ROFL, I was going to advise you stay out of the line of fire, and then I read what kind of shot you meant.
Honeypie, there are going to be lots of things you're going to hate and fear in boot camp and beyond, and you are not going to get out of them. You need to get a sense of perspective about what you'll live through and what really matters. An injection in the behind may be embarrassing, it may be painful, but it will be OVER pretty quickly (if it happens at all), and you will live through it. Go into it like a man. Consider it practice for other times when you can't get out of something and can't let your fear show.
You let on that you're afraid of something, and they will use it against you at some point. Hazing happens.