Muzzle Energy
The kinetic energy a bullet carries as it leaves the barrel, measured in foot-pounds — a common proxy for a load's power.
Muzzle energy is the kinetic energy of the bullet at the instant it exits the muzzle, calculated from bullet weight and velocity and expressed in foot-pounds (ft-lbs). It's a convenient single number to compare the relative power of different loads and calibers — a 9mm might make ~350–400 ft-lbs, a .308 over 2,500.
Energy isn't the whole story: bullet construction, expansion, and shot placement matter enormously for terminal effect, and some jurisdictions set minimum energy for hunting certain game. Still, muzzle energy (alongside momentum and sectional density) is a useful starting point when comparing cartridges or choosing a hunting load.