Why a Firearm Moves Backward
Recoil is the rearward motion of a firearm that accompanies firing, and it is a direct consequence of the conservation of momentum. As the projectile and the propellant gases are driven forward, the firearm must move backward with equal and opposite momentum. Recoil energy is the kinetic energy contained in that rearward motion, and it can be estimated from a handful of physical values.
Conservation of Momentum
Momentum is mass multiplied by velocity, and the total momentum of the system is zero before firing. Afterward, the forward momentum of the projectile plus the ejected gas must be balanced by an equal rearward momentum in the firearm. This balance is the fundamental law from which every recoil calculation is derived.
The Contribution of Ejected Gas
Recoil is not driven by the projectile alone. The propellant gases leave the muzzle at very high velocity and carry significant momentum of their own, adding meaningfully to the rearward push. This is why a light projectile driven by a large powder charge can generate more recoil than a heavier projectile pushed by a modest charge.
Why Firearm Mass Matters
The forward momentum is fixed by the projectile and gas, but that same momentum can be absorbed by a heavy firearm at low rearward velocity or a light firearm at high rearward velocity. Because recoil energy depends on the square of the rearward velocity, a heavier firearm converts the same momentum into markedly less recoil energy, which the shooter feels as a gentler push.
Free Recoil Versus Felt Recoil
The calculated figure is usually free recoil energy, the energy of the firearm recoiling freely without being held. What a person actually experiences, felt recoil, also depends on stock design, recoil pads, grip, and how the energy is spread over time. Free recoil describes the physical event; felt recoil describes the human sensation of it.
Worked Example
Suppose a projectile weighing 150 grains leaves at 2,800 feet per second with roughly 45 grains of gas ejected at high speed, fired from an 8-pound firearm. The forward momentum of the projectile and gas is matched by the firearm's rearward momentum, giving it a recoil velocity of about 12 to 13 feet per second and a free recoil energy in the neighborhood of 20 foot-pounds, felt as a firm but manageable shove.
A Common Misconception
Many assume recoil energy simply equals the projectile's muzzle energy, so a hard-hitting projectile must kick just as hard. In truth recoil is set by momentum, not energy, and the ejected gas contributes substantially. A firearm's mass also spreads the same momentum into far less felt recoil, so muzzle energy and recoil energy are related but distinctly different quantities.