As a laywer, I think its an interesting question whether or not the use of the likenesses of existing, real firearms would be a fair use or a trademark violation. At first glance, this seems like a pretty straightforward situation - EA likely cannot legally use actual trademarked names and likenesses of existing firearms without acquiring licensing rights from the license holders, as in, the gun manufacturers. The logic is exactly the same as to say, a car in a racing game.
Likeness/design are well protected in the firearm industry and has been enforced by law with increasing regularity. I doubt they care about most smaller games using their IP, but someone large like Activision and EA making money off their property would likely cause a lawsuit.