This is a touchy area… What studios I have worked for in the past have done is to use the identifier of the gun. For example instead of using Colt M-16 you can just call it an M-16. Or if you have a Beretta 9MM you could just call it a 9MM. Its all about not using the name persay but using something that people will recognize and every knows what an M-16 is, or at least has an idea of what it is when that name is used.
*Not a Lawyer…