Recoil

Recoil is the rearward motion of a firearm as a projectile and propellant gases move forward. Its momentum follows conservation of momentum, while its felt force depends on how the firearm transfers that motion to the shooter.

Conservation of momentum: A physical principle stating that the total momentum of an isolated system remains constant. The firearm moves backward to balance the forward momentum of its projectile and gases.
Momentum: A physical quantity equal to an object's mass multiplied by its velocity. Projectile and gas momentum determine the firearm's compensating rearward momentum.
Recoil spring: A spring in a firearm action that absorbs movement and helps return the mechanism to battery. It slows and reverses moving parts in many recoil-operated firearms.
Muzzle rise: The upward rotation of a firearm's muzzle during firing. It often accompanies recoil but is a rotational effect, not the firearm's rearward translation.
Impulse: The change in momentum produced by a force acting over a period of time. A longer recoil impulse can feel less sharp even when total momentum is similar.
Kinetic energy: The energy an object possesses because of its motion. Recoil energy describes firearm motion but is not the same quantity as recoil momentum.
Recoil pad: A cushioning component fitted to a firearm's buttstock to reduce impact on the shoulder. It spreads the recoil impulse over more time and contact area.
Blast overpressure: A pressure wave in air caused by a rapid release of energy, such as a muzzle blast. It can feel forceful near a firearm but is distinct from recoil motion.
Recoil operation: A firearm mechanism that uses recoil movement to cycle its action. Some firearms harness the rearward movement that recoil produces.
Newton's third law: A law stating that interacting bodies exert equal and opposite forces on each other. Firing involves opposing forces on the projectile and firearm.

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