Gun ReviewsRifles6 Facts About AR-15 Direct Impingement Vs. Gas Piston

6 Facts About AR-15 Direct Impingement Vs. Gas 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 “poops where it eats”. But is that really fair? Here are Richard Mann's 6 factual observations in the AR-15 direct impingement vs. gas 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.

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Richard A. Mann
Richard A. Mann
A former police officer and serviceman with the U.S. Army, Richard A. Mann is one of the most well-respected firearms authorities writing on the subject today. He serves as contributing editor for several magazines, including NRA’s American Rifleman and Shooting Illustrated, Varmint Hunter, and Combat Handgunner, and is the editor for Gun Digest’s Cartridges of the World 13th Edition.

22 COMMENTS

  1. Hard for me to believe this article when the author doesnt even use the terms correctly. This is simple stuff yet lost on this author. First off, they are BOTH “gas” guns. They are also BOTH “gas impingement” as gas impinges on something in either case. Correct terminology would be gas piston and DIRECT gas impingement. Hard for me to follow what the hell this guy is talking about.

  2. Bah, I’d like to see that tested elsewhere on the accuracy question: the gas tube – or piston – has absolutely nothing to do with the accuracy/tolerances/construction of the barrel or the characteristics of the powder or round. May as well say an upper with a fixed front sight is less accurate than the one with the fold-down front site. Reliability with gas-tube guns has been brought into question enough that SOF has purchased piston driven guns.

    As to the others, pistons do run cooler and cleaner. On higher end rifles, they usually have an adjustment you can set for regular or suppressed fire.

    • Barrel harmonics how do they work? That forward mounted piston effect barrel movement a whole lot more than a gas block and tube does.

      • How do you know it’s not affecting it in a good way?
        Do you know anything about the vibrations of a barrel tuned for positive compensation or are you regurgitating information by one of Eugene’s disciples?

    • All true and accurate information Wraith67. I will add some opinionated comentary.

      There is business and there is marketing. They brainwash many of you into thinking a DI’s more accurate. (Yeah, DI, I don’t care what the patent says. There are Piston Operated AR-15s and DI Operated AR-15s for the balance of this writing. I often find Stoner’s disciples regurgitating irrelevant minutia in an attempt to disqualify a poignant overarching relevant point. Their specious responses are nothing shy of worthless.)

      Bottom line — Supposedly, the AR-15 is a battle/service rifle and not the Hubble Telescope. The criticality placed on tolerances is nothing shy of ridiculous in so many ways.

      AR-15s are cheap enough to remedy any failed part relatively inexpensively. Unless, you were stupid enough to by a SOLGW or other high-end DI AR-15.

      I ask, what’s your expectation of a DI rifle? What’s the expectation of any firearm? To fire when you pull the trigger, (reliability), and a reasonable expectation of accuracy given a myriad of variables that the specific firearm comprises.

      For a lot of chaps, the AR-15 is a luv affair with cleaning, tweaking, accessorizing, and attempting to accurize an inadequate implement ill-suited to perform that task. All of this with an intermediate cartridge fired from a 16” or less barrel with about the best ballistic coefficient being approximately .274 (G1). What do you want to do with your AR-15 make luv to it or shoot it?

      Every time I hear, “Chad says…”, I think of the following.

      “It works fine if you clean it! Answer: A service rifle should still work fine even when you don’t have the time to clean it. Like when people are shooting at you. If it gets too muddy you should be able to open the action, piss into it to rinse the mud chunks out of it, and be back in the fight.”
      — Excerpted from the Mad Ogre

      So really, if you’re wrapped tightly, all you need is one big No-Go gauge when it comes to a DI operated firearm. If no one’s ever told you before, let me be the first…unless you’re a sponsored AR competitor, you’ve wasted your money on DI rifle.

      Trading arguments like weight and accuracy against reliability is inane. Weight — From chaps that hang everything but a clock on their AR. Accuracy, how much accuracy — a 32nd of a MOA from guys who shoot no more than 100 meters with their beloved, decked-out, DI AR-15?
      Did you know Colt makes the Advanced Piston Carbine (ACP) for Law Enforcement? I wonder why?

      Face it, you don’t have a piston AR-15 because you’re either too cheap, brainwashed, or both in combination.

      The POF piston operated AR-15 used in full auto mode for most of Iraqveteran8888’s test, fired approximately 2,900 rounds. That’s 2/3rds more rounds than the DI’s. I have NOT seen all the melt down testing so, correct me if I’m wrong. Now, guess who purchased a POF 415 edge? Yes, the writer of this diatribe.

      Did you ever wonder why googling AR-15 cleaning tools, selecting images, and BINGO, a myriad of tools is presented before your eyes? Mind you, this is supposedly a self-defense, modern sporting rifle, battle/service rifle — disciples, you may take your pick. And, look at all the marketing involved in solvents and lubricants.

      To my knowledge, the last DI Rifle to be manufactured besides Stoner’s was the French MAS-49 rifle and that was manufactured 70+ years ago. So, two in the last 70+ years. What does that tell you? Be honest with yourself. And face it, if DI rifles were the cat’s meow, the Germans, Belgians, or Finns would have manufactured one by now.

      In my opinion, if you had any sense at all. You’d sell all your DI rifles and buy at least one good piston operated AR-15 platform. Throw 90% of your specialized cleaning tools away and cut that loss or keep ‘em around to remind yourself to have an open mind and think for yourself. Don’t believe the nonsense these so-called experts are feeding you. You’re simply marks for their profits.

      • Well said. I have found that the best thing you can do to accurize your AR is to dump the milspec trigger and install a drop in trigger with a 4lb pull. Milspec triggers are gritty feeling that will not break at the same pull weight every time the trigger is pulled.

        BTW I call ARs Barbie Dolls for Big Boys. Guys buy them and dress them up and go out and play with them. Ironically these are the same people who complain about how heavy their 7 lb rifle feels after they add all of the add ons.

  3. I did alot of searching before I decided on what AR to buy.I didn’t know at the time there were 2 different variants of AR rifles as far as how they operate.After handling about 50 different AR rifles I purchased the ruger sr556e.I did so because of the outstanding quality of the gun.I found with just about every other AR I handled that they all had parts that were loose whether it was triggers guards etc.The Ruger didn’t have one piece that had any movement at all.It was heavy which I personally like for less recoil.And after having it for 5 years and about 5000 rounds later I have never had a jam or had to replace anything on it.And I have cleaned it maybe a dozen times.The accuracy he talks about is a crock of crap.I can hit any target out to 500 yards and below 200 within an inch.And cosy wise I spent 1100 on the gun which to me isn’t alot fo a great AR platform.

  4. 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.

    • Dude, I’m glad you like your rifle, but you are really full of shit with your comments regarding 600 rounds as fast as you can and having a bolt that is cool to the touch. Seriously, no one needs to read or hear such stupid crap.

  5. 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.

  6. 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.

  7. 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.

  8. “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.

    • 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.

      • AK is certainly a good anti-personnel gun but you’re Viet-Nam rhetoric must reference the old VN-era 55 gr ammo

        My 5.56 running the same 62gr tactical ammo the FBI uses works very effectively while permitting carrying much more ammo per kilogram than the AK

    • 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.

    • “Never seen a DI gun with an adjustable gas block. ”

      I have. In fact, I have one. It’s called a Larue Tactical OBR.

    • Seriously?? Never seen one? Do you ever check the parts sellers on the internet? There are probably more than 20 manufacturers of adjustable DI gas blocks. Check Midway, or Brownell’s, Seekins Precision, Superlative Arms, JP Adjustable, and just about ANYBODY that makes any type of gas block makes adjustable ones, and each in a half dozen styles and types. Primary Arms sells 19 different types. You need to get out more, or get a pizza and surf the web for gas blocks. Jeez.

    • Hmm – I built both a 5.56 and 300 Blackout with DI adjustable gas blocks – both function perfectly – I also used a side-charging upper on the 300 with no fouling or blowback when suppressed and a T-changing handle with special venting that insures no undue blowback when suppressed on the 5.56 (even using the binary trigger) – both very easy to tune with the adjustable gas blocks

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