My idea would be that it increases the maximum capicity but without spawning with more ammo but whenever you pick up ammo it gets stashed in the backpack increasing the ammount of ammo you're carrying up to the double amount. Also whenever you die you should drop an ammo pack depending on how much you had with you.
But also I think it should only exist for the fffensive classes, it would be really damn op with an Engineer, you could spam mini sentries all day with it.