Blowback (firearms) - Advanced Primer Ignition Blowback

Advanced Primer Ignition Blowback

Advanced Primer Ignition (API) was originally developed by Reinhold Becker for use on the Becker 20-mm automatic cannon. It became a feature of a wide range of automatic weapons, including the Oerlikon cannon widely used as anti-aircraft weapons during WWII.

In the API blowback design, the primer is ignited when the bolt is still moving forward before the cartridge is fully chambered Image. In a plain blowback design, the propellant gases have to overcome static inertia to accelerate the bolt rearwards to open the breech. In an API blowback, they also have to do the work of overcoming forward momentum to stop the forward motion of the bolt. Because the forward and rearward speeds of the bolt tend to be approximately the same, the API blowback allows the weight of the bolt to be halved. Because the momentum of the two opposed bolt motions cancels out over time, the API blowback design results in reduced recoil.

To increase performance of API blowback firearms, larger calibre APIB guns such as the Becker and Oerlikon use extended chambers, longer than is necessary to contain the round, ammunition for APIB firearms come with straight-sided cartridges with rebated rims (the rear of the cartridge case is smaller in diameter than the front). The last part of forward motion and the first part of the rearward motion of the case and bolt happen within the confines of this extended chamber. As long as the gas pressure in the barrel is high, the walls of case remain supported and the breach sealed, although the case is sliding rearwards. This sliding motion of the case, while it is expanded by a high internal gas pressure, risks tearing it apart, and a common solution is to grease the ammunition to reduce the friction. The case needs to have a rebated rim because the front end of the bolt will enter the chamber, and the extractor claw hooked over the rim therefore has to fit also within the diameter of the chamber. The case generally has very little neck, because this remains unsupported during the firing cycle and is generally deformed; a strongly necked case would be likely to split.

The API blowback design permits the use of more powerful ammunition in a lighter gun that would be achieved by using plain blowback, and the reduction of felt recoil results in further weight savings. The original Becker cannon, firing 20x70RB ammunition, was developed to be carried by WWI aircraft, and weighed only 30 kg. Oerlikon even produced an anti-tank rifle firing 20x110RB ammunition using the API blowback operation, the SSG36. On the other hand, because the design imposes a very close relationship between bolt mass, chamber length, spring strength, ammunition power and rate of fire, in APIB guns high rate of fire and high muzzle velocity tend to be mutually exclusive. API blowback guns also have to fire from an open bolt, which is not conducive to accuracy and means they can't be synchronized to fire through a propeller.