[Fletchette]

After reading the M4gery thread, it seems like a lot of people do not trust direct gas impingement as reliable, often citing the M14 as a better rifle (short-stroke). I know Vietnam era M-16s had problems but most of that was the powder they used and the lack of chrome-lined barrel - fixed long ago.

Also, the MAS 49/56 is a M-14 class weapon,and very reliable when used in it's original 7.5 French. The fact that the French made it is it's only negative, in my opinion. It uses direct gas impingement.

Is there someting I am missing? Maybe the lack of being able to clean the small diameter gas tube?

:confused:

[natedog]

Direct gas impingement uses gas directly from the barrel to cycle the action; the gas is vented into the bolt carrier key, which drives the bolt carrier rearward, rotates/unlocks the bolt, and the whole thing moves back, ejects the round, moves forward and feeds a new round. The 'problem' is that unburnt powder residue and other unpleasentries from the cartridge are deposited into the action; the bolt lugs, bolt face, carrier key, bolt recesses, etc. That, combined with the (relatively) tight tolerances of the AR15 can cause the following malfunctions.

A) Gum up the ejector, causing failure to eject,
B) Gum up the extractor, causing a failure to extract,
C) Decelllerate the bolt carrier enough that it won't have enough force to strip off a round and lock into the barrel, or
D) The bolt won't be able to lock because of the excess amounts of fouling in the lug recesses.

I say 'problem' because it really isn't an issue; using chrome lined parts, well lubricated rifle, good ammo, and a 5.56mm chamber it takes thousands of rounds to dirty it to this point, basically making it a moot issue. No one carries or fires thousands of rounds in combat at one time, and outside of tortue testing, most gun owners don't, either.

[Fletchette]

Hmmm. I did not consider the powder fouling in the breach area, but I can see the point. With regards to AR-15/M-16 weapons, this fouling is deposited in the rotating bolt area. However, the MAS 49/56 has a tipping bolt mechanism which locks into the receiver near the rear of the bolt -well away from any fouling. I wonder why such a locking mechanism went out of style, it is much easier to machine.

[natedog]

It probably went out of style the same time machined steel receivers went out of style; with newer guns, like the AR15 and G36, polymers and aluminum alloys are used, and the bolt locks up with the (steel) barrel. A tilting mechanism wouldn't be possible with alloy receivers.

[Fletchette]

I wonder if anyone has retro-fitted a short-stroke piston onto an AR-15 platform. The Daewoo is a nice rifle, but with the lack of replacement parts it fails the SHTF test.

[SpookyPistolero]

I can't remember who makes it, but I know someone did or does produce an upper assembly that uses a piston driven gas system. Always seemed like a good idea to me, and not just reinventing the mouse trap. If I were going to get an AR, it would be with that piston upper in 6.8 SPC. That would be a sweet combo.

[Gewehr98]

The 6.5x55 Swedish AG-42B Ljungman was also a direct-gas impingement system. In the AG-42B, the gas tube simply blows the bolt carrier straight back, the exhaust gas goes no further. Clunky, but relatively clean compared to more modern designs. ;)

http://mauser98.com/ljungmanbench.jpg

[natedog]

Gas-piston uppers are flooding the market now. Off the top of my head, HK, Leitner-Weise, Kurts Kustom, DSA, and POF all make gas-piston AR15 uppers.

[mustanger98]

Under battlefield conditions, dirt/dust, mud, water and what all else don't help with the tight tolerances. Like Mr. Kalashnikov said, you can throw a handfull of sand in an AK-47 and it'll still run because the tolerances are so loose, but you can't do that with an M-16.

I heard from a Vietnam vet who said just so much as a heavy dew and the M-16's barrel will hold water. If you're coming up out of the swamp, you have to blow all the water out of the barrel and gas cylinder. If the enemy is right there when you come up out of the water, you don't have time. Better to have a AK or M-14 because those systems, for whatever reason, allow the water to drain in a hurry.

According to Culver's "Saga of the M-16 in Vietnam", part of the problem with the direct gas impingement system was the powder. They had been using an extruded powder- I forget which one right off- and it worked fine, but for cost reasons and that they had a deal with DuPont, they switched to a ball powder. This ball powder burned slower and had a higher port pressure. The gas would run down the gas tube back to the bolt carrier key faster than it properly should and attempted to extract the spent shell case before the cycle of obturation had completed. The shell case, still stuck to the chamber wall, would not extract therefore, the extractor would many times either jump the case rim, or tear through it. This is why a soldier would use his cleaning rod as an ejector rod and it cost a lot of lives. They remedied this by chrome lining the chamber. While it allowed the spent shell case to slide out of the chamber faster, it also enhanced the battering the reciever was taking from the higher pressure making the rifle cycle faster than it should have from using the wrong powder for the system.

Not being able to clean the small diameter gas tube does, in my opinion, present a problem, whereas an M1 or M14 gas cylinder can be cleaned as often as anybody cares to go through disassembly.

[Bartholomew Roberts]

The main fault of the direct gas impingement system is that it is sensitive to variations in ammo and barrel length to a greater degree than many semis. It does direct fouling back into the action; but the fact is that in most well designed weapons like the AR15, a single man isn't capable of carrying enough ammo to foul the action to the point of malfunction, let alone shooting it in a single engagement with no chance to clean.

The problem with the ejector binding up isn't directly related to direct gas; but more of a side effect. If you shorten the barrel in a direct gas system, you also increase operating pressures. Likewise, a slightly hotter cartridge has the same effect. What binds the extractor is not fouling from direct gas; but the result of the increased pressure causing the brass to extrude into the ejector pin hole. As the gun attempts to extract while the brass is still extruded (due to the faster cycle of a higher pressure direct gas system) it can bite off tiny chunks of extruded brass that may eventually bind the ejector in a spring-loaded ejector system similar to the AR15.

On extractors, I've never seen an extractor fail because of direct gas fouling. However, the pressure side-effects of direct gas affect the extractor as well. If the pressure goes higher due to gas port erosion, shorter barrel or amoo fluctuation, a direct gas system will try to extract before the brass has shrunk back from the chamber walls and while pressures are still too high. This will result in fails to extract; but not because of fouling in the extractor.

Like most gas systems, an underloaded round that does not generate sufficient pressure will cause a direct gas system not to cycle. In addition, blockages within the direct gas system (blown primer in the gas key) can obstruct the action.

I've simply never seen a bolt so covered in direct gas fouling it can't lock up. If you do not clean a direct gas system, it will eventually happen but considering the AR15 system has gone at least 7,000 rounds with no cleaning, I really don't see this as an issue. Foreign debris is another matter entirely and effects all rifle systems equally. If the rifle is not designed to keep foreign matter out of the bolt/lug locking area, you are going to have problems.

As for gas piston systems, there are a lot of gas piston systems out on the market to correct perceived deficiencies in the direct gas impingement system for the AR15. Many of these trade problems related to direct gas impingement for new problems such as bolt bounce, increased weight or recoil, carrier key shear, and decreased accuracy.

Currently though, there are a few short stroke gas piston systems that are reputed to be very promising. Among these are the Colt M5, the Leitner Wise gas-piston system which is rumored to be the basis of the M5, and the HK 416 gas piston. The HK gas piston is reported to still have issues with shear and bolt bounce to some degree.

The Colt M5 is still mostly unknown due to NDAs. We know it finished second in the recent SCAR competition and that it is a short-stroke gas piston mod of an M4. Beyond that, we don't really know much.

The Leitner-Wise system is the most public and comes from a company renowned for two things - developing extremely innovative small arms engineering and being totally unable to deliver those same products on a production basis.

Leitner-Wise manufactures a self-regulating short-stroke gas piston system for the AR15 that is weight neutral and can be retrofitted to existing direct gas systems. Leitner-Wise claims the following benefits of their system:

1) Reduction of recoil
2) Reliable function regardless of barrel length
3) Reliable semi-auto and auto function with ammo ranging from subsonic to high-pressure NATO loads
4) Total system weighs the same as existing gas tube
5) System can be converted back to direct gas simply by replacing parts
6) 40,000 rounds with no cleaning (lube added) and no stoppages
7) Lifetime warranty to include free upgrades.

Leitner Wise is currently marketing this system to the public and will convert existing systems from other AR manufacturers, including magnetic particle and proof testing of the bolt and barrel.

The negative side to this is that Leitner Wise has offered many innovative products on the marketplace before. Inevitably, the samples of these products met with much praise but the production versions left something to be desired. Some examples...

1) Leitner-Wise offers a .22LR conversion that receives much praise in the gun press. They take no money from anyone but form a waiting list in 1999. The people on that list are still waiting today, even though LW maintains the product on their website.

2) Leitner-Wise wins a $20M contract to provide the Coast Guard with the .499LW AR-15. Effectively, this is an AR15 shooting a round with .45-70 ballistics with only a change of bolt, barrel and magazine. A year later, semi-official Coast Guard publications report unacceptable reliability issues with the rifle (a direct gas version) and LW has delivered only a few rifles on the contract.

Recently (February-March 2005) LW has delivered new .499LW rifles using the new gas-piston system to the U.S. Marines for testing and evaluation of their use at vehicle checkpoints. The Marines want the rifles for the same reason as the Coast Guard - they are very effective in stopping vehicles.

Leitner-Wise has recently finished a special purchase at AR15.com on the gas-piston and gas-piston conversion sets; but has already been unable to meet the desired delivery schedule. They have experienced several setbacks such as a plant-destroying fire and demand to fufill U.S. Marine interest for evaluation systems. However, this failure combined with the past history has made some nervous but hopeful.

To add to the drama, Leitner-Wise has even more systems in development that promise even more potential (want an AR15 with a continuous rail system that can qucik change any standard AR barrel OR LW-gas piston barrel AND maintain zero? How about a closed/open bolt LMG lower?).

I am on the list for one of the Leitner-Wise systems and it was a cheap investment since at the get go they made it clear they would not take any money from me and they would not release any system before it met their standards. I have to admit that given their past record, I am skeptical; but given the promise of what they claim I am extremely hopeful they will succeed.

[Bartholomew Roberts]

Under battlefield conditions, dirt/dust, mud, water and what all else don't help with the tight tolerances. Like Mr. Kalashnikov said, you can throw a handfull of sand in an AK-47 and it'll still run because the tolerances are so loose, but you can't do that with an M-16.

Tolerances work both ways. The tighter external tolerances of the AR15 serve to keep dirt out of the bolt/lug area of the system. Two different approaches to the same problem - the AK allows loose tolerances to move dirt from the bolt/lug lockup and the AR seeks to prevent it from ever reaching the bolt/lug lockup. However, I'll guarantee you that neither one performs well once a certain amount of dirt is in the system and that the difference isn't all that decisive.

I heard from a Vietnam vet who said just so much as a heavy dew and the M-16's barrel will hold water. If you're coming up out of the swamp, you have to blow all the water out of the barrel and gas cylinder. If the enemy is right there when you come up out of the water, you don't have time. Better to have a AK or M-14 because those systems, for whatever reason, allow the water to drain in a hurry.

First, let me say how much I appreciate your willingness to acknowledge second-hand information from a source you feel is reliable. Too often, we lose sight of the goal of sharing knowledge and don't relate our sources of information accurately enough.

Second, the water drain issue with the M16 has nothing to do with direct impingement gas systems and has to due with the small caliber of the bore and a thing called "Capillary action (http://www.madsci.org/posts/archives/feb98/887637827.Ch.r.html)". The smaller bore of the M16 takes longer to clear of water because the bore itself is smaller and water exits less quickly. The larger bores of the M14 or AK47 are not as affected by this issue.

This ball powder burned slower and had a higher port pressure. The gas would run down the gas tube back to the bolt carrier key faster than it properly should and attempted to extract the spent shell case before the cycle of obturation had completed. The shell case, still stuck to the chamber wall, would not extract therefore, the extractor would many times either jump the case rim, or tear through it.

And this is really the only fault of the direct gas system, it isn't very tolerant in changes of pressure. To be fair though, very few autoloading systems are tolerant of changes in gas pressure, regardless of whether they are direct gas, long-stoke gas piston, or short-stroke gas piston.

[Agrippa]

Don't forget about the ZM products that YHM is selling. The LR300 op-rod upper looks very promising.

http://www.yankeehillmachine.com/store/lr.html

[Fletchette]

And this is really the only fault of the direct gas system, it isn't very tolerant in changes of pressure. To be fair though, very few autoloading systems are tolerant of changes in gas pressure, regardless of whether they are direct gas, long-stoke gas piston, or short-stroke gas piston.

Very interesting conversation. Thanks for all the info.

Since were on the subject of different lock-up mechanisms, has anyone ever made a recoil-operated (like the M2 or Barrett semis) 5.56mm? No gas system at all. If the tolerances are held tight, there is little reduction in accuracy (consider the Barrett). Perhaps roller-locks have pre-empted this evolution.

[owen]

Flechette,

Yes there is a recoil operated battle rifle, the Johnson M1941. If you find one for sale, I will pay $900 for it.

It was a compettor of the M1 Garand, and the Marine Raiders used it in WWII. After the war most of them went to Denmark (I think). One of the main problems is that they didn't work so hot when the bayonet was fixed. One of the nifty features was a magazine that could be topped off with a stripper clip, without opening the bolt, ala Krag.
Johnson at Guns America (http://www.gunsamerica.com/guns/976501427.htm)

[JohnKSa]

DSA makes an AR-15 style rifle with a FAL gas system and piston.

[RGL01]

I have experienced and observed problems with the AR15 due to the direct gas impingement system. IMO the M16 never was and never will be a robust rifle design. Suffice it to say I sold my Armalite M15A2, and will stick to my Polytech M14/S as my first line defense rifle.

[owen]

way to be specific RGL01