roadrunner
WKR
Maybe we should start referring to bullet performance in horsepower. That should be entertaining
The bullet might only need 250lb force to mushroom/fragment but that doesn't necessarily mean if it started out with 350 it should exit. There is plenty of energy 'lost' (transferred) to the elastic nature of tissue that doesn't directly increase the size of the wound or aid in incapacitation. The increase of the drag on a mushroomed bullet uses a lot of that remaining energy as well.
250 ftlbs to go 20". That means after 20" there is a remaining 100 left to do whatever. That 250 accounts for the elasticity of organic media. As the bullet goes deeper, there are opposing and equal forces. It may indeed only take 75 or 100 ftlbs to deform the bullet.
It's the total energy of the system; the conservation of energy.