Regarding "given the complexity of the reaction of living tissue and what if any vital organ is hit, that there really is no simple math?" Look at it this way

Big animal, heart full of blood - whether a small(er) bullet traveling fast or a larger, heavier bullet travelling slower,
the effect is that it causes a pressure spike in the heart - which is of course already at high pressure as it is full of blood - and maybe a spike up to the brain - which as Kevin Robertson in his book describes so well, causes drop dead effect.

Then do the same with an empty heart - that's when I prefer a heavy, larger bullet.

Just my HO.

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