i don't buy my son guns to play with... nor do i allow him to simulate with other objects...
When we see guns on TV, (and he does, as lots of boys do, think they are cool), we usually end up having a a discussion about how guns are only for the army, and the police, etc.... he understands that these are people who are VERY grown up, and don't really mean to hurt with the gun, unless there is a lot of bad stuff going on.
He kind of gets it...
I bought him some army men (
you know the green ones, that come 100000000 in a pack for $0.75??? 
), to satiate the desire for the good guy/bad guy play he likes to emulate from television... The plastic army men have guns, and he can act that stuff out, without being the agressor, per se.
Consequently, he identifies guns with a specific purpose... He knows that they are not for him, but for grown ups who have certain JOBS. I
do watch him/LISTEN while he plays. If I don't like the way he's going, I gently intervene and redirect...
I am not sure how I feel about the idea that stores should be prohibited from selling toy guns... ???
Let me ponder on that... good question.