page_title: Blowback (firearms)
section_title: Advanced primer ignition (API) blowback
hierarchical_section_title: Blowback (firearms) / Advanced primer ignition (API) blowback
caption_attribution_description: English: Bolt cycle of the en:MK108 cannon (part I)
context_page_description: Blowback is a system of operation for self-loading firearms that obtains energy from the motion of the cartridge case as it is pushed to the rear by expanding gas created by the ignition of the propellant charge.
  Several blowback systems exist within this broad principle of operation, each distinguished by the methods used to control bolt movement. In most actions that use blowback operation, the breech is not locked mechanically at the time of firing: the inertia of the bolt and recoil spring, relative to the weight of the bullet, delay opening of the breech until the bullet has left the barrel. A few locked breech designs use a form of blowback to perform the unlocking function.
  The blowback principle may be considered a simplified form of gas operation, since the cartridge case behaves like a piston driven by the powder gases. Other operating principles for self-loading firearms include blow forward, gas operation, and recoil operation.
context_section_description: In the API blowback design, the primer is ignited when the bolt is still moving forward and before the cartridge is fully chambered. This requires a very careful design to ensure the proper balance and equalization of forces between the projectile weight, propellant charge, barrel length, bolt weight, and return spring strength. In a simple blowback design, the propellant gases have to overcome static inertia to accelerate the bolt rearwards to open the breech. In an API blowback, they first have to do the work of overcoming forward momentum to arrest 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.
  According to Anthony G. Williams, the "API blowback principle is used virtually in all open bolt sub-machine guns" (technically, although generally these are known as "simple blowback" weapons due to the very late ignition of the cartridge compared to specially designed API blowback guns like the MK 108), although "the relatively low pressures and velocities mean that extended chambers and rebated-rim cartridges are not required" for sub-machine guns. In heavier weapons, advanced primer ignition (API) was originally developed by Reinhold Becker for use on the Becker Type M2 20 mm cannon. It became a feature of a wide range of designs that can be traced back to Becker's, including the Oerlikon cannon widely used as anti-aircraft weapons during World War II.
  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, and ammunition for APIB firearms come with straight-sided cartridges with rebated rims (rims that are smaller in diameter than the cartridge itself). 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 the 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 than would be achieved by using simple blowback, and the reduction of felt recoil results in further weight savings. The original Becker cannon, firing 20×70mmRB ammunition, was developed to be carried by World War I aircraft, and weighed only 30 kg. Oerlikon even produced an anti-tank rifle firing 20×110mmRB 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 also prevents synchronized fire through an aircraft propeller arc.
  According to a United States Army Materiel Command engineering course from 1970, "The advanced primer ignition gun is superior to the simple blowback because of its higher firing rate and lower recoil momentum. However, favorable performance depends on timing that must be precise. A slight delay in primer function, and the gun reverts to a simple blowback without the benefit of a massive bolt and stiffer driving sp
caption: MK 108 cannon bolt cycle (part I)
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