A blowback action is a gun design where the bolt moves under pressure from propellant gasses to cycle the action while the barrel remains stationary. Additionally, inertia supplied by the mass of the bolt supplemented with spring tension provides the force needed for locking; no mechanical locking mechanism is present.
Controlling the response of the cartridge case to the propellent gas pressure is the basic design criterion of blowback weapons. The case responds by tending to move rearward under the influence of the axial force generated by the gas pressure on its base. Meanwhile, because of this same pressure, the case dilates to press on the inner wall of the chamber. The axial force tends to push the bolt rearward, opposed only by the resistance offered by the bolt inertia and the frictional resistance between case and chamber wall.AMC Pamphlet 706-260
While spring tension may assist with locking, the mass of the bolt alone restrains rearward movement of the cartridge case under pressure from propellant gasses during firing. The force moving the cartridge case and bolt to the rear is the same as that moving the projectile forward. Thus, while reward and forward movement begin at the same time, due to the considerable mass of the bolt, the cartridge case and bolt accelerate much slower than the projectile. This lower acceleration keeps the cartridge case supporting by the chamber until propellant gas pressure falls to a level that will not rupture the case. If you are wondering exactly how big the bolt needs to be, the Ruger 10/22 bolt and average .22 LR case weigh ~2,839 grains and the projectile is 36 grains. That means the bolt and case are ~79 times heavier than the bullet. That is not to say that is how much bigger it has to be. It is just showing one example of how much bigger it is.
aka:
- simple blowback action
related:
guns using a simple blowback action:
Footnotes / Sources:
“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.
The Franklin Institute. Engineering Design Handbook Guns Series Automatic Weapons. Comp. Engineering Handbook Office of Duke University. Washington DC: United States Army, Material Command, 1970. Print. AMC Pamphlet 706-260.
Question:
- Do you know of a gun that uses a simple blowback action that is not on the list?