Recoil operation
 ANIMATION Browning AUTO 5 "cycle of one shoot", YouTube
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Recoil operation is an operating mechanism used to implement locked-breech, autoloading firearms. Recoil operated firearms use the energy of recoil to cycle the action.
The same forces that cause the ejecta of a firearm (the projectile(s), propellant gas, wad, sabot, etc.) to move down the barrel also cause all or a portion of the firearm to move in the opposite direction. The result is required by the conservation of momentum and is expressed in the formula:
which by the definition of momentum is calculated by:
In non-recoil-operated firearms, it is generally the entire firearm that recoils. However, in recoil-operated firearms, only a portion of the firearm recoils while inertia holds another portion motionless relative to a mass such as the ground, a ship's gun mount, or a human holding the firearm. The moving and the motionless masses are coupled by a spring that absorbs the recoil energy as it is compressed by the movement and then expands providing energy for the rest of the operating cycle.
Since there is a minimum momentum required to operate a recoil-operated firearm's action, the cartridge must generate sufficient recoil to provide that momentum. Therefore, recoil-operated firearms work best with a cartridge that yields a momentum approximately equal to that for which the mechanism was optimized. For example, the M1911 design with factory springs is optimized for a 230-grain (15 g) bullet at factory velocity. Changes in caliber or drastic changes in bullet weight and/or velocity require modifications to spring weight or slide mass to compensate.
Recoil-operated designs are broadly categorized by how the parts move under recoil.
Long recoil operation is found primarily in shotguns, particularly ones based on John Browning's Auto-5 action. In a long recoil action the barrel and bolt remain locked together during recoil, compressing the recoil springs. Following this rearward movement, the bolt locks to the rear and the barrel is forced forward by its spring. The bolt is held in position until the barrel returns completely forward during which time the spent cartridge has been extracted and ejected, and a new shell has been positioned from the magazine. The bolt is released and forced closed by its recoil spring, chambering a fresh round.
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Wikipedia