Iv had this happen before and its because one of the guns was damaged. The B25H's nose guns each carry 400 rounds. If you look at the damage list it doesn't list each nose gun individually. The list goes "Left Nose Guns, Right Nose Guns, Left Blister Guns, Right Blister Guns" Because each of those parts consist of two guns only one being damaged would leave you with 400 rounds (if you haven't used the guns when it was damaged) and the list showing it still working. Its not an ammo bug its a damage list bug. Maybe they could fix it by showing the part in yellow when only one of the guns are damaged.