recoil

Recoil is the backward momentum imparted to a device when it exerts a forward impulse on another object, a consequence of conservation of momentum. In physics and engineering, recoil describes the reaction force that accompanies thrust or propulsion.

In firearms, recoil arises when a projectile is accelerated forward within the barrel and the gun must receive an equal and opposite impulse. The forward momentum of the bullet plus that of the rapidly expanding propellant gases contributes to the total forward momentum that the gun must balance with backward momentum. The recoil impulse is approximately J ≈ m_bullet v_bullet + m_gases v_gases, and the gun’s recoil velocity is v_gun ≈ J / M_gun. The recoil energy is E_r = 1/2 M_gun v_gun^2. Felt recoil, the perceived kick, depends on gun mass, bullet and gas momentum, stock design, and grip.

Recoil can be mitigated through design and materials. Increasing firearm mass, lengthening the barrel, and using recoil-absorbing stocks or pads reduce felt recoil. Muzzle devices (brake or compensator) redirect gases to counteract backward impulse, while special propellants or gas-operated systems can lower initial pressure. In other contexts, recoil generalizes to any reaction force from propulsion, such as rocket exhaust producing thrust, where emitted mass and energy generate opposite momentum.

See also: conservation of momentum, impulse, thrust, impulse-momentum theorem.