i think it is all luck and the will of the deer to live. i have seen both shoulders shot and the deer still try to go with just the back legs. i have seen deer with nothing left of the heart or lungs but they still tried to go for for a few seconds. with head shots i have seen them jump around like a jumping bean for a short time. and even the spinal cord shots i have seen them spin in circles on the ground. there is no sure way to tell what shot is going to put a deer down in its tracks. i have seen them drop like a rock from a lung shot of a .308 and other times they may run 50 or 100 yards. biggest surprize i ever had was when i shot one about 70 yards with my .50 cal with a round ball he never moved. it was a slow moving projectile to the lungs and the bullet never even passed threw. there is just not any sure shot to make them drop on the spot. maybe bionicrooster is on to something.