How Does It Work: Direct Gas Impingement

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Direct gas impingement is an operating system that has been used only in a few production guns (all of them rifles). It is extremely simple, although perhaps not well suited to adjustability. It is also not the operating system of the AR series of rifles – we will cover those in a separate video next month.


  1. Umm, I have to disagree.

    In the MAS rifles, the gas tube blows back into a blind hole in the bolt carrier.

    In the AR system, it blows back into a short tube machined into the bolt carrier, which is basically a “blind hole” without the surrounding metal.

    As Sir Arthur Conan Doyle is reputed to have said, a difference, which makes no difference, is no difference.

    As for DGI only being used in rifles, look at the workings of the Desert Eagle gas-operated self-loading pistol sometime. Gas block; gas tube; bolt carrier (slide) with gas “piston” fixed at the front; rotating bolt.

    The only substantial difference is that in the pistol the gas tube and etc. is below the barrel rather than above it.



    • Eon, the AR-series is emphatically not direct impingement; that tube you refer to takes the gas and directs it into an expansion chamber behind the gas rings on the bolt inside the carrier, where it serves to assist the unlocking of the bolt itself, not just to drive the carrier rearwards.

      • Sorry Eon but from that point of sight, all gas channels are of blind holes ending with gas pistons… Are all considered as Dl…

      • I am very suspicious about the whole “direct gas impingement” classification.
        Can anyone looks at slide 14 there:
        showing cut-away drawing off 2Х35 and then choose exactly one of following options:
        – it is direct gas impingement
        – it is not direct gas impingement
        and inform me about that?

      • Kirk, since the gas enters the expansion chamber behind the bolt, doesn’t that drive the bolt forward, at least as much as clearance allows? Wouldn’t that resist bolt unlocking?

        • Nearly equals the gas pressure level at the both side as easining the bolt unlocking as light as manual tracking.

        • Blowing the bolt forward unlocks it via the cam pin, because it’s acting against the carrier.

          It’s an ingenious system, and one that Stoner ought to get more credit for. It puts all the energy into a straight-line path from the bore to the buttstock; there’s no tapped-off gas driving back a piston that serves to torque the weapon upwards ever-so-slightly. If you fire an M15/AR15 and then compare it to something like the Valmet M76, you can feel the difference in the recoil impulse when you’re paying attention to it.

          • After reading TinMan’s comment about higher pressure rounds, the bolt moving forward seems designed to increase dwell time thereby decreasing chamber pressure and easing extraction. Of course I could be wrong.

  2. The part which takes power from imparted gas is the no other piece but the mass of bolt carrier. Very good example of the Direct lmpingement.

  3. This is beginning to sound like the case of generic ladders vs. step-ladders. The Scotti Model X and the Nippon Special Steel Kawamura rifles operate the other way, with the gas pushing everything EXCEPT the bolt carrier. I could be wrong.

  4. Just to confuse the issue totally…

    Here’s what W.H.B. Smith and Joseph Smith have to say about direct-gas-impingement systems in Small Arms of the World, 9th ed. (1969);

    Swedish 6.5mm Ljungman Semiautomatic Rifle

    The Ljungman gas system is unusual in that it seeks to avoid the use of an intermediary thrusting piston for its actuation.

    As the weapon is fired and the bullet passes by the gas port about one-third of the distanced from the muzzle, gas is tapped off through a hole into the gas cylinder on top of the barrel in standard fashion. The gas is directed back through the gas cylinder where it impinges upon an extension of the bolt carrier whose forward end passes into a mating receiver cut.

    The gas thus delivers its thrust directly to the face of the bolt carrier itself.

    (The unlocking process is described at length, we’ll skip it here.)

    French Model 1949 and 1949/56 Rifles

    When a cartridge is fired, propellant gas is tapped of through the gas port in the barrel. This gas blows back through the gas tube directly into a hole in the top face of the bolt carrier. The gas tube protrudes a small distance, so that a portion of it actually enters the bolt carrier.

    The bolt carrier moves to the rear and, after a slight “dwell” time, the bolt is cammed up and out of its locked position in the bottom of the receiver, and starts to travel back with the bolt carrier.

    Special Notes on the AR-10, AR-15, M16, and M16A1 Rifles

    The gas system of these weapons is similar to that of the Swedish Ljungman M42 and the French Model 49 and 49/56 rifles, in that no piston rod is used. The gas blows back directly into the bolt (in the other rifles mentioned, it blows back into the bolt carrier).

    The official U.S. Army exploded parts view of the M16 rifle on p. 653 lists the “gas rings” in the bolt head assembly as part # 61540, and calls them “Bolt Ring”, not “gas rings”.

    Since the AR system also has a bolt carrier (assembly #61826), I’d say any difference between the AR system and the Ljungman and MAS systems is minimal, at best. It’s all direct gas impingement, one way or another.

    Incidentally, DGI seems to be the only form of gas operation never even experimented with on a medium or heavy machine gun. Of course, I could be wrong, but none of my references mentions even one example. And the AR-10 LMG variant (7.62 NATO, belt-fed, with quick-change barrels) seems to be the only example of a light MG using the system.



    • Reportedly, the Chinese Type 77, Type 85, and QJZ-89 HMGs are direct impingement (the latter being a hybrid gas-recoil system, but with DI for the gas part).

    • Eon,

      The AR-10 LMG variant (7.62 NATO, belt-fed, with quick-change barrels) is not really an AR-10, it is a complete different weapon system. I have fired every LMG that the US Navy has, and none of them are the same weapon system as the AR-10. Eugene Stoner was quoted as saying: “This invention is a true expanding gas system instead of the conventional impinging gas system.”

  5. I’m with the AR is DGI crowd, but not any kind of gunsmith. The gas piston difference seems to me that it has a piston between the gas tube and the bolt (carrier). Whether it’s a short stroke and whomps on the bold (carrier), or long stroke and stays connected, there is something there that the AR lacks. The AR has more in common with the MAS rifles than any piston systems.

    To say the AR is not DGI because the gas goes into the insides and does something in addition to pushing the bolt (carrier) is a further distinction without a difference. The MAS unlocks the bolt just like all locked breach guns. If it’s not the gas doing that, then it’s magic.

    Or maybe the definition is too narrow. One might consider the long stroke piston as part of the bolt (carrier), since it is definitely not independent like a short stroke piston.

    If someone has a better reason for calling the AR a non-DGI system, something better than quibbling about what work the gas performs, I’d like to hear it. Probably have to wait for the video mentioned.

    And of course, I may have opened my yap just to prove what a fool I am 🙂 But this argument of mine, that the AR is DGI because it lacks an independent piston, is what I have asked before and never got a proper rebuttal on.

    • Felix, there is a piston; it’s just that it’s not out at the gas tap, it’s back in the carrier.

      There are two ways you can tell that the AR-15 gas system is emphatically not direct impingement: One is to clog that carrier key with something like a pipe cleaner, and then try firing it. Gun won’t work worth spit after that, because although the carrier will somewhat cycle with the key blocked, it isn’t putting enough energy into the system to make it function reliably. Second thing you can do is get rid of those bolt seal rings, or line them all up so they don’t seal properly; either way, you’re no longer getting enough energy into the system for the weapon to function reliably. I’ve seen both problems crop up innumerable times as an armorer in the Army, and every time, by clearing the carrier key or replacing/adjusting the rings in the gas system, I’ve been able to return the rifle to reliable functioning.

      I don’t know what the proper nomenclature for the AR-15 series of weapons should be, but direct impingement ain’t it.

  6. Gary Paul Johnston and Thomas B. Nelson, _The World’s Assault Rifles_
    “Where the Johnson’s rotary bolt was operated by short recoil, [Eugene] Stoner’s M8 bolt group operated by direct gas with its barrel remaining fixed. However, while direct gas operation was nothing new, the way Stoner employed it was unique.

    Amounting to a sophisticated gas cylinder, the M8’s bolt carrier received gas through a hole on its left side directly from a narrow gas tube that ran from the receiver to a port in the barrel. In this case, the gas tube was actually a long gas port. As the gas entered a hole in the left side of the carrier, it acted against the rear of the bolt and the inside of the cylinder/carrier much like the piston of an engine … Using the established operating principles as defined by Col. George Chinn, this unique operating systemis perhaps best described as “long stroke piston and cylinder via direct gas.” [!]

  7. R. Blake Stevens and Edward C. Ezell, The Black Rifle M16 retrospective Vol. 1 “… Stoner’s gas system utilized a simple open pipe, a concept first used in the Swedish Ljungman Gevär 42 [sic! France pre-WWI!], and the later French 1944 and 1949 MAS semiautomatic rifles. In these relatively rudimentary applications, the gas piston and spring of a conventional gas-impingement system were replaced by the jet of hot gas itself, which travelled back through the hollow gas tube and impinged directly onto the face of the bolt carrier. The kernel of genius in Stoner’s gas system was that the AR-10’s gas tube, running along the left side of the barrel under the handguard, fed the gas through lined up ports in the receiver and bolt carrier wall into a chamber formed between the tail of the bolt and the surrounding bolt carrier. This forced the bolt carrier back. After about 1/8″ of movement, the port in the carrier no longer lined up with the port in the receiver, and the further flow of gas was cut off. The momentum already imparted was sufficient to keep the bolt carrier moving, which unlocked the bolt by rotating it, due to connecting cam pin …”

  8. Kirk is correct. Direct gas impingement is where the gas simply strikes (impinges on) the bolt carrier, imparting momentum which drives it back.
    The Ljungman is the classic example OTHER than the MAS series that utilizes this method of operation.

    In the case of the AR, gas flows from the tube, through the gas key and INTO the bolt carrier, where it expands behind the tail of the bolt. This makes it a sort of internal-gas-piton-inside-the-bolt-carrier method of operation. If it merely struck the bolt carrier and momentum/inertia pushed the bolt carrier rearwards, THEN it would be direct gas impingement.

  9. Let’s ask the inventor:

    Eugene M. Stoner, Los Angeles,Calif., assignor to Fairchild Engine and Airplane Corporation, Hagerstown, Md., a corporation of Maryland

    This invention is a true expanding gas system instead of the conventional impinging gas system. By utilization of a metered amount of gas from the barrel, the automatic rifle mechanism is less sensitive to different firing pressures caused by variations in the propelling charge. It is therefore still another object of this invention, to provide a rifle mechanism which is not affected by variations in the propelling charge.

  10. There is a huge difference between an ar10/15 and the true gas impingement of the MAS. If blowback/delayed blowback/roller or toggle delayed are distinct operating sydtems, than the ‘expanding gas’ system (stoner’s words) is ceertainly distinct from DGI. Having to manufacture gas rings into the bolt is pretty clear evidence that the systems have differences in operation. In an AR the tapped gas does more than just push the BCG back, and there are additional parts required to satisfy these requirements. An often neglected part of the AR vs DGI distinction is that the forward/backward force of the AR is truly in-line with the barrel, whereas the MAS action is imparting force slightly off of center of the barrel. Stoner, being such a big fan of in-line action, probably saw this as a definate advantage for his system.

  11. Very similar to the Ruger Mini-14, except the bolt carrier on it extends all the way to the gas block on the barrel. The same spigot on the gas block, the same blind hole on the bolt carrier, no adjustments.

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