Operating Systems 201: Ljungmann vs. Stoner Direct Impingement

In this 201-level post on the devices and mechanisms that automatic firearms use to do their work, we’ll be discussing a distinction between two very similar types of gas systems. These are what’s commonly known as the “Ljungmann-type” gas system (called so after a Swede, even though in reality a Frenchman invented it decades earlier) and “Stoner-type” gas system. Both are forms of what’s called “direct impingement”, a term which has two different meanings, one of which we’ll discuss today

  • Direct gas impingement (often just “DI”): Commonly refers to a gas system whereby propellant gases are bled through a long tube reaching from a gas port, mounted somewhere on the barrel body, all the way back to the breech to meet the moving parts group.

The other use is more broad and only appears in US Army technical documents, so we’ll ignore it for now. Since both Stoner-type and Ljungmann-type gas systems are direct impingement, that means they both use a long gas tube and both bring gas rearward either up to or beyond the uprange end of the barrel. However, there the similarities end. Ljungmann-style DI simply directs the gas from the tube directly against an impinging surface on the moving parts group. Stoner DI is more complicated, as it routes the gas from the tube to a channel in a bolt carrier, where the gas is then allowed to expand between the bolt and bolt carrier. YouTuber Bloke on the Range explains below exactly how that works, what the difference is between the two systems, and what advantages the Stoner system enjoys:

For those who do not want to watch the video, you can see a clear image of this process in an image from Richard Crommet’s patent on the ICE-15 carbine:


You can see how the gas (59) is tapped through the tube (60) into an extension of the bolt carrier (10) called the “carrier key” (15), where it expands inside a toroidal chamber (11) created by the space between the bolt (8) and the bolt carrier. This unlocks the bolt and forces the moving parts group as a whole to the rear.

As Bloke noted above, the Stoner-type system enjoys the advantages of very even forces on the moving parts group, forces that are almost totally in-line with the bore itself. This almost eliminates stress on the rifle’s receiver, which allows the designs like the AR-15 that use Stoner’s system to have very lightweight receivers with no strengthened receiver rails. In contrast, the Ljungmann-type system creates off-axis forces that require tough receiver rails, which increases the weight of the weapon’s receiver.

Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


  • PK

    To me, it’s always seemed as though Stoner looked at a few different systems for operating firearms and thought to himself “but why can’t I use them all?”

    Damned if he didn’t make it work. Direct-impingement… but with a piston in the carrier made of the bolt which the gas works on. It’s always fascinated me, just by how very strange it is. Not exactly intuitive, either.

    • Jwedel1231

      He was a damned genius, for sure.

      • PK

        I’m not sure I’d go that far, but either way he was a whole lot smarter than I’ll ever be! It seems like every time I really look at how the entire system interacts between the parts, the more impressed I am.

    • gunsandrockets

      Yeah, it always struck me as a way of combining a bolt+bolt-carrier group with a White type gas expansion and cut-off piston.

  • The bloke in the video also neglects to mention that the Stoner system also has the benefit of pushing the bolt forward as the gas causes the carrier to move. This gives a slight contribution to breaking bolt lug to barrel extension contact making rotation a little easier and the extractor is not as highly stress in bolt unlocking. Personally I do not consider Stoner’s M16 to be DI at all, just relocation of the piston.

    • Jwedel1231

      Good point about pushing the bolt forward (equal and opposite reactions, after all). Where did you come across that, or did you realize it yourself?

      • PK

        Mongo is heap big medicine when it comes to engineering. I wouldn’t be surprised if he thought of that himself, although (now that he’s pointed it out) it seems straightforward.

        • thanks for the complement PK.

          I believe I figured it out myself when I was drawing a free body diagram and explaining the differences between a piston above the barrel and Stoner’s piston system to a friend. I don’t remember ever reading it anywhere.

    • I don’t think it does, actually, due to residual pressure in the chamber. I am not 100% sure, however, and I don’t know how you’d empirically test it.

      • therealgreenplease

        It acts to move toward equilibirum pressure between the bolt face and the bolt lugs. There is still net positive pressure on the bolt lugs but it’s less than you would have in a traditional piston system.

      • Rations of pressure and surface area. Net force will still be on the lugs but it is partially countered by the force on the piston.

        • Could you write that formula, pretty please?

          • PxA=F
            P= pressure being applied to the area
            A= area the pressure is acting on
            F= force

            Fbe=Force on barrel extension lugs
            Fc= Force from chamber thrust
            Fb= Force from bolt due to gas system, recoil spring all factored with inertia of the carrier mass > this is a fairly complex item to solve
            Fbl= Force bolt lugs

            Before the bolt opens, you have to look at the slice in time (as delta T approaches zero) and your pressure in the chamber and the pressure in the gas piston of the carrier. Since the bolt has not moved the system is static equilibrium at that slice in time. Therefore the forces must balance. Some one could write one hell of a dissertation on the system dynamics at the time of firing alone.

  • FWIW: The Austro-Hungarian inventor Baron Adolf Odkolek von Augezd has what appears to be the first published patents for direct gas impingement. While Bloke cites the US patent filed 1900, it appears to be the same that Odkolek filed in the UK in 1899:



    • PK

      Fascinating! Thanks for the information, much appreciated.

    • There were secret French developments that if I recall correctly preceded that design by a few years, but that’s still very cool!

      • The Rossignol B1 didn’t appear until 1901. I believe his earlier repeating rifle was manually operated.

        Odkolek would have been known in France via his interactions with Hotchkiss.

    • pbla4024

      Odkolek was from Prague, therefore Czech 🙂

      • Alas, Odkolek died nearly two years before the creation of the independent Republic of Czechoslovakia. I believe he would have been identified as a Bohemian-Austrian at the time of his birth.

        • pbla4024

          Czech are in that area since 6th century, not since 1918. He could not have been identified as Austro-Hungarian at birth, because Austro-Hungaria only existed since 1867.
          He would not be Bohemian-Austrian, Bohemia is only one of three parts of Lands of Czech crown together with Moravia and Silesia. The family name is typically Czech, even second part (von Újezd). He spoke German for sure and probably Czech as well. Story is same for Krnka and he is usually identified as Czech.
          So many answers are partially correct and none are fully correct 🙂

  • Strongarm

    Stoner system, as clearly stated in his patent text, is not a direct impingement. Gas expands inside the carrier as ascending the dwell time very much like the M14, and starts pushing the carrier with a dumpening effect creating minimal stress on the locking engagements. This may be best described as “In line piston system”.

    • guest

      hear hear!

      • Strongarm

        US Patent . 2951424.

    • ostiariusalpha

      Stoner didn’t write the technical contents of his patent, or so Nathaniel F. has recently asserted to me. Maybe he was just pulling my leg. ?

      • Strongarm

        Carefully read the patent written the rebly below.

        • ostiariusalpha

          Oh, I’ve read it many, many a time. I keep a physical facsimile of it in a folder with other firearm patents. And I was just ribbing Nathaniel a lil’ bit, you’d have to take it up with him if you disagree with his view on the authorship of the patent.

          • Strongarm

            Never mind. Do not take it serious.

      • What evidence do you have that Stoner’s patent was inventor-drafted? Keep in mind that would be the exception, not the rule.

        • ostiariusalpha

          Final legal formatting of the text is the purview of the patent attorney, they do not select the engineering language of the document; now that, I contend, would be the exceptional case.

          • On the contrary, that is exactly what they do, as they must draft the document itself. Why are you under the impression they do not?

          • ostiariusalpha

            Because the inventor is the one that has to explain what principles and mechanisms his invention uses to the patent attorney in the first place, Nate. It’s really that simple. The attorney isn’t going to mention that such-and-such component seals or leverages or radiates unless the inventor has told him that it does so. The patent attorneys job from there is to properly format this technical information for use as a legal document in case of litigation. That’s all they legally can do, it is not a creative writing course, otherwise they’d be opening themselves up to a lawsuit from their client for misrepresentation.

          • I think you have a very poor understanding of the job patent attorneys actually do. However, this argument is neither relevant to whether Stoner-type gas operation is DI, nor to the indisputable fact that Eugene Stoner did not write the text of US Pt. 2951424, his patent attorney did. I do not have the time nor the acute expertise to easily refute what you said, so I’m going to leave it here.

            But I bet someone else will come along and set you straight. 🙂

          • Indeed. It is highly unusual that an invention report is sufficiently detailed and of sufficient quality to use directly and just touch up and format.

          • Strongarm

            In the three utilary firearm patents I have had, attorneys never wrote anything out of the technical frame I had figured.

          • ostiariusalpha

            ? Balderdash!! The patent attorney is a necessity for EVERY JOT & TITTLE!! There’s no way they’d just let you write it out yourself!

            *Goes and reads the MPEP*

            Oh, wait, never mind. Fooled again by Nate! He is just too convincing for his own good.

          • You from TTAG or something?

          • As someone who can immediately tell the difference between an inventor-drafted patent application and a competent-attorney-drafted application, you’re making me LOL here…..

          • Strongarm

            Do not exagarete the merits of patent attorneys and especially in field of firearms. Most of them do not know even the basics. If you need one, you have to make a very large search and selection and get ready to pay more than you expect and in case of studying starts, be ready to make numerous corrections and a result lower level than to be reached. Most published patents include wrong terms like “ejector” instead of “extractor”.

          • Of course, but did you write the actual patent text or did they?

          • Strongarm

            I do not know what other patentees made, but in mines, all the text excepting claims were same what I wrote and even the drawings were fair copies of my original drafts. Most of the merits of patent attorneys should be included in preparing the claims. However, inventions and innovations should not be in the known classificiations. But it should be a minimal platform embracing the whole and according to my opinion, Stoner’s system should not enter to the description of DI.

          • “That’s all they legally can do, it is not a creative writing course, otherwise they’d be opening themselves up to a lawsuit from their client for misrepresentation.”

            Err, what? That’s not “misrepresentation”. that’s not how it works at all…

    • Stoner’s system meets the qualifications for both the vulgar and US Army definitions of DI. It also is similar in many ways to Ljungmann style DI, and so is therefore worth putting in the same category.

      But you are free to call it whatever you want.

      • Strongarm

        Wish Stoner knew that. These definions seem also overviewed. Wonder if they call Ruger Mini14 in DI or not.

        • I don’t think I have ever seen a definition of DI that Stoner-type gas operation didn’t qualify for, have you?

          Stoner’s patent doesn’t say his invention isn’t DI, by the way. It says it is not the “conventional impinging gas system”. Which is true, it’s different than Ljungmann DI.

  • kyphe

    Most people I know do not class the stoner system as DI, it is quite a clever hybrid system. Ofc many simply think of DI as having no transfer piston.

  • IndyToddrick

    Didn’t cover the unlocking. But I did gain a deeper appreciation for the smoothness of the original AR design.

  • Broz

    I, too, noticed the barrel flex the the IWI advert on Full 30…I watched it closely numerous times just to convince my self that I saw what i saw…there’s an awful LOT of flexing in the Tavor…not that it’s a basd thing necessarily…

  • Jeff

    Great video. I know very little about AR rifles and this made a lot of sense and explained the gas system well.

  • DIR911911 .

    really would have been nice if the guy looked up all his terms before doing the video. he used “piston ” incorrectly pretty much every time he said it. “fixed piston” . . . you mean the receiver? pistons move by definition.

    • No, no they don’t. You can have a fixed piston and a moving cylinder, or a moving cylinder and a fixed piston.

    • No, no they don’t. You can have a fixed piston and a moving cylinder (e.g. AR18, SA80…), or a moving cylinder and a fixed piston (e.g. AK, SIG 55x etc)…

      Try googling the phrase “fixed piston”. you’ll find plenty of engineering examples from pumps, firearms, hydraulics and so on where there’s a fixed piston and a mobile cylinder.

  • wjkuleck

    From Stoner patent # 2951424:

    “This invention is a true expanding gas system instead of the conventional impinging gas system.”

    Every time I read that the Stoner system is “direct impingement,” I feel Gene making another rotation in his grave. Sigh.