This subject was mentioned in the post on the single shot rifle. That post has gotten way too long in the tooth, so I'm not going to add to it. Some seem to think "the perfect shot" means the games goes down instantly and dies right there. That happens in some perfect shots, but not all. A perfect shot is one in which the animal will die shortly after being hit. I'm talking seconds or minutes or in some cases instantly. Why do some animals hit straight through the heart die and drop instantly and others run off a 100 yards or more and die? Answer: it depends. It depends on what the animals heart was doing when it was shot. If it was in the contraction mode when hit, the blood pressure will spike and may burst blood vessels in the brain and down it goes. If it was not beating at the moment of being hit, it may run off even if shot through the heart. There are other factors involved too, but I think the above is the biggest. I gleaned this from an article I read in Outdoor Life years ago. I believe it is a solid explanation.