Being a Vet. means having some weapons training. From what I've read, George seems to be more of a wannabe. My personal guess is George accidently made his own head wound. A common reaction to a stressful situation is to grab/slap the back of one's head. If you are still holding a handgun, that wouldn't go so well. I'd bet his wound matches with a surface of his weapon.

I have a friend who is a concealed carry instructor. He says that he teaches that if you have to fire, you empty the clip, in rhythm. He said that bang, bang, bang, bang, could be seen as evidence that after shooting, you walked over and made sure that the person was dead.

worried and in eminent fear of losing his life, why he only shot once.
Regarding the head wound and your hypothesis that he caused himself, he was seen putting his hands on his head in concern after murdering that boy. They didn't talk about the force he used to put his hands on his head but that is quite possible. The only thing that I think that isn't probable is the way he claims things went down.