Most videogames take on ammo:
If you pick up bullets they automatically jump into your infinite supply of mags!
If I were to make a game a mag would be a separate item from the actual ammunition, and you could have 12 mags max per gun, and each one would load realistically, (See: Half-life & Half Life 2 shotguns)
and be put into the gun one at a time.
Example:
use mag 4 on Mac-10
fire off X amount of bullets
switch to mag 6
empty
switch to mag 4
you do NOT have a full mag because of the bullets you just shot out of mag 4.
