5/8" @ 1000 yards
Knowing, and learning your rifle, how to reload for optimal results as well as the skeletal structure of the game you are shooting, gives one the ability to put down the animal one is seeking.
Knowing what the round will do to the animal upon impact, in the case of headshots, you are not trying to kill an animal by shooting it in the brain, I know a lot of people walking upright without much of one, that isn't what keeps one alive.
Headshots, or shooting an animal in the head, when the impact of the bullet goes into and through the skull, it breaks the spine and in most cases takes the head completely off the spine, dropping the animal in it's tracks.
Again, a sniper doesn't shoot for the heart or lungs, even with the big .50 cals they're using today, always, all, Headshots.
I'm not asking you or anyone here for their moral opinion, I don't give two rips about that side of the coin. But again, I think my argument stands that fewer headshots = less game lost.