bighit, most interesting post. Makes a lot of sense to me.
As a Deer Warden I have to dispatch a good few deer. My method is a explosive brain shot. It can take a good while for the body to realise its very dead which is no help when there are witnesses to Bambi's demise.
I consider air rifle pellets be it .177 or .22 as a stiletto stabbing rather than some explosive grenade going off inside. Shot placement is everything. I suspect it takes at least 18ft/lbs to generate some kind of meaningful hydraulic destruction effect.