Table of Contents
What Are Foot-Pounds?
Foot-pounds (ft-lbs) is the imperial unit of energy commonly used in ballistics. One foot-pound is the energy needed to raise one pound by one foot against gravity, equal to about 1.356 joules. In firearms, ft-lbs measures the kinetic energy of a projectile, indicating its potential for doing work on a target.
Muzzle energy in ft-lbs is determined by bullet weight (in grains, where 7000 grains = 1 pound) and velocity (in feet per second). Heavier and faster bullets carry more energy. This metric helps compare cartridges for hunting suitability, terminal performance, and legal minimum energy requirements.
Formula
This simplified formula directly gives ft-lbs from grains and ft/s. It is derived from KE = 1/2 mv^2 with appropriate unit conversions.
Common Calibers
| Caliber | Weight (gr) | Velocity (fps) | Energy (ft-lbs) |
|---|---|---|---|
| .22 LR | 40 | 1200 | 128 |
| 9mm | 124 | 1150 | 364 |
| .308 Win | 150 | 2820 | 2,648 |
| .30-06 | 180 | 2700 | 2,913 |
FAQ
How much energy is needed for deer hunting?
Most experts recommend minimum 1,000 ft-lbs at impact distance for deer-sized game. Many jurisdictions require specific minimums. The .308 Winchester with 150gr at 2,820 fps produces 2,648 ft-lbs at the muzzle, well above this threshold even at several hundred yards.
Is energy or momentum more important?
Both matter. Energy determines tissue disruption. Momentum (mass x velocity) affects penetration depth. Heavy, slower bullets penetrate deeper; lighter, faster bullets create wider wound channels. The ideal depends on the intended application and target.
How does energy change with distance?
Bullets lose velocity (and energy) due to air drag. A .308 loses roughly 30% of its energy by 300 yards. Ballistic coefficient (BC) describes how well a bullet resists drag; higher BC means less energy loss downrange.