If you think about it, it really is quite an interesting engineering problem. I know some folks have addressed this with penetration depth in balistic gelatin. A number I hear tossed about is 13-18" depth is a good number.
The Kinetic Energy only tells part of the story. If a fast moving bullet overpenetrates, and goes completely through the target, then all of that extra KE is not used up in the target. A common engineering equation is the enegy balance equation: basically states that the sum of energy imposed equals the sum of energy exhausted. So if the bullet doesn't stop in the bad guy, the extra KE is wasted and does you no good. The bullet must penetrate far enough to reach vital organs, but not so far as to exit the target. Kinda makes me think of the sport "curling" for some reason...
Sorry if this has been discussed before. I don't see it mentioned much anywhere. This post probably makes no sense - I'm sleepy...
Thanks for this interesting thread Tact.
http://www.rathcoombe.net/sci-tech/b.../wounding.html
Good article discussing this topic. Makes a good point about engineering efficiency vs. real world effectiveness.

You do make a good Point Rangerbluedog. I agree with you. This is why I don't understand why the UN and Geniva Convention mandate only ball ammo in battle. Its supposed to be considered more humane. I see it to be a greater potential for wounding rather than a quick kill. I wonder how many soldiers foreign or domestic have been shot and survived because of non expannding rounds. I know I would rather live than die, but a non expanding round induces a greater possibility for unnecessary suffering. After all arent soldiers trained to kill not wound? What is the point of having such a powerful round if all the energy is wasted?