my girls would not even know what a gun is. I wont ever let them play with them though. DD1 picked up her cousins once (in was a toy rifle) and tried to sweep the floor with it.
I think somehow your DS has learnt about guns and is just mimicking what he sees. I don't think it is any different to my DD1 constantly seeing Angelina ballerina dance and she tries to copy. It is just something your DS has seen liked the look of and wanted to do.