AR-15 Gas Piston System

[SonOfRoost]

I'm building an AR-15, and I'm considering putting a short stroke piston system in it. I don't have the upper receiver yet, or the stock, I do however have a free float carbine length quad rail for the forestock. Would someone please explain all of the ins and outs of the gas piston system for me, and explain what I'll need to do in order to make it all function properly? Thank you in advance.

 

[BigMag]

Did you say gas piston?

 

[Justin]

Most people buy piston guns for reasons that strike me as being 90% emotional and 10% practical.

 

[steven58]

If you want a piston AR as a novelty item and or a work of art I say damn the critics and just go for it! whichever one you choose will probably work fine.

However, if you are laboring under the mistaken belief that piston guns offer significant advantages over direct gas impingement ARs that are not offset by their own design limitations then i respectfully submit that you need to do more research.

 

[MCMXI]

Most people buy piston guns for reasons that strike me as being 90% emotional and 10% practical.

It occurs to me that the anti-gun lobby could use that argument about any firearm purchase although they probably wouldn't suggest that a firearm could even be 10% practical.

 

[Stack]

I never had a piston AR but I have a pant full of DI Ar's. The main reason I don't is because the best AR parts manufacturers and armorers I know feel as though the systems create more problems than they remedy. A properly set up AR is expensive enough for me already. I love the parts interchangeability that a gas gun offers too. Just my opinion on the matter. That said, if you can't live without one I suggest buying a complete upper from a seasoned outfit such as perhaps Adams Arms. Rainier is a good dealer to consider.

 

[taliv]

from a completely theoretical standpoint, piston systems are adding moving parts and adding weight to the front of the gun. plus, you get a little of the "carrier tilt" issue which can wear your upper receiver. in exchange for this, the action will stay a little cleaner and a little cooler.

the facts seem to be that you need to properly lube your rifle regardless, and if you do that, it will continue to run no matter how dirty it gets. i.e. the sludge from carbon fouling will not cause a stoppage. (this is repeatedly demonstrated at high-round-count carbine classes)

so its a trade off. a little cleaner and cooler and a little heavier with more parts to potentially break and carrier issues.

the deal breaker for me is the availability of those custom parts (since there is no standard for gas piston on an AR). usually made by tiny companies, may not be in business tomorrow... i'm not saying don't get it. just buy a spare if the gun is important to you

 

[Stack]

from a completely theoretical standpoint, piston systems are adding moving parts and adding weight to the front of the gun. plus, you get a little of the "carrier tilt" issue which can wear your upper receiver. in exchange for this, the action will stay a little cleaner and a little cooler.

the facts seem to be that you need to properly lube your rifle regardless, and if you do that, it will continue to run no matter how dirty it gets. i.e. the sludge from carbon fouling will not cause a stoppage. (this is repeatedly demonstrated at high-round-count carbine classes)

so its a trade off. a little cleaner and cooler and a little heavier with more parts to potentially break and carrier issues.

the deal breaker for me is the availability of those custom parts (since there is no standard for gas piston on an AR). usually made by tiny companies, may not be in business tomorrow... i'm not saying don't get it. just buy a spare if the gun is important to you

Yeah, What Taliv said.

 

[MCMXI]

in exchange for this, the action will stay a little cleaner and a little cooler.

The action will stay a lot cleaner and quite a bit cooler. I've shot over 1,000 rounds through my P415 and P308 without any cleaning at all and the BCGs are spotless. The only parts that get fouled is the gas plug, piston and first third of the operating rod closest to the gas port. I'm not claiming that this is important, merely convenient.

I will say that reliability is my number one concern and I will agree that a properly built DGI AR should be more than reliable enough for just about anyone under most conditions. I will say that in very dusty environments, a weapon that requires no lubrication to function reliably is advantageous. I was involved in a program with the M249 where the objective was to coat the internals so that no lubrication was required.

 

[Voltia]

I won't buy a piston AR since the entire rifle is set up for DI. It's not a question of engineering of what system is better, it's simply what the entire platform is designed around. If the AR were piston driven, the recoil spring would be elsewhere and the upper end of the gun would be entirely different.

While I'll reserve my professional engineering judgment on DI vs piston systems for another time, I will comment that the worst engineering comes from someone trying to "improve" someone else's design. As Colonel Cooper, I believe, said about double action semi-autos, piston ARs are a "perfect solution to a non-existent problem."

 

[MistWolf]

The standard AR already has a piston. It is part of and inline with the bolt to eliminate offset recoil forces to reduce muzzle climb. Eugene Stoner in his original patent states his gas system is not a conventional direct impingement system.

PS- True DI systems still have a piston. On the Ljungman it's part of the carrier

 

[taliv]

The action will stay a lot cleaner and quite a bit cooler. I've shot over 1,000 rounds through my P415 and P308 without any cleaning at all and the BCGs are spotless. The only parts that get fouled is the gas plug, piston and first third of the operating rod closest to the gas port. I'm not claiming that this is important, merely convenient.

like i said, it's a trade off. i'm not interested in debating exactly how much cleaner or cooler, but it doesn't run clean enough or cool enough to change any TTPs. so what difference does it make?

 

[ApacheCoTodd]

Cool, more moving parts and another set of critical dimensions. Is it at least adjustable?

Guess I presented my opinion as downs and ups and chose to ignore the relative non-starter of cleanliness or heat transfer.

 

[SonOfRoost]

Well, I was asking about ups and downs, most of you are giving me negative opinions with little reasoning. I see a lot of articles and hoopla about how much better gas piston ARs are, and I thought I would ask around on my favorite gun forum. I've never fired a short stroke AR before, and I've never seen a reliability test of a DI AR, but I have fired dozens of them. However, I've seen countless reliability tests of the piston system, if someone could direct me to a side by side test, that would be nice. I've been of the opinion that DI is just as reliable, but I never see a test about it, and I'm always reading articles about superiority in piston systems. I'm not saying it's better, I'm asking whether it's worth my time to look at them, I've been thinking DI is the way to go since I started the project, but all of the things I've seen and read lately are hyping pistons. Someone lay me out a list of what each system does, a list that I can go verify somewhere because a lot of this seems very biased, not all of it, just most of it. I've always liked the DI ARs, but I'm not beyond looking at other options. Convince me.

 

[steven58]

I don't know about head to head testing that is fair to both sides. Hopefully someone will be along soon with a few links.

you know that old bit of advice of not buying a car model in its first year of production? Well the standard AR has been in use since the early 60s and by now all of its issues are known quantities. We know when to replace each part. We know what wears and how. What parts break and how. This is super valuable real life data based on hundreds of thousands of rifles in use all over the world.

Pat Rogers has had several articles on his loaner ARs that are used in his carbine courses. He has several at over 25000 rounds and one that is at 42000+ rounds with no cleaning just lube. http://www.ar15.com/archive/topic.html?b=3&f=118&t=526041

There are numerous articles flogging piston guns for a reason. They are the flavor of the week and the companies that make them want to sell them. Gun magazines love to write about the latest new thing. It helps them sell magazines and advertising.

I bought a BCM middie and a POF piston rifle at about the same time. shot side by side both are very nice. Both feel different from each other with the BCM being slightly smoother and the POF slightly more accurate.

However at 1700 rounds or so on both rifles, the POF, due to carrier tilt wear at the mouth of the receiver extension, suddenly allowed the buffer retainer to come loose tying up the gun. The BCM at the same round count showed the EXPECTED slight wear on all internal mating surfaces.

The POF now has a special extended receiver extension and a special ball bearing roller cam pin (yes, MORE moving parts) that should, in theory prevent this in the future. Will it? Will it cause other problems? What problems might crop up? who knows, its an experimental system at this point. Come see me in 40 years when it's as grown up as the standard AR.

When POF, or any of the piston AR producers go out of business getting spares for that proprietary gas system will be no fun!
Best quality parts for standard ARs are ubiquitous and the variety endless.

If you want to go high tech, and build a next generation AR you don't put in a piston. Build your AR with the latest coatings on the bolt and bolt carrier, use a fluted nitrated hammer forged barrel instead of chrome lined, try a monolithic upper and throw a Geissele trigger in it.

 

[kozak6]

The AR gas piston is a brand new fad. There isn't a singular, well established AR piston conversion. It's a Cambrian explosion of completely different, proprietary and basically experimental designs, all shoehorned into an action that wasn't designed with their use in mind. Some are designed by beancounters to maximize parts compatibility at the expense of wear and function, and others are a little better thought out.

There isn't a lot of side by side testing out there.

The general consensus is that a well built DI AR is probably going to be good enough.

 

[JustinJ]

One of the first challenges you may face is piston systems generally only work with a limited number of hand guards. If you plan on doing an at home build there aren't a whole lot of options. Of the top of my head i can think of Adams Arms and Osprey. The Adams Arms includes an adjustable gas block while the Osprey uses any standard gas block. The Osprey can be used with a standard adjutable gas blocks though. Both replace the carrier. Adams Arms generally has the best reputation and many AR manufacturers either use them directly or have copied their design. One piston AR i really like though is the long stroke PWS as the only thing it has done is replace the gas key with a long stroke piston rod which seems like it would be far less prone to breakage than most other designs. The PWS is however not available in a conversion kit although i think it used to be.

As others have said the piston adds additional parts which are heavier and can break but the same can be said of just about every other military pattern rifle. My biggest concern with AR piston systems is the carrier channel. Piston guns have rails for the carrier to travel on and without them you get the AR carrier tilt. Supposedly this has been resolved by reconfiguring the carrier back end but i couldn't say how true this is. I've heard claims though that some designs have shown no signs of it after thousands fo rounds. The fact is no other succesful or future military pattern rifle uses the DI system but said rifles are also designed to have select fire. Ultimately i've come to the conlusion that DI is not a good thing for short barrel rifles when used at high rates of fire. The temperatures inside the carrier get insanely hot and i suspect will bake the carbon/lube sludge into a very hard substance that will be adhered to the bolt and carrier. A rifle lenght gas system is probably much less prone to this problem as gasses cool very rapidly and there is more room for gas expansion. However, since we are almost certainly talking about semi auto only don't think a piston system offers any real advantage unless you plan on running a silencer. I suppose even in semi auto a piston will reduce the speed at which lube burns off but again in semi auto it shouldn't be that fast to begin with.

 

[buckhorn_cortez]

I think it's really funny how the DI people get all worked up to convince everyone within shouting range that "DI Good," "Piston BAD." In fact, Armalite worked on an "upgrade" to the original AR design - the AR18 - a short stroke piston powered rifle.

SIG makes piston rifles, FN makes piston rifles, HK makes piston rifles, the M1 is a piston rifle, the M14 is a piston rifle - I think piston rifles have been proven to work.

The idea that there's more parts to break is like arguing that you should only buy 4 cylinder automobiles because cars with more cylinders have more parts to break. There is NO proof that anyone can give in piston rifle longevity other than anecodatal stories which don't count as real data.

I own BOTH types of rifles and can only say they're different, and one is not "better" than the other despite all of the protestations and "proofs" given by the DI disciples.

Whether they'll admit it or not, the piston does run cleaner - a lot cleaner. If that's important or not is up to you and not someone else quoting "statistics" about running xxx,xxx,xxx,xxx number of rounds through their rifle with "no problem." Or, a friend of a friend's first cousin's best friend's uncle who said he heard about a guy who went to a carbine class and met someone who had run yadda....yadddaaaa...yadddaaaa....

Here's what I know from owning two different types of guns.

1. Weight: My piston gun IS slightly heavier than my DI - by 6 oz. The weight difference is dependent on how you have the gun configured as well. Top rail only, full length quad-rail system, type of stock, barrel profile, etc. The point is - there's more to weight than "Piston Heavier."

Given each gun being configured exactly the same - a piston system will be marginally heavier. If carrying six ounces more is a big deal - as we say in bicycling, "Want to eliminate 6 ounces from your bike?" "Lose weight."

2. Carrier tilt: in a well designed piston rifle, carrier tilt is a non-issue. This includes AR style rifles. The Barrett REC 7 does NOT have carrier tilt because the bolt + carrier are designed to eliminate it.

3. Proprietary parts: YES, piston systems have proprietary parts. The idea that "I can interchange parts in my rifle with any other DI" is nice, but, is it necessary? That's something only you can answer. I can easily interchange parts in Glocks and even 1911's - but, I've never done it, never had to do it, and never wanted to do it. So, I'm not sure what type of point this is making.

If you're into building rifles (and I'm not) - that may be a germaine to the discussion, othewise it's just another specious talking point masquerading as "important data."

In a way, that's like saying "I'm never buying and FN SCAR because you can't interchange parts with an AR, HK, or SIG." Whatever. If parts interchange is a make or break buying point for you fine - it's not for me.

I'm probably NEVER going to shoot a rifle enough to break anything on a quality product anyway.

4. Your Quad Rail: now there's a real problem overlooked by the DI proponents as they hop up and down attempting to stomp piston rifles into a goo spot. Your quad rail will probably not fit on a piston upper, so you might want to check that prior to buying any type of piston conversion system or piston upper. I know on the REC 7, the only quad rail system that fits is the Daniel Defense Omega X-Rail specifically made for a piston rifle.

If using your quad rail is important, buy a DI, you can expect it to fit that.

There, now all of the DI aficiandos should feel better because, in the end, I had to cave in and recommend a DI upper. Yes, I did, but not for any of the "reasons" given previously, only on a pragmatic choice basis of matching what is already owned - the quad rail.

 

[ATLDave]

Since so many people are dumping on piston systems, I would only add this as an under-discussed factor: Piston guns are much easier and faster to clean after shooting. Because the piston keeps the hot carbon fouling from "plating" the bolt and carrier, everything in the receiver can just be wiped clean. Even if all that black, crusty gunk in a DI gun never causes a failure, it takes time to scrape it off. In a piston gun, that stuff never gets into the action and only build up, if at all, on the piston head or cup - which is far more tolerant of it, less complex in shape, and therefore can be cleaned faster and less frequently.

If you have infinite free time or just really dig scraping around in your gun with a dental pick, then this is a non-factor. If you have a demanding job, kids, other hobbies, and/or other demands on your time, cutting cleaning time in half or better may be a meaningful difference.

 

[451 Detonics]

The AR works fine without the gas piston...if it ain't broke don't fix it applies here. Biggest problem is every gas piston gun made uses a railed bolt to keep the bolt from tilting from the off-center strike of the piston. The AR does not use a railed bolt so that off center hit will always be an issue. John Noveske is rather well thought of when it comes to AR...his thoughts..

via email from John Noveske: "Also, we should mention the poor choice of platform for the piston conversion on a round receiver bore as found on the M16/M4 system. All other piston type systems out there utilize a railed receiverdesign, like the M14, AK-47, M249, FAL and so on. The round receiver bore design used on the M4 is only acceptable for the standard op system. The carrier and bolt expand on axis with the bore under the normal gas impingement cycle, but on a pistongun , you run into off center impulse issues with carrier tilt and incorrectly designed carrier contact points. Some designs attempt to address the carrier tilt problem with over sized carrier tails and rollers. I do not believe the receiver extension should be used in this manor. I know many people are very happy with their piston weapons. This is not meant as a knock on the piston conversion systems out there, but as a philosophical dialogue focused the new physiological relationships applied to the M16/M4 platform through the introduction of an operating system which has traditionally been applied to receivers with rails for the bolt and/or carrier. I would rather see an entirely new weapon system designed for the piston from the ground up. I believe there several outfits currently working on this."

Another claim is they run cooler...perhaps in a full auto during extended firing...for a semi auto you would never notice.

Cleaner? nope...instead of cleaning the bolt you now have to clean a piston. To do so requires additional disassembly and the chance, especially in the field, of a part getting lost or stepped on and bent. You want an easy to clean bolt have Robar apply their NP3 finish...carbon will wipe off with a soft rag.

You might want to read this before proceeding with a piston upper...

http://www.defensereview.com/novesk...ecce-carbine-john-noveske-interview-part-one/

 

[Cesiumsponge]

Consider there are already many mediocre DI AR manufacturers that can't even take the time to stake their gas keys or ensure their rifles aren't overgassed. This is on an established, standardized platform. I have no reason to believe a dozen small piston companies with propriety designs of their own shoehorned into the AR platform each "got it right". The statistical sampling is too small to establish which of these designs is better than the next. I'd give it 5 years for the free market to pick a winner. Maybe by then the piston shoehorn trend will be dead and the industry will be pushing roller delayed blowback gas system retrofits for the AR.

When the proposed benefits of less fouling are touted, I point that the overwhelming majority of people don't shoot their guns enough to even plug up a DI rifle. There also isn't a realistic scenario where you'll ever go 10,000 rounds without lube or a scrub, even if you're a secret squirrel. Easier cleaning is not really a priority on a working gun. You don't have to pick out every last patch of carbon to get any rifle functioning.

To be perfectly frank, I'd buy a platform designed around a piston system if I wanted a piston gun. And I do want one. There are plenty of awesome piston guns. I just don't see a need to retrofit into the AR platform.

 

[steven58]

Rifles designed to have the piston outside of the receiver such as the M1, M14 et al have steel rails for the bolt or bolt carrier to move in. These rails are necessary as the operating rod that connects piston to bolt must be offset slightly to bring the impulse from parallel with the chamber to inline with it. Rifles with this system are among the finest battle rifles in history. They work fine as they were designed to.

Stoner's design places the piston in the bolt carrier directly in line with the chamber. One of the advantages of this is you don't need tough steel rails for the bolt carrier to operate in as the operating forces are perfectly in line with the operating parts.

The AR system has no rails because it does not need them. Unless of course you decide to move the piston from where it's designed to be and put it out front parallel to the barrel. Now you just introduced a bunch of forces that the rifle was not only not designed to handle but was designed to avoid.

Now you have to start including "anti tilt" carriers, special carrier keys and other assorted widgets in an attempt to reverse engenier an inline bolt system into an offset bolt system.

My advice is if you want an "external" bolt system then buy one of the many modern or classic offerings that were designed from the ground up to be such. Don't spend a ton of money on a "pistonized" AR as it is not an improvement of the system.

 

[MCMXI]

like i said, it's a trade off. i'm not interested in debating exactly how much cleaner or cooler, but it doesn't run clean enough or cool enough to change any TTPs. so what difference does it make?

Consider that some piston AR manufacturers have been trying to develop a system of which the piston is just one part. It's a no-brainer that reducing temperatures in a dry system is more important than in a wet system. Picatinny Arsenal has been trying to develop lubricious, corrosion resistant coatings for the M249 (and other weapons) for a reason, namely that lubricants and fine dust don't play well together. The thing that irks me most about these GP vs DI discussions is that the naysayers will try to make the argument that gas piston ARs are merely a marketing ploy or a "solution to a non-existent problem" but in actuality, they're a response to a real-world issue. Just because you haven't experienced the problem first hand doesn't mean that the problem is non-existent. The current attempts at solving this issue may not be the most elegant but engineering always has been and always will be an iterative process.

 

[Tirod]

Like any AR15, choose a piston maker with some care. If you want it, no sense getting hurt.

Some say they run cooler and cleaner - what actually happens is the heat and gas is moved to a different area. Total residue and BTU's expended are identical, after all, it's the same cartridge and ammo in either. Swap a DI to Piston and it's the same physics going on in the cartridge case, and the same amount of gas residue.

All you did was move the piston from the carrier to the barrel. What happens - as tested by more than one owner - is that the bolt and carrier run about 80 degrees cooler, but the barrel will be about 400 degrees hotter at the piston during extended fire. Not my figures, others used infra red temperature guns to detect it.

You will likely need an aluminum handguard to run it extensively. Include that cost in the overall project.

Another consideration is that since it's "cleaner," it doesn't need to be cleaned as often. That means the gun gets less maintenance after shooting, and that means if a part is broken, you don't know it. If it's a duty gun and you need it to be 100% reliable, then it's required that YOU do your part, tear it down after each use, wipe it down, inspect ALL the critical moving parts. If you don't, then it's not the gun or designers fault you failed - because they absolutely built it to a price, and that means you don't get perpetual indestructibility. That doesn't even happen with DI guns of the Top Tier. Parts wear out regardless.

Acton springs, extractors, extractor springs, bolts, and firing pins are identical pretty much regardless of where you built the piston. They all fail after thousands of rounds, it's part of the built to a price design. Not inspecting the gun after each time you shoot it just leads to not discovering the failure.

Don't buy into the concept that you "don't need to clean a piston gun," and all will go right. Accept that, and you accept failure when you don't want it.

 

[taliv]

well, when they develop a coating lubricious enough that you don't need lube, let me know.
and when some piston "AR" mfg succeeds in developing a system, let me know.

just don't try to pass off the current generations of miracle coatings and piston retro-fits as being elegant solutions to that problem.