Say if you had a Famas with 180 bullets plus the clip you are using and then you picked up another Famas on the floor as your secondary (and both these Famas's are exactly the same) and say the second famas had 150 bullets including the clip already loaded then both the famas's will say 330 bullets plus the clip but when you shoot one of the Famas's the ammo count goes down for both of them

Pretty cool! Sure wont run out of ammo for a while