Operating Systems 101: API Blowback Operation

Previously on Operating Systems 101, we discussed the straight blowback method of firearms operation; today we’ll be talking about its close cousin, API blowback. “API” stands for advance primer ignition, which refers to the chief way that the mechanism differs from its pure cousin. We’ll get to that in a bit, but first we need to discuss the concepts of open bolt and closed bolt operation.

  • Closed bolt operation: Where the firing operation of a weapon begins with the bolt in the closed position, with a cartridge already in the chamber.
  • Open bolt operation: Where the firing operation of a weapon begins with the bolt held to the rear, and incorporates feeding of the cartridge into the chamber.

A closed bolt weapon operates the way that most already understand: Firing of the weapon begins with the round already in the chamber, and when the trigger is pulled, that round fires and the cycle of the firearm begins. In contrast, open bolt operation begins with the moving parts group (or “bolt”) held to the rear, such that when the trigger is pulled, it first must begin to travel forward under spring pressure, strip a round from the feeding device, and push it into the chamber before firing can occur. The disadvantages of this system are obvious, chiefly that the length of time between pulling the trigger and the round discharging is much longer, impeding accuracy, but also that the weapon’s center of gravity shifts significantly in between pulling the trigger and firing. However, the system also has advantages, one of them being that it makes possible the API system of operation.

In an API blowback open bolt weapon, when the moving parts group moves forward after being fired, it has significant momentum. If this mass were allowed to bottom out against the weapon’s receiver, it would have no value, but API blowback harnesses that momentum to resist part of the thrust generated by the cartridge’s ignition. To do this, the ammunition is ignited not when the breechblock is home against the barrel, but before, so that the thrust of the round must first stop the breechblock’s forward motion before it can begin to force it to move rearward.

  • Receiver: The frame or body of a firearm, in which the moving parts group moves.

At 14:26 in the below video from Forgotten Weapons, you can see the API blowback mechanism of the ZK-383 submachine gun in action in slow motion. Note how the ignition of the cartridge precedes the breechblock’s stop against the barrel, and how the moving parts group is immediately driven to the rear from the thrust on the cartridge base:

API blowback affords a mechanism as simple (in many cases, simpler) than pure blowback operation, while reducing the bolt mass needed for reliable, safe operation. In small arms this effect is modest, but that hasn’t stopped API blowback from becoming the foundation of the vast majority of the world’s submachine gun designs, and it makes possible incredibly cheap and quick to manufacture fully automatic weapons.

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.


  • Jesse Volk

    This was an interesting one, I hadn’t understood the distinction between this and standard open bolt blowbacks.

  • Evan

    Nathaniel, I’m really enjoying this series. Thanks for putting it together.

  • Cymond

    So … is there any way to tell if a subgun is simple blowback or API blowback? (Aside from looking it up online or using a slow motion camera.)

    • Cymond

      Also, Nathaniel, it would be great if you could explain/illustrate telescoping bolts in a clear way. I’ve always had trouble visualizing it, and don’t have access to one to inspect myself.

      • Hi Cymond.

        I think I will write up that post tonight.

        Keep the recommendations coming!

      • Steve Truffer

        Look up a teardown of an uzi, and a teardown of a sten. The uzi has a telescoped bolt, the sten does not.

    • For small arms, the best way to tell is by looking for a fixed firing pin. Otherwise, you need high speed.

  • Riot

    Anyone know the most powerful cartridge that is used in an API firearm?

    • There is no limit; there are API blowback 30mm autocannons (e.g., the German MK108).

      If you mean firearm as in a handheld weapon, then I am not sure. Gordon Ingram’s .50 Ingram Magnum might be it.

  • MichaelinPA

    Top Marks!

  • I really enjoy this TFB series. Talking about open bolt vs closed bolt, can you describe how similar (or different) a full auto Uzi is from a semi only Uzi, since the former is open bolt and the latter is closed bolt?

    • Xanderbach

      The Semi-auto Uzi uses a striker-fired mechanism, not unlike a glock. From a closed bolt, cartridge ready in the chamber- Trigger is depressed, releasing a striker with an attached firing pin. Pin ignites primer, thrust from cartridge cycles bolt, which ejects spent cartridge, strips a new one from the magazine, and locks striker to the rear, all in one motion. Bolt continues forward, with a new cartridge being readied in the chamber. Trigger is released, resetting the sear. Continue until ammo runs out.

    • Rick, you’ll be glad to know that Alex C already wrote a thorough explanation of this very topic, complete with pictures: http://www.thefirearmblog.com/blog/2013/06/20/open-bolt-explained-tale-uzis/

      • Wow, thanks for directing me to that other article. TFB has an awesome staff of writers. Keep up the good work!

  • Cymond

    Ahh, that helps.

    So to make sure I got this right, the mass is needed for blowback operation. Telescoping is basically a way to shift the bolt’s mass forward, saving length at the back.

    I figured it was simple, but text-only explanations fell short.

    • George

      You seem to have it.

      Bolt mass is a factor of the bolt thrust impulse and bolt mass. Commonly simplified by balace with bullet mass and velocity (and secondarily powder mass and effective velocity), bolt mass, and bolt allowable kickback velocity. 4 m/s is conservative. 6 m/s is not uncommon. Micro-Uzi and MAC -10 were higher. Too high and it’s hard to avoid blowing cartridges out because pressure hasn’t dropped fast enough. Plus, tge faster and harder it hits the frame, the harder the kick.

      Once you know mass, given operating goals (say 9mm +P+ will have max ROF 800 RPM and bolt velocity 7 m/s), you look at configuring the mass. If you want to hold it one handed, then centering the cocked position bolt mass over the handgrip is a good idea. And that implies some is forwards…

      That goes directly to telescoping as the configuration.

  • David Harmon

    Geez, if it had a muzzle brake it would be nearly perfectly controllable.