While I also prefer an exit wound, it isn't vital in killing game or even recovering game. It is the damage the bullet does to the vitals that kills game and not the number of holes in the hide. A hit that resulted in a dead deer in 15 yards is a very good hit. Not every animal will drop at the shot. Most will run since the instinct to flee is natural to them and they will cover some ground before the brain runs out of oxygen. You can blow the heart completely in pieces but the brain will still function for a little while. This is why it is very important to follow up every shot and be aware that you may find very little or even no blood at the beginning. I once tracked a bull elk almost 200 yards and never saw a single drop of blood on the ground but he was deader than Hollywood morals when I found him. Some animals simply bleed internally. It is rare, but it does happen sometimes.

The only way to ensure every deer shot drops right there is to break both front shoulders, spine them or brain them. None of them are ideal shots as far as I am concerned. Both the spine and the brain are small targets and can easily be missed and a shot that takes out both front shoulders ruins too much meat. In my opinion a shot that is just behind the shoulder about 1/3- 1/2 the way up the chest will take out the lungs and maybe the heart and will quickly kill the animal without ruining too much meat. If an animal is quartering the bullet should be placed in such a manner that the far shoulder will be hit. Such a shot has to penetrate the vitals before reaching the shoulder without regard as to what the angle is.

Remember, every shot is unique and every animal is unique and you can't expect them to react the same way every time.