How Does it Work: Gas-Delayed Blowback

Gas-delayed blowback is a relatively uncommon operating system used in handguns. It is not an efficient mechanism for high-pressure rifle power cartridges, but works well with something like 9mm Parabellum. It tends to provide benefits of light felt recoil and better-than-average accuracy, in exchange for overheating much more quickly than other systems.

20 Comments

  1. This system is one reason the H&K P7 will function reliably with such a wide range of loads. Just don’t go over 124 grain bullet weight. The gun will shoot those fine, but heavier bullets mess up the timing of the slide opening and will accelerate wear.

    • The Steyr GB-80 and its less-than-successful clone, the Rogak/LES P-18, both worked on the gas-delayed blowback system.

      They operated on fundamentally the same system as the German VG1-5 “Volkssturmkabiner” of 1945; a gas trap “cup” near the muzzle with gas bleed holes in the barrel, allowing firing gases to “push” forward on the cup (attached to the slide) until pressure had dropped sufficiently to allow the slide to open safely.

      The pistols had a single recoil spring surrounding the barrel, much like many straight-blowback pistols such as the Browning M1910 series and the Walther PP/PPK.

      As a Gun Digest writer reported in 1976, the Steyr firing hot-loaded military SMG “carbine” type 9 x 19mm loads had significantly less felt recoil with them than a comparable weight locked breech pistol such as the MAB PA-15 would. The higher gas pressure delayed the cycling of the action, which is literally what “gas delayed blowback” means.

      Note that the Desert Eagle automatic is a gas-operated, locked-breech design with a rifle-type rotating bolt, not a gas-delayed blowback. Its operating cycle closely resembles that of the Swedish Ljungman AG-42 rifle and its Egyptian copy, the Hakim.

      The Wildey automatic pistol is also a locked-breech, gas-operated system using a three-lug rotating bolt, operated by an annular gas collet surrounding the barrel. The closest approximation to its action in a rifle would be the Bang system, also used in a modified form (with an actual linear piston) on the German Mauser G41 rifle of World War 2.

      cheers

      eon

      • With slight difference that in steyr and hk barrel port is in the middle and/or near the chamber, so “push forward” is not extremely evident.
        Also, does steyr have fluted chamber ?

        • Actually, the GB-80/P-18 had the gas ports near the muzzle, like the VG1-5, so that maximum gas pressure would impinge on the “cup”, which was attached to the front end of the slide by a bayonet lock, again like the VG1-5.

          Among other things, since the recoil spring surrounded the barrel, using it as a guide rod, this prevented hot gas from hitting the spring and over time drawing its temper.

          Never having taken a GB-80 apart, I don’t know if it had a fluted chamber or not. I would guess “not”, since the whole point of he system was to delay breech opening, not accelerate it, which the HK style fluted chamber does as a side effect of reducing needed primary extraction pull.

          cheers

          eon

          • Dont know what are talking about, since in Ians video, thats GB 80, and you can see where the ports are.

            Rogak p18 had even slightly different style, check Ians video on that, too.

      • The MAB PA-15 does not operate with a locked breech, but rather by delayed blowback as well with a rotating barrel modeled after Josef Nickl’s work at Mauser and later CZ culminating in the vz. 24/vz. 27.

  2. This working system lets the blowback for all needed auto loading tasks but prevents its negative usage effects at manually cocking and shooting comforts. It is not a semi
    locking system like lever or roller type blowback. It is only a buffer in effect of related page rts in recoiling stage. In lower chamber pressures like,.45 ACP or higher velocity bullets like. 40″ it does not work. It needs suitable muzzle velocity, suitable chamber pressure with suitable barrel lenght to provide optimum performance of the rounds used.

  3. It’s rather questionable if an opening half way through the barrel, from where the spilled gas has to fill a relatively large space (like in the first pistol shown) can really act to delay the opening of the action until pressure dropped to safe level. Probably it acts just like a buffer, preventing the slide to slam into the frame at a speed dangerous for the integrity of the gun (and detrimental for the comfort of the shooter), while the action is largely a pure blowback.

    • Dont think at all in gb80 it can act like in vg 1-5 as a buffer, since the gas is not trapped inside gas chamber after the round lefts the barrel.

      Thats due to the port locations and that only short portion of barrel near muzzle turned to match the inner diameter of the gas chamber, barrel behind has smaller outer diameter, meaning that in slide recoil the gas vents out of the system through both there around the muzzle, and via 2 port holes.

      In vg 1-5, gas fills the chamber that in recoil quickly travels past the port holes, so the gas is trapped inside until the very end of the travel, till the g. chamber comes to the turned smaller diameter barrel portion, that serves as a relief for gas to escape.

    • That may be why Walther advertises the system in the CCP as “softcoil”. I’m not sure how useful it is since my petite wife is OK with the recoil of a similar sized S&W Shield which uses a conventional tilting barrel action.

  4. Behavior of the Gas Brake Function of HK P7 and Steyr GB seems rather different. Steyr has longer barrel and wider gas ports drilled at a distance from the chamber. Both need certain time fractions for gas expansion and begin to aimed tasks. Steyr shold have at least 1 to 1.5 mm free blowback empty case backward travel and does not have chamber flutes. İf it is remembered that case extraction continiues all th way long to its lenght and not limited in the stage when the highest pressure existing in the barrel, the function of chamber flutes might be understood as unstickedly matching the initially acceraleted empty case speed with gradually slowing slide velocity which seeming to happen earlier in P7 than Steyr. That means; At Steyr, case extraction occurs when the gas brake becomes effective and slide velocity begins to slow… IMHO.

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