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.