I did it once, to see if I could. Granted, the conditions were as follows:
Extremely accurate heavy barreled .22-250 with Barnes "X" bullets.
Rock solid rest and known range of 110 yards.
20x scope, broad daylight. No wind to speak of.
Waited till it was directly facing away, with it's head down. Timed it so that the trigger squeeze started when it put it's head down to feed. Shot right between the legs into the bottom of the head.
It was nerve-wracking and I won't try it again. Used to shoot pigs in the head, till I did some home taxidermy on a pretty big skull and figured out just how small the brain is. I guess that the majority of them that I shot in the head just had enough damage and bone fragments going through whatever that they died on the spot. Never had one run off when doing that, but I know people who have. Figure I'll preserve my perfect record at this point and call it good. Well, last fall I did shoot one more pig in the head on purpose. But at a range of 50 feet or so, that eyeball looks pretty big in the scope!
I'll admit to doing it because that is the truth. But I'm not a good enough shot to make it a habit. Has to be very close range and an absolute sure shot. We don't get those too often.