Perhaps something in-between could be true, the bullets' casings are empty ones that are loaded with the correct innards upon request (changing ammo setting for example, then also immediately as the next round needs loading after firing). That way you could still run out of a certain type of ammo. There also could be a sharing of resources in that situation, like all rounds draw their initial propellant from the same explosive reservoir as Hi-Ex, just in much lower doses, and incendiary could be the same explosive with an extra chemical added or air-ratio changed or something.