IMO, three things are drasticly going to affect this number. #1) is the rifle caliber you are using, #2) is where you hit the animal, and #3) is bullet selection. everything else is not nearly important. with my 300 mag, MY percentage is quite high, i voted 61-70%. part of this is because i shoot, not as much as i would like, but a lot compared to many hunters. the second is having a caliber that is capable of delivering enough hydraulic shock to the animals body. and, of course bullet selection does play into this. a fmj bullet will blow clean through almost any animal, but will now transfer much hydraulic shock. you need a bullet that will RELIABLY expand WITHOUT blowing apart inside the animal. when you get all three, working together, then everything works in harmony, and bang, down, its all over, but the cleaning. i do not think that there is any way (other than a bazooka) of guaranteeing a one shot drop everytime. but you can maximize your efforts. it is up to you to do the home(range)work.