EDIT: This should probably be in the Business section(I think at least), but I missed it. A bit late to realize this I suppose. I've noticed in many games they do not use the actual names of the guns. Does one need a licence to use the actual name of a gun in a commercial game? Just asking :D

The answer to your question can be found in the forum FAQ here.

I previously asked a similar question HERE regarding the legality of using proper names, etc. The consensus seemed to be "it depends". (I know that's not very helpful.) So, though the FAQ provided by Obscure covers item names, the A2 answer is also along the lines of "it depends"...

So that leaves Answer #1 - talk to a lawyer :(

Try contacting the makers of the weapon. never know, you could get lucky and score free models and skins of their weapons after a polite email.