Thomas T. Hoel
Barrel Group Basics
All versions of M60 barrels are exceedingly well made, fabricated from the very highest quality base materials, and combined with their full chromium bore plating and Stellite® throat liner, make for a barrel that can endure a tremendous volume of usage, and even abuse. They are likely one of the finest firearm barrels ever used on a US weapon, and despite any other shortcomings with the basic weapon design, the barrel tube is not one of them. Despite the high quality of the basic barrel tube, there are several practices the owner-operator should be accustomed to performing to preclude incurring any damage(s), and to maintain the expected lifespan and utility of the barrel group components. Unlike some other machineguns with quick-change barrels, the M60 barrel is a semi-permanently joined assembly of the basic barrel tube, the barrel locking extension, the complete gas system, and (usually) the complete bipod assembly. The several component functional groups contained there in all require specific service and care procedures.
There are two distinct gas systems now used with M60-series machineguns. The original gas system required considerable user attention during usage and maintenance. The newer product-improved E3/E4 gas system strived to eliminate most of these concerns. The correct tool for servicing the standard M60 gas system is the “Combination Tool, Machinegun, M60” (PN 7790680); for the E3/E4 variants a different combination tool is required (PN MC4035). Both are specific to the hardware found on each model.
For those guns currently equipped with an original style barrel group utilizing the first variant, or “standard,” gas system the following practices are mandatory. The most important area of attention on the standard gas system is the retention of the two end caps. These are the front-end gas cylinder extension plug (PN 7269030), and the rear end gas cylinder nut, or bushing (PN 7269031). Both are retained in their screw-thread fittings by a special locking-type washer known as a “key washer” (PN 7269035), though this key washer alone is never sufficient to retain the vent plug on the front end of the gas cylinder during firing use of the weapon as was discovered in military service. To prevent unscrewing of the frontal vent plug, it is required that the locking key washer be safety-wired to prevent it loosening when the gun is fired; this has also been recommended in military usage for the rear locking key washer additionally, though for civilian usage this can usually be omitted if the washer is not damaged. The only correct wire to use for this procedure is aircraft-grade stainless steel wire of 0.032-inch diameter. The recommended (correct) tool for installing a new wrapping of safety wire is an aircraft mechanics style “safety wire pliers” that contains a screw-thread mechanism to provide uniform twisting of the wire rope strands without imparting strains that may cause the wire rope to break while twisting. For civilian applications, regular hand pliers may be used with care if the user is experienced. The wire rope should wrap around the front (or rear) plug area for at least half the diameter of the gas cylinder against the direction of rotation, and be secured by wrapping each strand around one of the two gas cylinder legs, then tied off with a short twisted segment with the direct end bent over to prevent unraveling.
This gas cylinder type uses a centrally located service porthole to provide access to the gas port in the barrel underside for cleaning. This service porthole is capped off with a threaded gas port plug, which in the earlier version must also be safety-wired to prevent unscrewing during firing. The safety wiring procedure is identical to the above, and the early plugs (PN 7792093) have a 0.040-inch hole drilled the plug head for the wire to pass through. To eliminate this safety wire requirement on this service port plug, a new design of plug was fielded (PN 9362515) in the early 1980s that incorporates a simple split-type locking washer that was sufficient to hold the plug tight in its well. This eliminated one safety wire area, and was intended to facilitate more frequent inspection and reaming of the gas port to keep it clear and reduce weapon down-time; with this newer style of port plug the gas port may be reamed (cleaned) without disassembling the entire gas cylinder by letting the gas piston slide fully forward exposing the port in the barrel. The use of this new style plug in a standard variant gas system is highly recommended for the civilian owner-operator.
The E3/E4 variants introduced a marked improvement in terms of ease of service in their new design gas system as it completely eliminated the need for any use of safety wire to retain the component parts as they employed interlocked keying. Additionally the need for a separate barrel gas port service plug was eliminated, by designing the cylinder extension to allow access to the barrel gas port when partially disassembled. The E3/E4 gas system removes most of the maintenance concerns of the original design, while considerably simplifying the overall assembly group.
Many of the operational difficulties encountered with a typical civilian M60 center on the gas system as seemingly the gas system in many of these guns is either never cleaned, or cleaned too often. There is no set interval for cleaning of either version gas system, in terms of numbers of rounds fired, but it must be periodically serviced to keep the gun functioning. The recommended method for determining when to fully disassemble the gas system and render complete cleaning is to hold the barrel assembly horizontal, then tip it end to end through approximately a 60° arc, and if the gas piston slides freely within the cylinder it is still in firing condition, not requiring immediate service. Only when the piston cannot be heard to slide freely does the gas system require servicing action. The gas port however, may be reamed (cleaned) at shorter intervals using the combination tools’ reamer to prolong service intervals of the entire gas system if it appears it is becoming caked with carbonized firing residues. A general rule of thumb is to do a full gas system service at 2,000-2,500 rounds fired, or by the above measure. This is, of course, highly dependant upon numerous factors such as ammunition used, etc. (These interval recommendations are of course for normal use of the weapon in the civilian realm, as military protocols are considerably different due to the nature of combat readiness requirements.)
When it is time for full servicing of the gas system, a proper powder solvent should be used to dissolve as much of the carbon residues present as possible, combined with manual clearing of the barrel gas port, gas cylinder bore, the several gas holes in the gas piston head, and the gas vents in the cylinder forward end. Both the gas cylinder bore, and the gas piston itself, are fully hard chromium plated so vigorous use of a stiff bristle stainless steel or brass brush can be employed without fear of damages. After cleaning, make sure that both the gas cylinder bore and the gas piston are completely dry and free of any oil or lubricant. Reassembly of these components with a coating of lubrication is neither intended, nor desired, and to do so may actually increase the tendency for carbonization in the gas system.
CAUTION: After cleaning and prior to extended storage, for preservative purposes only, apply a light film of protective oil, such as CLP, to the gas system components prior to storage to prevent any possible corrosion. Prior to the next use, the gas system and all components must be clean and free of all preservative oils. Never fire the weapon with any oil or other lubricant inside the M60 gas system as it was designed to be fired completely dry. (In the civilian realm, it is generally acceptable to leave the rear gas cylinder nut un-safety-wired to facilitate access to the cylinder bore and piston.)
Threads on all component parts of either variant gas system should be thoroughly cleaned of any foreign matter prior to reassembly to facilitate future removal and wiped with a light, preservative type, oil (CLP), or an anti-seize lubricant compound. Care must be taken to replace the standard type gas piston with the closed-end head facing toward the rear, or the gun will not function beyond the first shot. This potential dilemma has been eliminated with the E3/E4 variants by using a fully reversible piston. Following reassembly of a standard M60 gas system, the required safety wiring applications must be restored to all threaded joints as outlined above (exception for the rear gas cylinder nut), though never attempt to re-use old pieces of safety wire. On E3/E4 variant gas systems the only required safety check is to verify the integrity of the interlocking key washers at reassembly. For both type gas systems, locking washers with broken tangs should not be reused. The owner-operator should consider obtaining a supply of these as spares, as they can break their tangs easily if excessive force is applied during maintenance operations.
The barrel tube itself is essentially easy to maintain, with service limited to thorough cleaning of the bore lands and grooves following normal procedures. A combined powder solvent/preservative compound such as CLP is ideal for use in such plated bores. When cleaning the bore only, and not simultaneously doing a thorough gas system servicing, the barrel assembly should be inverted and angled upward so the cleaning solvents will not readily enter the gas system through the gas port located on the barrels’ underside. If care is taken in this manner, several bore cleanings can be done without excess solvent causing problems in an otherwise still serviceable gas system. If the bore is cleaned without inversion of the barrel assembly, the gas system components will invariably require a thorough swabbing and drying, to remove solvent that has passed through the gas port, to prevent undue accumulations. Due the combination of the Stellite® chamber throat liner, and hard chromium bore plating the average M60 barrel can achieve phenomenal amounts of service use with minimal care and maintenance. For example, many surplus barrels now in use with civilian guns bear manufacture dates in the early 1960s. Since belt-fed machineguns such as the M60 are rarely used for “accuracy” shooting, a barrel that is otherwise sound should not be considered unserviceable due to eventual wear of the bore lands. Even very well used M60 barrels that register high marks on a GI throat erosion gauge are still usually perfectly serviceable for civilian usage provided they are structurally sound. The M60 barrel tube itself has one primary critical area to judge if it is safe to shoot, and this is verification of the integrity of the Stellite® liner, located just forward of the cartridge chamber. Stellite® is an extremely hard, tough, yet durable alloy with very good resistance to high temperatures and erosion that has allowed extending the useful life of machinegun barrels several times over. The liner is permanently installed at manufacture, and cannot usually be removed or replaced. An owner-operator should learn to become familiar with the normal appearance of the liner in his barrel(s), and be alert to changes in appearance, which may signal it is no longer safe to shoot. The liner will have a small circular parting line with the rest of the bore that is clearly visible a short distance beyond the chamber; this gap is required for heat expansion during use and as such the measurable gap visible should only be checked on a cold barrel. With a liner, any visible cracking or chipping is cause for rejection of the barrel tube. The same applies for longitudinal separations that become visible. The danger is that the liner will usually fail in large sectional areas possibly causing pieces of the liner to form dangerous bore obstructions. Also, never fire a lined barrel where a visible area of the liner is missing, this can cause projectiles to be gouged or otherwise damaged as they travel down the bore, with the same potential for causing bore obstructions with further use.
As a result of the employment of a quick-change type barrel requiring a fixed headspace setting, the use of spare barrels for the M60 require proper consideration. Since “headspace” settings are fixed at time of manufacture of the barrel, and are a result of a new barrel assembly being checked against a new test-bolt gauge, the headspace will change with any use of the regular bolt and barrel over time. (There is no way to adjust headspace on a M60 barrel assembly after manufacture.)
For whatever reasons, many M60 owners like to have at least one spare barrel assembly for their gun, and that being the case proper determination of headspace values with each separate barrel (and bolt) is critical to safety. The only way to accurately determine if the chamber headspace is within acceptable tolerances is to measure each bolt with each barrel using the appropriate test gauge. M60 barrels are chamber reamed to 7.62x51mm NATO cartridge specifications, which is not directly equivalent to the more common commercial chambering of .308 Winchester. As such, the use of commercial .308W chamber headspace gauges is not an ideal substitute (NOTE: Commercial gauges are not routinely manufactured with a grooved cut for the extractor and may need to be modified, or else the bolt may need to be stripped prior to use. Clymer(r) brand headspace gauges are the exception.) The military issues two NATO chamber headspace gauges, a “GO” gauge dimensioned at 1.6415-inch (part #7274786/NSN 4933-00-069-8676), and a “NOGO” gauge dimensioned at 1.6455-inch (part #7274790/NSN 4933-00-647-3698), along with a special test bolt gauge (part #7799699/NSN 4933-00-653-9550) that allows determination of an out-of-spec part directly.
Commercial gauges may be used if the military parts are not available, however the above tolerance dimensions should never be exceeded, and a barrel and bolt combination should never close on a commercial .308W dimensioned “field gauge.” Determination of headspace may be done by hand manipulation of the individual bolt body into the selected barrel assembly, by attempting to close and lock the bolt head manually. If proper headspace using the gauges cannot be obtained using a particular combination, it may be necessary to swap bolt bodies or barrel assemblies until a combination is found that will pass both gauges correctly. If either a bolt body or barrel is ever exchanged, the headspace testing must be re-done in bolt-barrel combinations to assure safety. It is therefore prudent to prominently mark, or otherwise identify easily, the spare barrels that are mated in headspace to the bolt body currently in the gun. CAUTION: Many M60 barrels that are currently being sold as “new” are in fact surplus units that are in fact quite “experienced,” and have been freshly re-parkerized only for sales purposes to the unwary or uninformed. Despite any attractive appearance to the contrary, the only way to ascertain the condition of an unknown barrel is through the proper usage of a chamber throat erosion gauge, and muzzle wear gauge. There exists a special throat erosion gauge for the M60 barrel, the M8 Small Arms Gauge Kit (part #5910297/NSN 5280-00-313-9485), which is difficult to obtain but it is the most comprehensive tool available for the job. In lieu of the M8 gauge, a more common M14 rifle chamber throat erosion gauge may be utilized with some care to allow for interpolation of the readings.
At the least, a “field expedient” examination of any potential barrel purchase should include a thorough inspection of the bolt locking cam ways for evidence of peening or galling (or visible cracks), an inspection of the chamber face for same, and a quick check of the flash hider pin holes; any evidence of excessive peening or drill markings here is evidence of a likely replacement of the flash hider. Conversely, a stripped M60 barrel with a freshly parkerized appearance, and bearing more than one drill pass through the flash hider threads is clearly a rebuild at one time or another. Lastly, the entire gas system should be firmly attached and the gas cylinder should be closely examined for any evidence of corrosion inside.
Fortunately, all versions of M60 bipods are simple and relatively sturdy designs. Extensive application of common sense is the best way to prevent damage to the legs of the bipod, which lose a considerable amount of their support and strength when fully extended. Use of the bi-pod should be limited to only providing support of the weapon, and nothing else. Proper maintenance of the bipod assembly, of either variant, is essentially limited to proper cleaning and a preservative coating of oil at normal maintenance intervals. If ever required, higher-level maintenance and repair procedures can be consulted in the appropriate military manuals, such as TM 9-1005-224-23 & P.
The last area of maintenance concern on the barrel assembly group is the barrel extension locking collar, and its bolt locking cam ways. Due to the design of the operating system of the weapon, we shall consider this area of the barrel assembly during the next topic, the Operating System. As will be seen, this is a critical area for concern both from a functional and safety standpoint.
This article first appeared in Small Arms Review V7N4 (January 2004) |
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