head is kinda small and hard. personally, i would aim for the torso: bigger target and just one bullet can be deadly.
but in this case i think the attackers shot in his general direction rather than aimed at a specific point. unless they deliberately shot the knee, the guy refused to cooperate, they shot the hand, he still refused and so on. but then why bother with dialogue when you can kill first and then rob the body?
Shooting someone in the head depends on the distance. At close range he would not miss. I do agree that one bullet would be enough. Believe me, even when the head is small and hard, the
bullet will penetrate.