycled with the handle in the folded position, though leverage is considerably reduced when used in this fashion, and if not careful the rear of the cocking handle may impact the front of the receiver. We only recommend cycling the action with the cocking handle fully extended and locked in place!

Aimed shots, while holding the front sight up into the line-of-sight, are actually more difficult than instinctive aiming. With such a short overall length, and with both hands so close together high in front of the face, the gun must be held out at arms’ length to gain a reasonable sight picture. It is simply a matter of getting used to, but very acceptable shot patterns can be had out to reasonable distances. The only problem with holding the gun in this manner is that the arms tend to be forced to absorb all the recoil forces, whereas held low in front of the body allows more of the body mass to help counter the stout recoil of such a short BBL weapon. There will of course be those who just have to try and shoot it like a normal handgun, one handed and fully extended, though they will be quick to remember that this is still a shotgun! Recoil when fired in this manner can be described as….stout, even with game loads. Aiming in this fashion is probably only realistic out to 5 feet at most, and to fire in this manner defeats the purpose of it being a pump action repeater! We tested our demo Super-Shorty with several common ammunition types, in order to gain a wide-ranging opinion of it’s capabilities, and manners. Over two field sessions we tested 2 varieties of buckshot loads, and four commonly available field loads as follows. On the first expedition we loaded and fired several magazine tubes each of:

-Winchester WW12D,

No. 7 1/2 shot, 1 oz. shot, 3 1/4 Dram equiv.
-Winchester WW12P, No. 6 shot, 1 1/4 oz shot, 3 3/4 dram equiv.
-Remington HV12-6, No. 6 shot, 7/8 oz shot, MAX dram equiv.
-Winchester XS123, T shot, 1 1/4 oz shot, MAX dram equiv., 3 in. magnum

As expected, recoil forces increased as either shot weight or powder charges increased, though for all practical purposes the Remington HV No. 6 shot provided the most ‘pleasing’ combination, and one we could essentially shoot for several boxes without any discomfort. While some would balk at contemplating using anything less than a “buckshot load” in a shotgun for self defense purposes, it must also be remembered that with this weapon, engagement range is measured at a few feet at most. As such medium range shot sizes, such as 4 and 6, in front of a common field charge would provide ample performance!

To explore the upper realms of usefulness, we then explored two common true buckshot loads. Firstly we chose the very popular Federal H132-OO; a OO Buck .33cal, 9 pellet load in a 2 3/4 in chambering, their “Tactical Buckshot Load”, which is actually a reduced charge loading compared to normal hunting loads. (This round is widely sold to LE Agencies for their duty guns) It provided a definitely enhanced effect, with considerable recoil though still manageable. The effect of those 9 pellets at 5 feet is devastating. With the field loads we could easily ‘strong arm’ the gun during the recoil effects but it became clear that when firing even these reduced charge buckshot loads a well-balanced stance was necessary to help with recoil control. The Super-Shorty, though, remained well-mannered, with maximum recoil displacement being a sharp upward transition of the muzzle. A controlled return to point-of-aim while cycling the action to load the next round was completely under control and rapid. The gun is, of course, cylinder bore, so shot patterning will be expected to expand quickly from the bore. Patterning with this load was tested at 3, 7 and 10 feet, with maximum shot displacement at 10 feet patterning in an elongated upward right side deflection approximately 9”. Serious engagements would likely occur under this range and as such the patterning of this load is quite acceptable for defense purposes. Extensive practice with this weapon using buckshot is highly recommended. (We contemplated having the chamber back-bored to ease the recoil loading, and the barrel could be adapted to a screw in choke system, though it is doubtful if this would increase the overall effectiveness of the weapon at longer ranges due to the inherent sighting issues and required firing hold.)

The last load we tested was the Federal P156-OO; a OO buckshot .33cal, 12-pellet load, 2 3/4in chambering listed as a magnum MAX dram equiv. charge. This shell was clearly too much power to recommend it for serious shooting in such a light, short weapon. It was fun for a round or two, but recoil forces quickly bit into the web of the rear hand and caused the sliding cocking handle grip to become difficult to cycle easily as the rear hand was badly out of position when trying to immediately cycle the action. In terminal ballistics, the difference between the Federal Tactical buckshot loading and this one would not be noticeable at the intended ranges.

To summarize, the Serbu Firearms Super-Shorty AOW shotgun is an amazingly well conceived, highly concealable weapon. It has legitimate self-defense capabilities, and is surely one of the most striking looking firearms available today. It is one of those weapons that just oozes personality and calls out for attention!

While we only tested the original 12-gauge chambering, the Super-Shorty AOW shotgun is currently available in both 12-gauge and now 20-gauge, which is probably even a more desirable chambering considering the gun’s physical size envelope. (The 20 gauge model is built on the Mossberg Model 500 action, but is identical in all other aspects.) Retail price for the standard model is $675.00, the Deluxe version with front sight installed and with sling swivels is $725.00, priced the same in either chambering. Either version transfers on a $5.00 NFA Tax Stamp.

Serbu Firearms, Inc.
6001 Johns Road, Suite 144
Tampa, FL 33634
(813)854-1532