All the "scientific" formulas you will pull out will not matter as they won't take into account the loss of energy in the bullet ripping apart. The kinetic energy equation only works if there is no energy loss, including friction & etc.. Same for a ballistic knock down equation, that assumes all the energy is going into the "target/animal".
If you really wanted to figure it out, you could setup a target on a "frictionless" surface and measure the acceleration & velocity at which it moves just after being shot with a high speed camera. Compare the actual energy transferred to that target, vs the theoretical kinetic energy that should have been in the bullet and you'll see how much you lost in the transfer.