Recoil energy

Momentum is conserved. The bullet and the ejected powder gas go one way, the rifle goes the other. Add the bullet's momentum to the powder charge's momentum multiplied by a gas velocity factor, divide by the firearm weight, and you have recoil velocity. Square it, multiply by the weight and divide by twice gravity, and you have free recoil energy in foot-pounds.

The gas factor accounts for propellant leaving the muzzle faster than the bullet. 1.75 is the usual figure for smokeless rifle powder; around 1.5 suits handgun loads and 1.25 suits black powder.

Free recoil assumes a rifle floating in space. Real recoil is filtered through stock geometry, recoil pad, comb height, how the rifle is held, muzzle devices and how fast the impulse arrives. Two rifles at the same 20 ft·lbf can feel completely different — a straight stock with a soft pad is gentle, a drop-heeled lightweight with a steel buttplate is not. A brake can cut felt recoil sharply while the free-recoil figure barely moves.

• Under 10 ft·lbf — most shooters can shoot all day and spot their own impacts.
• 10 to 20 ft·lbf — typical deer rifle territory; manageable with technique.
• 20 to 30 ft·lbf — magnums; flinch develops quickly without a brake or a heavier rifle.
• Over 30 ft·lbf — big bore. Practise with reduced loads.

Weigh the rifle as you shoot it, scope and full magazine included, and use your actual charge weight rather than a book maximum. Then run both loads through the recoil calculator and compare them against something you have already shot.