To promote team cohesion, I think a player should be able to take only 1 AR/M16/M4/Mk18/etc. or AK magazine from a teammate's backpack using F. If the player is out of primary ammo, however, he will not have magazines to give up in his backpack. Riflemen might also serve as Assistant Gunners and be the only class from which a Gunner can get an extra belt or box of ammo. Those with Exotic Weapons are just out of luck and will have to rearm from the dead or at ammo boxes. I think this will reflect the commonality of assault rifles and their magazines in military and paramilitary forces.