The basic principle behind a recoil operated action is Newton's third law of motion. In simple terms, it says "For every action, there is an equal and opposite reaction." Thus, when the bullet leaves the barrel, there is a recoil by the firearm in the opposite direction. Due to the law of conservation of momentum:
mass of bullet * velocity of bullet = mass of firearm recoiling parts * velocity of firearm recoiling parts.
The velocity of the bullet is high, but since the mass of bullet is much smaller than the mass of the firearm, therefore the velocity of the recoiling firearm is correspondingly smaller than the velocity of the bullet.
In recoil operated firearms, the entire firearm doesn't recoil when the bullet is fired. Instead, only a portion of the firearm is allowed to recoil, while the rest of the firearm remains motionless relative to the recoiling parts. The recoiling parts and non-recoiling parts are connected together by a spring, which is used to return the recoiling part back to its original position. Unlike blowback operated firearms, the bolt is held locked at the time of firing. This means heavier cartridges can be used with recoil operated firearms.
Similar to the blowback action, it is desirable for the bolt to start moving only after the bullet has left the barrel and the gas pressure in the chamber has fallen to safe levels. This is because: (a) a tight gas seal must be maintained until the bullet has left the barrel for greater range. If the bolt moves back early, it will reduce the gas pressure in the chamber and barrel and reduce the force propelling the bullet out of the barrel (b) It is not good for high pressure gas to blow through the internal mechanisms of the gun and rearrange everything on the way out.
As we've seen previously, in a blowback action, this effect is achieved by using the inertia of the bolt and spring pressure to hold the bolt in place at the moment the weapon is fired. The bolt is not locked in any form of blowback action. In all recoil actions, this effect is achieved by locking the bolt to the chamber and delaying the unlocking until after the bullet has left the barrel.
There are three major types of recoil operated firearms:
- Long Recoil Operation: This is mostly used in automatic shotguns.
- Short Recoil Operation: This action dominates in automatic machine guns and semiautomatic pistols.
- Inertia Operation: This is a newer action that is used in some bigger shotguns.
In the next few posts, we will study these various actions in some detail.