Recoil
Where the kick comes from
Pick a gun and a load. Get the recoil energy, the gun's speed coming back at you, the momentum, and the muzzle energy. Then compare two loads and settle the argument from the duck blind.
Don't want to cut a shell apart? Leave these alone. The estimates come from the load's energy. How much they matter.
- Recoil energy
- — ft·lb
- Gun speed back
- — fps
- Momentum
- — lb·ft/s
- Muzzle energy
- — ft·lb
- Power factor
- —
Don't want to cut a shell apart? Leave these alone. The estimates come from the load's energy. How much they matter.
- Recoil energy
- — ft·lb
- Gun speed back
- — fps
- Momentum
- — lb·ft/s
- Muzzle energy
- — ft·lb
- Power factor
- —
The math, and every assumption it makes
Free recoil, the SAAMI way. Everything that leaves the muzzle carries momentum forward. The gun gets the same momentum backward.
ejecta momentum = (projectile + wad) × muzzle velocity
+ powder × gas velocity
gun velocity = ejecta momentum ÷ gun weight
recoil energy = ½ × gun mass × gun velocity²
muzzle energy = ½ × projectile mass × muzzle velocity²
power factor = bullet grains × fps ÷ 1000- Gas velocity. SAAMI: 1.75× muzzle velocity for rifles. 1.50× for pistols and average shotguns. 1.25× for long-barrel shotguns. Hatcher: a flat 4,700 fps.
- Powder estimate. The payload's kinetic energy divided by a per-platform factor: shotshell 85, rifle 52, pistol 60, rimfire 70 ft·lb per grain. These were fitted to typical published loads. Expect ±25%. Low-pressure straight-wall cartridges (.45-70 and friends) burn more powder than this predicts, so override it.
- Wad estimate. 18 + 14 × ounces of shot, in grains. 1⅛ oz gives about 34 gr. Real plastic 12-gauge wads run about 30–50 gr.
- Free recoil means a gun floating in space. Your shoulder, the stock, the pad and the action shape how it feels, not how much there is (see step 5). Muzzle brakes and ported barrels do reduce it. They aren't modeled.
- Presets are rounded, typical factory numbers, not specific products. Check your box.
What leaves the barrel
Recoil is the gun's half of a trade. Whatever goes forward, the gun pays for backward.
Three things leave the muzzle. The shot or bullet, the wad (in a shotshell), and the powder. The powder doesn't vanish. It becomes hot gas, and that gas leaves the barrel faster than the shot does.
Each part carries momentum: its mass times its speed. Add them up. That total is exactly the momentum the gun gets, pointed at your shoulder.
The gun gets the same push
Same momentum, heavier gun: slower gun. And energy goes with the square of speed.
Momentum is fixed by the load. The gun's speed is that momentum divided by the gun's weight. Double the gun's weight and it comes back at half the speed.
Recoil energy is half the mass times the speed squared. Half the speed, squared, is a quarter. Times twice the mass, that's half the energy. So recoil energy falls in direct proportion to gun weight.
Heavy and slow kicks harder
For the same energy downrange, a heavier projectile puts more recoil into you.
Muzzle energy is ½ m v². Recoil depends on momentum, m v. Hold the energy fixed and make the projectile heavier. It has to go slower, but it slows down only as the square root. Momentum still climbs.
So recoil tracks mass more than speed. A heavy load at moderate velocity kicks harder than a light load at high velocity with the same punch at the muzzle.
The pressure ceiling
The heaviest loads are always the slowest. That isn't a choice. It's a limit.
A shotgun chamber has a maximum safe pressure. SAAMI puts it around 11,500–14,000 psi for 12 gauge, depending on shell length. That's a fraction of a rifle's.
The work done on the shot is roughly pressure × bore area × barrel length. The pressure is capped, and the bore and barrel are fixed. So there's a ceiling on the energy a shell can give its payload. Add shot and the energy has to be shared out, so velocity drops.
Below, every dot is a typical factory load. They hug a curve of constant energy, because that's where the ceiling is. The gray lines are equal recoil in — gun.
Felt recoil is a delivery problem
The calculator measures how much. Your shoulder also cares how fast.
The gun's momentum has to go somewhere. Your shoulder stops it. The total push (force × time, the impulse) is fixed by the load. The peak force isn't. Spread the same impulse over twice the time and the peak drops by about half.
That's what a recoil pad does, and what a gas-operated semi-auto does. The gas gun spends part of the shove driving its bolt back, which stretches the delivery out over a longer time. Neither one makes momentum disappear. Conservation doesn't allow it. Only a muzzle brake or a ported barrel actually reduces recoil, by throwing gas sideways or back.
A hard plastic buttplate on a fixed-breech rifle is the worst case. All of it arrives at once, through a small, hard surface.
Why calculators disagree
Same load, same gun, five common methods. Here's how far apart they land.
Every recoil calculator makes the same three choices, usually without telling you. How fast the gas leaves. Whether the wad counts. And what to assume about powder you didn't weigh. Here are your two loads under each choice.
| Method | Load A, ft·lb | Load B, ft·lb |
|---|
- mass × speed
- momentum
- gun speed
- recoil energy
- how fast it arrives
Your shoulder gets the momentum no matter what. The rest is how the gun hands it over.