I didn't even mention all the factors that COULDN'T be predicted by a formula, though some other people have mentioned a few, like the unpredictable variations in "media" that occurs when bullets hit animals.
Then there's the fact some bullets will blow off most of the frontal mushroom at higher impact velocities, and that not all bullets of the same make (whether Hornady Interlocks, Nosler Partitions, or Swift A-Frames) are all built exactly the same way in different calibers and weights. They're all tweaked a little for different intended (or expected) uses, whether in core hardness, jacket thickness, or whatever.
This why most major bullet companies I know of go through three stages when bringing new bullets to the marketplace. The first is designing the bullet at the factory in accordance with what they already know about expanding bullet design. Then there's testing them in various kinds of "indoor media," between changes in design. Finally there's going out and shooting a bunch of animals so they know what the bullets actually do Out There.
Many also further tweak bullets after more reports come in from the field, or manufacturing methods are changed for greater efficiency.
It would interesting to see what sorts of formulas would accurately predict penetration versus expansion in every situation. I'm sure the bullet companies would want to hear about it, because it would sure make such intensive and expensive "product development" obsolete.
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