![]() The bolt is held back in its rearwards position before firing. The API Blowback overcomes these problems in a mechanically simple way. Simple blowbacks only work with low-powered cartridges (in practice, the smaller pistol rounds) because with powerful rifle or cannon ammunition the bolt would have to be so heavy for its inertia to hold it in place that the rate of fire would be extremely low - and if the gun were pointed upwards in the AA role, the weight of the bolt would overcome the recoil spring, causing to slide backwards. The recoil spring then pushes it forwards again, chambering the next cartridge. The momentum of the heavy bolt continues to carry it backwards, ejecting the fired case. So by the time a significant part of the cartridge case has moved backwards and is no longer supported by the chamber walls, the pressure is too low for it to burst. The bolt is much heavier than the projectile and does not move very far before the projectile has left the muzzle, causing the gas pressure to drop sharply. ![]() On firing a simple blowback, the high-pressure gas generated not only thrusts the projectile up the barrel, it also pushes the cartridge case, and therefore the bolt, backwards. The massive recoil of the 20 mm cartridge was quite effectively tamed by the straight blowback operation and the mass of the bolt creating much less vigorous kick than regular delayed blowback actions.ĪPI Blowback mechanisms are similar in construction to a simple blowback, in which the bolt or breechblock is held against the breech face only by its own inertia and a recoil spring. The blowback action Oerlikon SSG36 reveals the extra long chamber in which the cartridge totally sinks along with the case head. The layout of these weapons was different, as they had the mainspring behind the bolt instead of wrapped around the barrel, giving a particularly modern, uncluttered appearance. Oerlikon produced two shoulder-fired anti-tank rifles using the API blowback principle the SSG and SSG-36, in 20x72RB and 20x110RB respectively. Swiss Oerlikon SSG36 20mm anti-tank rifle ![]()
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