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pistol-whipped,
Jibjab's formula gives the correct transformation of a bullet's mass and velocity values to the kinectic energy value. But it is a little unclear what that formula comes from. It is an algebraic simplification of the fundamental formula for kinectic energy, which is 1/2 x mass x velocity-squared. Jibjab's formula contains constants of 1/7000 to convert the bullet's mass from grains to pounds and 1/32.2 to convert those pounds to "slugs" (which are the English system units of mass, although most people don't realize that). The answer (for bullet weight in grains and velocity in feet per second) comes out in foot-pounds. Of course, if you were doing this for a shotgun load with the bullet mass in onces, you would need to divide by 16 instead of 7000 to get pounds.
One fact to remember is that you need to reduce the velocity from the value at the muzzle to the value at the target to get the energy that strikes the target. That depends on the target's range and the bullet's ballistic coefficient. There are tables and computer programs that can give you the velocity reductions.
But, you need to know your starting velocity for your handloads. The reloading manuals give you values for teh muzzle velocity OF THE TEST GUN, but yours may differ by as much as a couple hundred feet per second. You really need to shoot your loads over a chronograph to get the answer to your question for your own handloads.
SL1
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