It's called a weapon, private,
This is my weapon (hold up rifle)
This is my gun (grab crotch)
This is for killing (Hold up rifle again)
This is for fun (Grab crotch again)
You learn the serial number of your little sister and you keep an eye on her at all times because, if a drill instructor got his mitts on your weapon he would take it apart and spread the pieces around to every drill in the company. Leaving you with the task of running around and asking for each piece. It never happened to me but, it was funny watching the other kids get trashed repeatedly until their weapon was reassembled.
Moral: Never admire a friend's gun. He might think you're ...odd.