If a kid can tell the difference, at a level appropriate to his age, between fantasy and reality, then that's what matters. Like Bones11b, a lot of kids played army, or cops and robbers, and also dreamed of hunting. In the fantasy world of army or cops and robbers, it is appropriate to point a toy gun at someone, and pretend to shoot them, because the game is an image of reality. Just don't put a real gun -- even a bb gun -- in their hands until they are old enough to learn the rules of safe gun use. And even then, keep it locked up, just like you do (or should) your own guns, if you have kids around. Then they will learn that real guns are used only as real tools or weapons, not as toys.
Really, this is nothing more than the same debate over whether violent video games cause violent behavior in teens and adolescents. It doesn't, in anyone sane enough to know the difference between fantasy and reality. Yes, there are a few who are so disturbed the distinction becomes blurred to them. We lock them up; we don't put the innocent at a disadvantage by taking the tools of self defense away from them. Unless you are an anti, of course. Then you outlaw first person shooter video games, and toy guns as well, if you can. And its because you (the anti) cannot tell the difference between fantasy and reality, and live in a dream world where everybody can be made to play nice with each other if we just make the right rules, or pass the right legislation. Yeah, right.