Recoil can be described mathematically by the physical law of the Conservation of Momentum. The law states: "If a force and its reaction act between two bodies, and no other forces are present, equal and opposite changes in the momentum will be given to the two bodies." Simply stated, this says that for every action there is an equal and opposite (in direction) action. It must be noted this approach does not consider any impact on free recoil due to the redirection of propellant gases by attached devices such as muzzle brakes or suppressors. Felt recoil (also referred to as “Perceived” recoil) can also vary by modifying the rate of application of force by devices such as recoil pads or damping devices and the influence of action type.

The momentum, therefore, of a free recoiling firearm is equal and opposite in direction to the momentum of the bullet (or shot charge/slug and wad column) and the propellant gases. Because the propellant gases are extremely difficult to weigh, for purposes of this application, the propellant gas weight will be equated to the powder charge weight.
Sporting Arms and Ammunition Manufacture's Institute

Revised: July 9, 2018