DarKeX More than likely you would encounter a problem if you started to sell your game. If it's free then gun manufacturers would only have a weak case in court (arguing 'fair use' on your part is guaranteed, it's how people get around putting music on YouTube).
However, if you're wanting to earn money from your game, and are also wanting to use the real names of the guns, you will have to get licencing rights from each individual gun manufacturer, and I remember reading a while ago that it isn't cheap. The alternative is to do what alspal suggested and use a name that is similar to the real name of the gun, that way you conveniently side-step the issue altogether, and save yourself a boatload of money too. I hope this helps.
Edit: I found this link discussing the issue, it's informative:
Garage Games Forum Topic