Originally Posted by heavygargoyle
Do we look at velocity or energy? I am looking at the specs for .357 sig and 10 mm auto on wikipedia.
I like Taylor Knock Out or TKO
TKO = Bullet Mass (gr) x Expanded Bullet Diameter (in) x Terminal Velocity (fps) / 7000
or momentum x expanded diameter
Taylor first described this measure of stopping power in his classic work "African Rifles and Cartridges." Taylor referred to number generated by the equation as the "Knock Out Value" or "Strike Energy." Common practice today is to refer to this value as the "Taylor KO factor" or simply "Taylor KO."
The TKO has no physical meaning and is strictly used as a figure of merit for comparing cartridges. Its main advantage is the ability to attempt to represent complex terminal ballistics as a number; specifically momentum x expanded diameter. This can be utilized to assign different wounding capabilities to projectiles.
It does have good general correlation. For example, the TKO of a .22Mag is about 2, a .38Spcl about 6, a .357 about 10, the 10mm, .44Spcl and .45acp all about 13, a .44Mag 17 and a .454Casull at least 30. Clearly the TKOs are, at least loosely, tracking stopping power. You can't really say that about energy because a .22Mag produces more energy than a .380acp AND a .38Spcl. Momentum works a little better but still has problems: .22Mags and .380acps have about the same momentum and some .38Spcl+Ps have more momentum than .357Mags.