Why does a gun recoil when it is fired?

Recoil means move away from (in horror, or to spring backwards which is what a fired gun does. The bullet goes forward at high speed and the much heavier gun goes backwards at bullet velocity * bullet mass / gun (and shoulder) mass A Lee-Enfield .303 rifle weighed 4kg and had no recoil suppression. The bullet weighed 174 grains or 11.275 grams.

Muzzle velocity 744 m/s Recoil speed is 11.275 * 744 / 4000 = 2m/s. If not held tightly against the shoulder it will break bones. Tightly held adds the mass of the shoulder, chest and arm, say, 25kg This results in a recoil speed of 0.289m/s which has to be and is counteracted by the muscles of the shooter.

A canon uses friction of the gun carriage to absorb the recoil. Modern rifles divert some high speed gas backwards and move the bolt backwards to reduce and slow recoil.

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