I wish there was a way to practice (of course there's not). My thought would be, instead of relying on just one shot, I would do a series:
1. Thigh, hoping to hit an artery, but likely to miss and still be conscious, but causing bleeding if nothing else.
2. Chest, right at the heart. More likely to succeed, but if not, causing a serious wound, and I would still have second or two to..
3. To the head. Even if I don't hit the right area, this, plus the other two shots, should do it.
Worst case would be to bleed to death which shouldn't take long.