I think Paul has it right on the money! Here's a little tid bit to add. Remember that for every action, there is an equal and opposite reaction. The total energy available to knock the deer off its feet is no greater than the energy expended driving your rifle or shotgun back against your shoulder.

I think the term knock down power is unfortunate. It implies that the bullet contains the energy to forcibly knock the animal down. That of course is not even close to reality.

Nice job Paul!