I did try searching "Amputee marksmanship" but I really don't think this is Googleable.
Suppose you are missing the thumb of your left hand, and your right hand in its entirety (some kind of crude prosthesis such as a hook on the right hand is possible, but nothing more sophisticated), is it at all possible to hold and fire a gun? Would it be easier with any particular kind of gun?
Character is about 20, would be learning to shoot post-injury, is not expecting to perform feats of marksmanship, but would just like to be one of several scary armed people. I haven't decided about handedness yet.
Edit: Any kind of gun will do, rifles are of interest as well as handguns.