Rhino, if you have a substantial hit to any part of the skull without effect then hitting it again won't do you any good.
1. Gun is too small, takes to long to get another.
2. Skull too thick.
3. The brain isn't home...
I know general anatomy for first aid not for shooting
Two to the center of mass. If that does not stop the threat then one, two, three or whatever it takes to get a head shot on a still moving target.
And in a gunfight the best thing is to shoot the gun out of your opponent's hand.