Would he have seen people doing that somewhere?? Docos? TV shows? Other kids? etc. If so, it simply sounds like modelling and imitation to me. If he has seen it somewhere that sounds like normal imitation. Kids copy everything and anything we do and say and if he has seen it somewhere then if you have guns around, it would seem natural for him to model it with them? That is JMO of course, I might be way off what is considered normal at that age, my kids don't have any here so I am not talking from experience but I know mine have come home from creche role playing and talking about locking up people in jails etc. for a while which again was imitation from what they had seen and been doing there.