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ArmaLite Prez Mark Westrom writes in response to the post ArmaLite Prez: “No technical advantage to an external piston system employing current ammunition”

Patrick raises some good questions.

Sounds like someone is behind on development, doesn’t it?

Well, actually the first external piston system was applied to the AR-18, and we continued to use it when we introduced the AR-180B. Actually, the external piston was used for legal, not technical reasons. ArmaLite had sold the Stoner system to Colt in 1959 and couldn’t use it. Today ArmaLite has prioritized a long list of R&D projects including an external piston system for those who want it, but other projects are higher priority. We’ll likely get to it . . .

FYI Mr. Westrom, it is called “direct impingement” because the exhaust gases directly impinge on the carrier bolt.

No, it’s called direct impingement because of an error going back as far as the ’60s in Smith’s Small Arms of the World, in which the AR-15 system was incorrectly compared to the Ljungman system. It is not a simple DI system, but a more sophisticated piston system. The piston, cylinder, and piston rings are in the carrier. It provides better “pneumatic leverage” than a DI system and much less blast in the face. This error has been repeated over the years without much need for correction until lately.

If there were no advantages, particulalry in sandy environment, and is less accurate, then why was the SCAR brought into the military?

SCAR was submitted to SOCOM to an audience that specifically disliked the Stoner system, not because the external piston was proven superior. ArmaLite attended a meeting in Orlando and briefed SOCOM on an AR-180 candidate in response to specific guidance from a friend at SOCOM who stated “The tube’s got to go.” I’ve been struck by the fact that I’ve never heard any SOCOM user praise any technical advantage in the SCAR, and it’s been apparently been pulled from service.

Apparently Mr. Westrom has never even seen a piston-driven system. He should check out the LWRC video where they pull it out of a bucket of water and shoot a full 30-rd mag to empty.

Well, actually we at ArmaLite have both seen many piston systems and done considerable technical evaluation and find that the disadvantages outweigh the advantages unless the relation of energy and cartridge dimensions won’t allow an internal piston that’s large enough. (That doesn’t pertain to most cartridges.)

That water bucket test has been shown by a number of producers, but does it describe a feature really useful, or sensible operating procedure? I think not. Nobody with any sense will assume that a soaked rifle can be fired without draining the barrel, and the M16 is no different than the externally-driven rifles in that regard. Most of the bucket demos you see omit dipping the muzzle into the water. Kinda flashy, but really proves nothing.

I wonder why he didn’t compare how clean a new SCAR or LWRC rifle stays compared to an AR-15?

Because ot really was a topic best handled separately. I’ll summarize it briefly:

1. A cartridge generates the same residue when fired, regardless of what it’s fired in. The crud goes someplace. No system is immune.
2. The issue isn’t which system is “cleanest” despite the white-glove traditions of the Services. The issue is whether the location of the crud causes a mission-ending problem. That is not the case with a properly maintained M16 and many other rifles… an easy task.
3. There is no free ride. The Stoner system vents a bit of powder residue in the upper receiver (virtually all of which is easly removed with a rag). The external pistons gather their own crud but weigh more and are less accurate. Ya takes yer choice.

I spent a career as an Army Ordnance Officer… a rare Armament Maintenance Officer at that. I too was death on the slightest bit of carbon in the mechanism until I noted the damage done to M16s during cleaning. We don’t shoot em to death, we clean em to death.

I ran a little unsanctioned test in Germany with friends in the Polizeisportverein in Frankfurt firing 2000 rounds thru each of two M16A1s that had been normally cleaned and properly lubricated. Each gun fired 2000 rounds with no additional cleaning, no additional lubrication, and NO MALFUNCTIONS. I was frankly surprised.

So I now follow the current Ordnance guidance re the M16 family. “Throw away the white glove.” You need to clean moderately, lube properly, (and use good magazines, the source of most problems with all self-loaders).

My conclusion remains unchanged: Given current ammuntion, whatever advantage an external piston system enjoys in supposed cleaniness doesn’t outweigh the advantages of Stoner’s system. The controversy is more an intuitive or snake-oil (marketer) driven issue.

I’ll have a few changes made to the tech note to clarify some of the questions.

A friend attended a major military conference at which the M4 was criticized by a speaker. A Master Sergeant attending stood and said “I don’t understand where this is coming from. In all my time I’ve never seen a problem. Have any of you?” Interestingly, nobody in the audience had. I can say that in all the thousands of rounds I’ve fired in M16s I never did either, and I never saw a malfunction by any other soldier that wasn’t caused by either user error, bad mags, a damaged cartridge, or normal parts failure, all of which pertain to all firearms.


Mark Westrom

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  1. In combat not gone have time open up m-16 clean keep shooting at you enemy who ak 47 well keep fire way well try keep m-16 clean enough make work. The fireing rang is not thrid world battle field where on fire rang call time out so can clean fix weapon out battle field that can get you killed. Most people support m-16 have been in lots combat with them or have spent time out in field with them where keep weapon clean enough work well be able survie fire fight alot hard do than at fire rang. Most people who keep selling m-16 gas system keep living in denyle that there some other systems out there that are better than m-16 system most world armys are going way from m-16 system useing new better systems. Being person who seen m-16 have clean them at gun rang that worked daily for ten years they where pain in ass keep clean run compare to springfield m-14 or ak 47 even ruger mini 14 rent.

  2. There are a number of different problems with the platform as a whole that tend to get conflated, not the least of which is the 55gr-62gr .22 caliber cartridge. A friend of mine had an AR during the “assault weapons ban” “sporting rifle with three or more of the following features (SRWToMotFF) ban”. We thought they were junk b/c he couldn’t get through a mag without multiple failures. It turned out that the alleged NATO green-tip from Olympic was a big part of the problem. I read that the Olympic was under pressured to run in the L85, don’t know if that’s true but the crap couldn’t even run in a Mini. He almost sold the thing, but gave it one last try with a $60 HK steel mag, lo and behold, the real problem was the craptastic gun-show orlites that he had been buying b/c the SRWToMotFF ban was making it hard to find decent mags. Even the gun-show “green followers” sucked. I found out later that most surplus mags were surplused because the armorers would throw away make available to the surplus market the mags that didn’t work. With Lake City and Magpuls I have almost never had a problem.

    Back to the cartridge. No matter what the guv’mint says, there is a problem with that round. The problem isn’t so much that it is only effective when it is going fast enough (i.e. out of a certain min. length of barrel and within a certain range). The problem is that it can’t defeat cover at any range. If your target is hiding behind a concrete or stone wall, you can destroy his cover with enough rounds of either 7.62 NATO or Russian. With 5.56 that wall will not come down.


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