How Does It Work: Lever Delayed Blowback

Lever-delayed blowback is a relatively uncommon action system, although it is applicable to a wide variety fo firearms and has been successfully used in submachine guns, rifles, and light machine guns. It uses a principle of mechanical disadvantage on a fulcrum lever to force a mass to accelerate rearward while the bolt remains closed. This allows the effective weight of the moving parts to be amplified, resulting in a lighter firearm.


  1. Apart from Hungarian Kiraly 39M and 43M SMG and FAMAS, I only remember two other guns using this system: the French AAT-52 MG and the Cristóbal .30 carbine (also designed by Pal Kiraly)

    • I think the 7.62 Sterling LMG uses a similar system. I am slightly obsessed with this gun as information about it is scarce to say the least.

      • Pete:

        Agreed about the Sterling. I thought it was an assault rifle rather than an LMG, but information is as you say scarce.

        I find lever delays interesting, though a bit hard to get my head around. They seem to work, but only just.

        • It depends on design in geometry, material properties, and production quality. The toggle-delay is just as picky. And then there is the elephant in the room for rifle-caliber weapons: does the design require lubricant on the ammunition?

          • What you pointed out is precisely what designers in most cases want to avoid. That is the reason why we see most commonly used tried and true gas piston solution.

            In relative comparison on level of tricky-ness the gas operated system is hands down the best (if you cannot afford blow-back) followed by short recoil and then follows all the rest.
            In case of FAMAS its designer had lucky hand; it outlasted even HK roller locking.

      • There was a page on here about that, once. It did/was supposed to; operate thus, if I remember correctly… But it appeared to be on its side, maybe it had side mounted mags… Although, I have forgot.

  2. “submachine guns, rifles, and light machine guns”
    French also tested using such principle and firing 12,7×99 NATO cartridge, namely 12.7mm Machine Gun Type M.A.C.
    states that is generally up-scaled A.A. 52 and some examples were made and tested, but
    The 12.7mm guns were put to test a few times, but the model was not adopted because it did not make any particular improvements compared to the same guns in this caliber already in service.

  3. The accelerator-lever trick of course works both ways. Both the Lahti and original (Schwarzlose-type) Auto-Mag pistols have accelerator levers in the frame ahead of the trigger guard, but they act to give the bolt an extra “kick” to open it rather than to retard opening.

    The Lahti accelerator is there to ensure reliable functioning in very low temperatures. The Auto-Mag accelerator, which was basically copied from the Lahti’s, is intended to ensure (more or less) reliable functioning with handloaded ammunition of varying pressures and etc.

    I’m surprised that the new Auto-Mag pistols are still short-recoil. The basic Schwarzlose-type action seems made to order for adaptation to gas operation.



      • According to Jeff Cooper, the Lahti is so massively over-engineered for the 9 x 19mm cartridge that unless you use the kind of hot-loaded SMG ammo the Finns and Swedes were accustomed to, its functioning without the accelerator is marginal. It’s worth noting that as a rule, even with the accelerator, Lahtis do not function reliably with typical U.S. commercial 9 x 19mm, which is loaded significantly lighter.

        As for the Auto-Mag, I suspect the main reason for the accelerator there is to encourage the initial rotation of the bolt in the unlocking sequence to break the cartridge case loose from the chamber walls. Without that, I would expect that case-head separations would be frequent, considering the typical pressures of the various AMP cartridges.



        • Okay, I find myself corrected. If I were insane enough, I’d suggest crafting a lever-delayed pistol or SMG chambered for 7.65×21 Parabellum.

          • Well, Kiraly’s lever-delayed model 39 and 43 SMGs were in 9 x 25 Mauser (basically the same as the later 9mm Winchester Magnum), and the Cristobal carbine was in .30 USC. Compared to those, one in 7.65 x 21 P would be a bit on the sedate side.

            Also, the Swiss MKMO SMG used a similar form of delayed blowback, except it used a separate breechblock instead of a lever to do the “delaying”. It was made in 7.65 x 21 P, 9 x 19 P, 7.63 x 25 Mauser and 9 x 25 Mauser. So caliber-wise, it pretty much covered all the bases;




        • Dear eon & Cherndog:

          Elsewhere on this channel you will have seen Mr. M’s Lahti video, in which he asserts that Finnish pistols cracked in the frame at the accelerator when loaded with full-power ammo, and that Lahti had to actually specify an underloaded 9 x 19 cartridge (925 f/s) to preserve the gun. After reading for years of the Lahti’s legendary reliability, this was a great disappointment for me to hear. I figure, Mr. M’s talked to Finns, he would know. The Swedish Lahtis minus accelerator I have no account of, durability, reliability or other, but it does make Col. Cooper right when he says the original was over-engineered.

          I was also disappointed to find that Lahti seems not to have invented the accelerator lever, as one is part of the design of John Browning’s 1917 machine gun. Perhaps someone preceded JMB.

          I am very glad to have seen this video, it very clearly explains the geometry of lever delay to me. Thank you, Mr. M.

          Now someone explain an inertia lock in words of two syllables or less in English.

          • The Schwarzlose, Lahti L35 and M40, and the first model, Schwarzlose like Automag..

            all used accelerators because of their separate light weight bolts.

            Because of their light weight, the bolts need extra speed in order to have sufficient momentum and energy to reliably complete their cycle.

            The accelerator captures some of the momentum and energy of the recoiling barrel and slide assembly, and transfers it to the little light bolt.

            In toggle locked short recoil systems like the vickers/maxim machineguns, Borchardt and Luger pistols and the myriad of Furrer designs from SMG to aircraft cannons…

            The accelerator is there, hiding in plain sight. An accelerator is implicit in the cam that bends the joint in the toggle.

            Both JM Browning and AJ Lahti, designed short recoil belt fed machineguns that lacked a toggle

            Both used explicit accelerators to both buffer the barrel at the end of its travel, and more importantly to pass it’s momentum and energy to help the bolt and the feed mechanisms to complete their cycles.

            I do love Lahti pistols, but over the years, I’ve come to regard their high reputation in some gun books, as being anomalous and incorrect

            They might look vaguely like a Luger, but they’re much bigger, much heavier, and a lot less comfortable to hold.

            Their weight is in all of the wrong places, and the accelerator housing and the bolt stop at the rear of the grip frame are under designed.

            Because of the light, fast moving bolt, the Lahti pistols have to have a very stiff magazine spring to be able to lift the stack of ammunition up and into position in the short time available. The spring is so stiff that the magazines are painful to load without the loading tool.

            And even with all of that speed, the bolt sometimes doesn’t have enough momentum left at the end of its cycle to lock and to take the slide fully into battery (Same thing happens with lugers as well).

            All of these problems are cured with a correctly executed, Browning one piece slide and breech bolt.

            Because the whole combined mass of the slide and the breech bolt travel the full distance, there’s no need to transfer energy/momentum through an accelerator
            And because of the extra mass, there’s enough momentum to complete the cycle without needing high velocities

            Because the bolt velocity can be lower, there’s no need for painfully strong magazine springs to lift the column of cartridges into position in ultra fast time before the bolt returns.

  4. Sorry, but Newton might have a bone to pick with Ian on this bit: “…the amount of energy required to move this bolt carrier is much greater than it would be if it were acting directly on the bolt {I assume just misstated} [carrier]…”
    Short of some possible increase in friction in the system or (some Blish magic) there can be no increase in “energy required”. There is deceleration caused by “mechanical disadvantage” in the same way that a short, sharp tug on a rope attached to a block&tackle reduces the impulse to a suspended load – the lever converts a shorter distance of travel to a longer one, thus increasing dwell time.
    Think I got that right. I know half that from Gun Jesus himself and the rest from undergraduate physics. So, you can call me anything, so long as you call me malpsandsquibs

    • I thought the video was good. Sure, the physics might be a little rough but for a gun guy it’s pretty good. The way I see it is the bolt and carrier acquire less energy than the impulse on the bolt face suggests. I know energy is conserved, but one has account for the short end of the lever arm acting on the receiver. When the round is fired the case head exerts a force on the bolt face. The bolt begins moving slowly rearward, the carrier faster, and the receiver even more slowly. To a rough approximation the short end of the lever arm and the receiver act as a brake, at least until they clear each other.

      • Agreed that this particular ‘how it works” is good. It got me thinking and got your reply which made me reconsider all of the moving parts involved, which inevitably lead me back to KISS. We can try to round the edges as much as we can but the wheel has been invented.

        • One day Google will tell robot Amazon shelf stackers they can have us as we can’t re-invent the wheel.

          Well we can. And we can kick some robot ass.

  5. On reflection,too much credit to Newton – Leibniz, Galileo…they all understood. Conservation of energy – it’s not just a good idea, it’s the first law.

  6. Amount of momentum taken from a discharged round is certain and unchangeable. Lever delay uses this value as shared by two piece bolt group through a lever initialy fulcrumed on the receiver. Smaller massed member of group closes the rear of breech and bigger mass unit carries the same as inside both propped against the lever as shorter arm for real breechblock and longer for its carrier. When dischaged, recoil force pushes the small mass unit causing to transmit the gained momentum to bigger mass with an acceralating effect through the frame fulcrumed lever, and since the most of gained momentum is wasted to provide speed to bigger mass carrier, part of the momentum remained over the small mass breechbolt provides very slow backward motion resulting desceralated opening for real breechbolt away the breech.

    This approach first explained in a 1906 US patent granted for Pal de Kiraly with his friend and happens nearly most technical auto loading systems of all and can not be described through simple drawings. lt is purely mathematical.

    • Mostly I agree. The pin in the Kiraly gun shown and the trunnions on the FAMAS are what I view as the fulcrums. Very initially the momentum of the reciprocating parts equals the momentum of the bullet and the powder. Wen the reciprocating parts interact with another element, that can change. For instance, the Bond Arms Bullpup pistol has slower slide velocity because it transfers some of that initial momentum into rotating the barrel. (My interpretation of your comment.) The Negev LMG has a slower rate of fire with belts than magazines because the reciprocating parts encounter more resistance with the belt and slow down more. Lever delayed guns end up with a slower terminal velocity of the reciprocating parts because of their encounter with the frame.

      You are correct on how lever delayed blowbacks work, but the pivot is on the bolt.

      • Thanks for your interest. The lever should be on the bolt for the continuation of reciprocal movement. But the lever fulcrum initialy recessed over the frame for realizing required leveration. lt accomplishes its aimed task and clears off the frame for bolt group to cock and eject functions as remaining inside the travelling members for taking its place at returning stage for the next shot. Leveration is accomplished by contact surfaces of both bolt members and lever and clearing off is carried on by rotation of the fixed axis on the bolt. Backward movement of bolt is computed and arranged for syncronized motion. Delay lever initialy rotates rather fixed fulcrum recessed over the receiver but rotation axis carrying the lever on, acts. Without a fixed fulcrum, no positively leverage should be provided. Roller delay system works on the same leverage principle with backwardly and verticaly moving lever carrying axis and again with receiver mounted fulcrum.

        • Ваше здоровье товарищ! It appears to me that you have wisdom you are willing to share – text does poor service. Ian is interested in employing a curator – you should apply…as should the guy who does 3D CAD for C&Rsenal

  7. Lever-delayed actions tend, I think, to be picky about the pressure curve produced by the ammo. Wartime Hungarian 9mm Mauser Export was loaded carefully and produced unusually consistent pressures, quite as good as civvie loads. This was necessary because the Kiraly 39M and 43M SMGs didn’t always function well with anything else. Or so I once read in a long-ago source that I couldn’t find now if you pointed a loaded gun at me. Heh-heh, just kidding.

    • I have often heard that the FAMAS, designed for a 55-grain round akin to M193, does not much tolerate 62-grain projectiles like SS109 precisely because the heavier bullet results in an unfavorable pressure curve. All of this in addition to vigorous extraction and ejection that tends to mutilate brass, hence the French preference for steel-cased 5.56.

  8. In all blowback designs, whether retarded or not, the head of the cartridge case and the bolt start moving back as soon as the bullet starts moving forwards.

    It’s not so much a problem of violent extraction

    As a problem of keeping the cartridge case from separating, or from bursting.

    The shear resistance set up between the body of the cartridge case and the chamber, at the moment when movement starts to occur, is equal to the stress normal to the surfaces X the tangent of the friction angle of the surfaces.

    By using steel cases, it’s possible to have a case with a higher tensile strength to resist separation

    And greater wall stiffness to better resist expansion, and so reduce the normal stress against the chamber

    Fluting the chamber allows some gas into the shear plain and that acts to reduce the effective stress acting normal to the plain.

    Lubricant (which is best avoided in smallarms) acts to keep the two metal surfaces separated from each other. If the lube works as intended, it reduces both the effecive normal stress and the frication angle to close to zero.

  9. Has anyone here heard of the new Radial Delayed Blowback?, it uses a rotating bolt with angled lugs to handle the cartridge pressure. When firing the bolt rotates to accelerate the bolt carrier until the cartridge pressure drops to a safe level to extract.

    On the subject of Radial Delayed Blowback, it looks like the US Army is working on this operation or somthing similar for Hyper Velocity firearms but using a full rotation:

  10. Bet you could run an AR type gas tube through the cocking handles tube “which goes into the bolt carrier” and have an NiTi bar… Horizontally… Anyway I am on about Nitinol again, but there must be something in guns we can but aren’t using it for; surely.

      • I still see potential in “atomic” propellant, via NiTi; in the form of a light gas gun – Cartridge. It changes “atomically” at like 60,000 psi why couldn’t that squish say helium and fire a bullet.

        I thought that was one of my better “random” thoughts personally.

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