Not exactly to the point but something I think plays a part. I had a friend that ran a home animal shelter, she called one day to have me dispatch and process a doe that had been brought in with a broken neck. The deer was alive but paralzed, you could clearly see the bulge in the neck and it did not move anything but its eyes and ears. However, if you startled it the animal would attempt to run while lying down, all four limbs would would take a running motion at full power. It was explained to me by her that the deer had a flight reflex that was like a subconcious event. was she correct, I don't know, but I saw what I saw.

I almost never kill an animal in its tracks. I use 270, 7mmSAUM, and 50 cal mz loader and shot placement is almost always behind the shoulder within a couple of inches of the crease defineing the shoulder. Because of this I now use heavy constructed bullets to insure exit wound. I know what someone will say(if you shot a lighter constructed bullet, with no exit all energy expended in animal, more dead-in-tracks kills, but I have alredy been there and don't agree). The biggest difference I see in a heavy constructed bullet and light, with my usual shot placement is that theyrun about the same distance but with no exit wound are harder to find.

If I use the technique of hitting shoulder or leg bone as it passes through the chest the animals seem to lie down immediately, but too much meat is damaged.

So is it the breaking of bones, or the shock that is transferred through the skeleton that in my experience takes down the animal. I guess shock, deer with broken hooves or even lower legs are common around dog hunters and they move very well on three legs.

Point 1 - Shot placement seems to me to make a big impact on Knock-Down.

I have read articles and thought quite a lot on bullet diameter vs knock down power. Easy math says that a larger diameter bullet will impose more energy to be expended at the moment of impact. I think this quick burst of energy to the mass of an animal hasa "Stunning" value, the larger the diameter the more "Stun", and remember area formulas, a minor diameter increase will be a large surface area increase.

Point 2 - Surface area of bullet on impact will dictate how much energy is transferred at that time.

I am glad you brought up this point, I plan that my next purchase will be a big bore, but used for southern whitetails.