AR-15 Gas Systems

Our Department of Defense might soon begin issuing a replacement for the M4 carbine and, if some people get their way, the entire M16-series as well. Considering most of the proposed replacements are quite similar to already-issued equipment it would be the first time the US military replaced small arms without an an obvious improvement or change associated with the new device. The Trapdoor Springfield used breech-loaded metallic cartridges and supplanted muzzle loaders.

The Krag repeater replaced single shot rifles. Springfields and Enfields were much faster to reload and fired a more powerful cartridge than the .30-40 Krag. The Garand gave us semi-auto capability and the M14 more than doubled capacity. The last change made was to the AR15/M16 and probably the most controversial. The thinking was that troops measure "firepower" in volume, not individual hits, and can’t reliably hit point targets past 300 meters on the range, much less in the field.

A full power rifle cartridge is considered needlessly powerful and cuts down on volume while a light cartridge and rifle lets the trooper carry more. There are valid reasons to agree and to disagree with this concept, however, if this is the basis of chosen doctrine, the M16 is a good fit. Most of the proposed replacements to the M16-series, our longest serving rifle, involve changes to the gas system, specifically going from a direct impingement-based design to a piston driven arrangement.

Let’s look at the two systems and how they compare. AR-15 History Research conducted by the US Army Operations Research Office (ORO) about small arms effectiveness concluded that the most desirable infantry small arms should be of .22 caliber, select-fire and with high velocity bullets effective up to 300 meters or so. This lead to Project SALVO in the 1950s and requests to the Armalite Division of the Fairchild Aircraft Corporation to develop a suitable weapon.

Eugene Stoner, then at Armalite, began to develop a rifle based on his previously-designed 7.62mm AR-10 while Remington began work on anew .22 caliber cartridge based on the .222 Remington. The newly christened AR-15 was demonstrated to then US Air Force Vice Chief of Staff, General Curtis Le May, who procured about 8,000 AR-15 rifles for Air Force security forces to replace aging M1 and M2 carbines.

Shortly thereafter, US Do D Advanced Research Projects Agency purchased 1,000 AR-15 rifles and sent them to the South Vietnam for field trials. The Army initially contracted for an additional 85,000 rifles (the XM16E1) adopting it as a limited standard rifle to fill the niche until the forthcoming Special Purpose Individual Weapon system was completed.

The SPIW never passed the prototype and trial stages and in 1967 the US Army adopted the XM16E1 rifle as "US Rifle, 5.56mm, M16A1." Every gun crank is aware of Vietnam horror stories with the newfangled "Matty Mattels." The causes for malfunction during the first mass issue were due to excessive fouling by ammunition (originally-specified IMR powder was replaced by ball powder with insufficient testing), the lack of a chrome-lined chamber, no issued cleaning equipment and the fact that the initial "space age" M16 rifles were promoted as needing no maintenance.

These issues were addressed and remedied by 1970 but stories of the original problems from over four decades ago still carry traction. The HBAR (top) is direct impingementbased in A2 trim while the 6940P (bottom) is piston driven using Colt’s Articulating Link Piston. The 6940P is Colt’s offering to replace the current M4. The big "problem" with the AR-15 today is the gas system.

Standard AR- 15s use a type of direct gas operation, the origins of which can be traced to experiments conducted by a French firm, Manufacture d’armes de Saint- Étienne, or MAS, in 1901. Early service rifles based on the design include the MAS-49 rifle and the Swedish Agm/42. In 1956 Eugene Stoner patented a similar design that utilized a gas tube acting on the bolt carrier itself. In effect, the bolt acts as a fixed piston and the bolt carrier contains a movable cylinder.

Gas is routed into the bolt carrier and pushes on the bolt inside, unlocking it and pushing it rearward. In Stoner’s primary patent for this system (Patent Number 2,951,424) the reasons stated for this approach are low weight, an inherent simpler mechanism by using the bolt and carrier instead of separate components in the gas system, and an in-line recoil. Based on the ORO specs, the design called for the most compact and lightest rifle possible.

Direct gas systems like Stoner’s have a number of things going for it, including light weight, simple construction with little to wear in the gas system, and

The key of the 6940P (right) is solid and is the point where the operating rod actuates it, as opposed to letting gas flow inside it, as in a standard AR-15 (left.) ) Not surprisingly, the piston-driven 6940P bolt is noticeably less fouled. the ability to free float the barrel as only the small gas tube with no moving parts is there to interfere.

Direct impingement-based self-loading designs have been found to really shine in longer-barrel (18" and up) usage where accuracy, particularly at longer ranges, is the goal. As with the design of any integrated system, there are compromises. Regardless of marketing hype, gas piston systems still need maintenance and the additional parts can fail. If ammo is going to create fouling with a direct gas system that same ammo will foul any gas operated system.

Yes, the direct gas flow to the bolt and carrier means fouling is going to end up there. Guess what? If you are burning powder that fouling is going to end up somewhere. One benefit of the AR is that the entire carrier and bolt can and should be kept moistened with lube and occasionally wiped off. Often, manufacturers will recommend keeping internal gas system parts dry to prevent excessive fouling requiring dry scraping.

Anyone who has maintained M240 or M249 machine guns is aware of this and the special ) The piston, attached to the spring-loaded operating rod, fits inside the piston sleeve. The entire assembly slides in through the front sight base and the sleeve is held in place by a self captive retaining pin (shown here fully extended.) Colt engineered this to make backwards and upside down assembly impossible. tools that were created to accomplish it.

By keeping the "piston" (bolt and the inside of the carrier) of an M16 moistened, the bulk of this fouling can be wiped away. Yes, there will be some remnants of fouling, but this causes no harm. You do not have to scrape it all off.

As these parts show, the bolt behind the gas rings in the 6940P has no fouling but the piston and sleeve does (left) while the bolt and firing pin of a standard AR-15 are more fouled. A number of high level High Power competitors have documented going an entire season of shooting their AR- 15s without cleaning, just keeping the parts wiped down and dropping in some lube between shooting sessions.

Members of various military shooting teams maintain issue M16A2 and A4 rifles in the same manner and only remove the bolt from the carrier for cleaning annually. Typical maintenance after alive fire session is to pull out and wipe down the fully-assembled bolt and bolt carrier, upper and lower receivers with a rag, run a single wet, then dry, patch through the bore and place a drop of lube on each rail on the carrier and inside each drain hole. The entire procedure takes two or three minutes.

With a proper functioning AR-15 this will yield absolute reliability and needs to as competitions shot with issue rifles allows no abilbis. A drill sergeant/instructor might freak at such a "dirty" rifle but it continues to function flawlessly. This approach mirrors the maintenance advice directly from Armalite in their Tech Report Number 29 and by TACOM, the folks that write Technical Manuals for the US Army.

Neither recommends scraping away every last remnant of fouling and states that improvised metal tools and picks remove the finish and cause damage. Wipe or brush away enough fouling so the mechanism continues to function. Clear it, pull out the carrier, wipe it and the insides of the receiver out with a rag and brush, relube as per the TM and reassemble. Two minutes, done!

Colt ALP In 2010 the Congressional Research Service released a report written by Andrew Feickert entitled The Army’s M-4 Carbine: Background and Issues for Congress. In this report, issues about the M4 were addressed and officially released to our elected officials in Congress.

It was noted, "The Army is undertaking both the M4 Carbine Improvement Program and the Individual Carbine Competition, the former to identify ways to improve the current weapon, and the latter to conduct an open competition among small arms manufacturers for a follow-on weapon." One major study published by the United States Special Operations Command and cited in this congressional report ( M-4A1 Carbine and Related Systems Deficiencies and Solutions: Operational and Technical Study With Analysis of Alternatives) claimed that the M-4A1 design was fundamentally flawed–due primarily to the shorter gas tube and barrel–and that a variety of factors "led to alarming failures of the M-4A1 in operations under the harsh conditions and heavy firing schedules common in SOF training and operations." SO- COM has since concluded that the M4-series carbine currently as issued did not meet special operations forces requirements.

Mr. Feickert’s CRS report suggestions for potential improvements, dubbed the the M4 Carbine Improvement Program, include replacing the direct-gas system with a piston gas system, strengthening the rail system and a heavier barrel for better performance during high rates of fire, among others. Colt’s entry for this improvement is the 6940P, a modular, 5.56mm, piston-operated, lightweight magazine fed carbine with a one-piece upper receiver.

Colt’s first piston design, the 1020 (M5 Piston Carbine), used the rear portion of the carrier to prevent carrier tilt. The 6940P, similar to the 1020, incorporates Colt’s Articulating Link Piston operating system that reduces the inherent stress in the piston stroke by allowing for deflection and thermal expansion. The ALP is specifically designed for ease of cleaning, disassembly, and assembly of parts.

While the company seems to be less enthusiastic about piston designs than other manufacturers, labeling them "Alternative Operating Systems" in marketing literature, Colt is not new to the concept. They fielded a pistondriven AR prototype in the late 1960s as the model 703.

Colt’s ALP design is an improvement on gas systems found in their previous M5 and 1020 releases that is designed to increase mechanical accuracy while reducing stress on the components during the cycle of operation. Inherent stress in the piston stroke is reduced by allowing for deflection and thermal expansion and claiming less wear than that seen in in competing piston-driven systems.

The ALP consists of a hardened piston sleeve surrounding the piston which reduces the cyclic rate and reinforces reciprocating parts, thus extending service life.

The modified bolt carrier features a stout contact for the piston where the gas key was previously and reinforcement of the carrier’s rails ("skids".) Also reinforced is the piston sleeve retaining Colt 6940P Articulating Link Piston Rifle (Speci ications subject to change without notice) 1 Charging Handle 3 Bolt Carrier Assembly 13 Takedown Pin, Lower Receiver 23 Buffer Assembly pin that further reduces wear to the cam pin slot in the upper receiver. Colt redesigned the extractor, making it stronger.

The pivot forward design increases purchase on the rim of the cartridge case. Extraction issues have been a common problem in current M4s suffering reliability problems and this should alleviate them. Barrel life is listed at over 15,000 rounds. Any change to issue equipment will require retraining on use. To Colt’s credit, this will be very minimal with the 6940P. The only change is in operator maintenance as there are additional parts—the piston system—that need to be removed for cleaning.

The ALP is held by a single, self-captive retaining pin that works just like the pins securing the upper and lower receivers. Pressing the retaining pin allows the entire ALP gas 42 Piston Sleeve Retaining Pin 43 Piston Sleeve 44 Front Sight Base 52 Sling Swivel, Detachable 24 Action Spring 27 Takedown Pin, Lower Receiver 29 Upper Receiver 32 Lower Receiver system to slide out through the front of the front sight base.

The piston and piston sleeve pull easily apart and are cleaned just like the gas ring area of the bolt and bolt carrier in a standard AR-15. For routine maintenance the operating rod can stay attached to the piston. If they should separate, snap the piston onto the operating rod and slide the piston inside the piston sleeve. Slide the entire piston assembly in through the front of the piston sleeve hole (inside the front sight base) until the piston sleeve is seating inside the sleeve hole.

Push the piston sleeve retaining pin into the front sight base until flush with the right side. Cleaning these additional parts is offset by a bolt that is now carbon free. With all the fouling contained in the ALP piston and sleeve, the bolt and 56 Piston 57 Operating Rod 70 Barrel

carrier receive none. Reassembly is just as simple and the parts are machined to make backwards or upside down reinstallation impossible. The direct impingement-based gas system found in full length AR- 42 Piston Sleeve Retaining Pin 43 Piston Sleeve 44 Front Sight Base 55 Piston Assembly 15s can be utterly reliable with proper, minimal maintenance. It is the basis of winning rifles in High Power Service Rifle, practical 3 Gun and our nation’s longest serving rifle.

However, good piston-driven systems might just 56 Piston 57 Operating Rod 58 Piston Sleeve Hole prove more reliable yet, especially in short barreled weapons like the M4, and Colt’s ALP is a solid contender there. As with any quality firearm, learn how to take care of it and it will take care of you.

AG Colt 6940P Articulating Link Piston Rifle (Speci ications subject to change without notice) Colt 6940P Articulating Link Piston Rifle (Speci ications subject to change without notice) 14 Bolt carrier 16 Firing Pin 17 Firing Pin Retaining Pin 18 Cam Pin 19 Extractor Pin 20 Extractor 21 Bolt 21a Bolt Lugs, Face 21b Bolt Lugs, Rear 21c Locking Lugs 26 Bolt Carrier Key 33 Gas rings 43 Piston Sleeve 56 Piston 57 Operating Rod