each of the guns has it's own mini replicator dedicated to producing it's own ammo, all it requires is power/nanites/both, that it gets fed through nano channels in your arm when you hold the weapon, there is no store, it channels it directly from the clips via your belt.
problem solved?
it's the only way i think they could claim it makes any sense anyway.
still stupid, and i still think it's not as fun, and doesn't have the depth of more than one ammo type. but then it does make map making/item layout easier for the devs.