Talking about shots and reactions to shot, especially in the heart, it is worth noting some field work done by a guy name Kevin Robertson. He is a licensed veternarian in Zimbabwe and is also a professional hunter that specializes in Cape Buffalo which are notoriously tough. Like many hunters he had noticed how differently animals can react to what is essentially identical bullet placement. Some run off and others hit the ground flopping. He wanted to find out a reason so he put his veterinary training to use.
He ran some experiments and did a whole bunch of after shot "surgeries" and came up with an interesting theory. He says what he thinks happens is if the bullet happens to hit the heart when it if full of blood in the pumping cycle just as the valve opens the blood goes surging straight up the arteries to the brain and basically causes a massive stroke which results in an immediate lights out scenario. He basis this on dissecting brains on heart shot animals and noticed that some of them had damage to the brain as well and since this is nowhere near where the bullet hit, a surge of blood is the only explanation he could fathom. If the heart isn't full of blood then the animal will kick and run as usual and drop after a measure of distance is covered. Note none of this takes into effect hitting bones etc...
While it would be impossible to prove this theory in the field, it does make some sense.