6 Facts About AR-15 Gas Impingement Vs. Piston

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Though ARs come in all shapes and sizes, there are primarily two distinct operating systems the platform employs, gas impingement and gas piston. To the untrained eye, they are almost indistinguishable.

Though ARs come in all shapes and sizes, there are primarily two distinct operating systems the platform employs, gas impingement and gas piston. To the untrained eye, they are almost indistinguishable.

Some say the gas impingement operating system is like a person who poops on the same table they eat from. But is that really fair? Here are Richard Mann’s 6 factual observations in the AR-15 gas impingement vs. piston debate.

With the gas impingement system, gas is diverted from the barrel through a tube and back into the upper receiver to operate the action.

With the gas impingement system, gas is diverted from the barrel through a tube and back into the upper receiver to operate the action. Click to enlarge.

As someone who tests and reviews guns for a variety of firearms periodicals, I’ve had the opportunity to test versions of both the gas impingement and piston-driven ARs. Here are my factual discoveries:

1. Piston-driven guns run much cleaner. Fire a 30-round magazine through a piston-driven AR and it will look just as clean afterwards as it did before you fired it.

2. Piston-driven guns run much cooler. You’ll have to shoot about 100 rounds through a piston gun and a gas gun to really feel the difference, but it is there.

3. On average, piston-driven guns are less accurate. This does not mean piston-driven ARs are inaccurate, but, looking over my test records, the most accurate ARs I’ve tested have been those that work with the gas impingement system.

4. On average, piston-driven guns cost more. This observation must be qualified with “it depends.” There are some very expensive gas impingement ARs and some piston-driven ARs that are not all that expensive. However, if you want to purchase the least expensive AR possible, it will be a gas impingement gun.

With the gas piston system, gas is funneled from the barrel to drive a piston that works the action.

With the gas piston system, gas is funneled from the barrel to drive a piston that works the action.

5. If you intend to run a suppressor on your AR, it has been my experience that the gas impingement guns are more suppressor-friendly, especially those with an adjustable gas block that allows you to control the amount of gas directed back through the gas tube.

6. Both piston-driven and gas impingement guns are very reliable. If, by magic, you inserted me in the pages of Bryce Towsley’s book The 14th Reinstated and I had to live through a social and economic collapse where there were roving bands of marauders, and if you told me I had to pick between a gas impingement or a piston-driven AR, I really would not care which one I ended up with. Except for three things: parts for gas impingement ARs are easier to find, much more plentiful, and less expensive.

Now, here’s the good news. The unmatched modularity of the AR allows you to, in a way, have your cake and eat it, too. If you own a gas impingement AR and want to try a piston-driven AR, just purchase a piston-driven upper receiver. Since the gas impingement and piston systems work independently of the lower receiver, you can alternate between both on the same lower receiver.

Gun Digest Shooter's Guide to the AR-15This article is an excerpt from the Gun Digest Shooter’s Guide to the AR-15.

9 thoughts on “6 Facts About AR-15 Gas Impingement Vs. Piston

  1. clamatowas

    I have the LWRC M6-IC I shoot suppressed with my SAKER.

    I have fired over 1k rounds and never cleaned it. When I take it apart and hold the bold it is a small amount of carbon on it, a quick wipe down with my shirt would take care of it, but I don’t even do that.

    I don’t needs parts for my AR because I will shoot out the firing-pin or barrel before anything else fails. In a survival situation when your looking for gun cleaning essentials (that run our faster then my parts ever will) to keep your DI running. Mine will still work.

    The LWRC IC series will shoot under water, injected with sand and mud, and covered in rust. They even blew one up and it still fired after they made a 3 sec fix where they bent the barrel … straight ish again.

    I can also fire 600+++ rounds as fast as I can, pull out the bolt and it will be cool to the touch. This means I don’t heat up the brass and cause it to over expand in the chamber when fired and semi weld it self to the barrel. What is a much to common and catastrophic failure DI guns have Piston do not. I have had this happen to me with a DI gun, and a bud of mine had this happen to him while under attack in a river-bed in Iraq, almost cost him his life.

    Last argument, there is a reason why ALL Special Forces use Piston Guns, like the SCAR and the SEAL custom made H&k.

  2. mstrmstr

    I’ve been shooting a DI gun for over 50 years. It’s a AK variant.
    I also shoot suppressed with a AR piston gun since I hate all that gunk back blowing in my face plus with the turn of the switch on my gasblock I can make it a quieter single shot.
    Each has a specific advantage and conversely a disadvantage but the truth is as long as I have one or the other in hand when needed I’m good.

  3. brotherzoo

    Not to be antagonistic, and I and my family own both systems, but in my observations:

    1. True

    2. True, although I find the difference after as little as one 20 round magazine fired.

    3. In my small collection the opposite is true, which is just as anecdotal as your statement was, and when I have added more than one brand of after-market piton kit to more than one brand of AR I have seen no change in accuracy.

    4. The DI guns that are that cheap are junk, and you can get a reasonably priced GP or DI gun for about the same price now.

    5. I’m curious what you found more friendly about the DI guns? If you do have an adjustable gas block, and you dial it down until it gets to the same level of blow-back as a GP gun, it will not function – by its inherent design specifications…hence the need for products like the Gas Buster charging handle.

    6. No argument there, except that a DI gun will crap out from being dirty and clogged long before either gun needs spare parts, even faster without a chrome lined chamber and bore, which is the first thing you don’t get with any cheap AR. GP guns also don’t need new gas rings, or blow gas tubes when they get “last ditch” hot.

    Anyone with half a brain has their own spare parts for any gun intended for a long-term survival survival situation anyway. Personally, I keep a fully assembled bolt, a couple more firing pins, lower parts kits, pins, springs, etc. in my fore and aft pistol grips (two Ergo grips will hold a good bit of carefully packed parts)…and if the situation is worse than that, the search won’t be for gas rings, or a sear pins. It will be for a functioning gun…or guns.

  4. JC Customs

    Eugene Stoner tried a piston on the AR in the 1960’s. He found that it didn’t work as good as the original design. A DI AR exerts a linear force on the bolt, gas pushes forward and rearward from the chamber inside the bolt carrier. A piston adds a lateral force to the system, pushing on the top of the carrier. This causes the tail of the carrier to tilt down, as the only bearing surfaces are from the centerline forward. HK tried adding material to the bottom rear of the carrier, to fix this. The other force, induced by a piston, is rotational. As the carrier is jammed to the rear, it tries to rotate against the cam pin. The bolt isn’t pushing forward with equal force, like in a DI gun. I’ve seen piston guns shear more bolt lugs, break bolts in half, and eat apart upper receivers. DI guns have their downsides, but I feel that cleaning a rifle isn’t as bad as replacing parts.

  5. jimmyjames

    “5. If you intend to run a suppressor on your AR, it has been my experience that the gas impingement guns are more suppressor-friendly, especially those with an adjustable gas block that allows you to control the amount of gas directed back through the gas tube.”

    Never seen a DI gun with an adjustable gas block. Most every piston gun I have seen or owned has gas valve specifically for this purpose.

    1. bhp0

      I would say time will eventually tell if the new piston driven systems last mechanically as long as the older gas impingement systems. I think too that the gas piston systems are the result of reverse engineering which in the past has proven to be an inferior way to design a firearm. Look at the failures of 9mm frame handguns that were reversed engineered to handle the harder kicking 40 S&W cartridge.

      If I wanted or needed a reliable survival rifle it would be either the AK-47 or the M14 both of which are much more reliable and powerful as opposed to the AR15 and its anemic .223 round. I cannot believe you stated that you would use the gas impingement system AR15 as it is no more reliable today than it was in Viet-Nam. I have plenty of experience shooting large numbers of rounds through this inferior system and it is the last gun on earth I would use if the chips were down.

    2. bhp0

      I would say time will eventually tell if the new piston driven systems last mechanically as long as the older gas impingement systems. I think too that the gas piston systems are the result of reverse engineering which in the past has proven to be an inferior way to design a firearm. Look at the failures of 9mm frame handguns that were reversed engineered to handle the harder kicking 40 S&W cartridge.

      If I wanted or needed a reliable survival rifle it would be either the AK-47 or the M14 both of which are much more reliable and powerful as opposed to the AR15 and its anemic .223 round. I cannot believe you stated that you would use the gas impingement system AR15 as it is no more reliable today than it was in Viet-Nam. I have plenty of experience shooting large numbers of rounds through this inferior system and it is the last gun on earth I would use if the chips were down.

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