If you are refering to "why?", then I probably can throw some light on the issue:
A few times people were complaining about this or that gun in the game being not correct, i.e. having some part that isn't found on the original gun (I think one of those discussions was about one of the sniper rifles in the game)
The answer by a developer was that the guns are representations of a class of weapon and not meant to be 100% accurate real life weapons. The sniper rifle in question was said to be a mix of two or more real life guns for example. (all info extracted from my faulty memory, so don't quote me on that)
EDIT: Ah, gazz answered as well. Naturally as a coder/designer he has to blame the artists