Don jon's magic bandolir

You transmute your your last bullet so it produces an endless supply of magical ammunition, which seems to leap into your firearm when you reach for it.

On each of your turns until the spell ends, you can use a bonus action to make two attacks with a frirearm that uses ammunition of that type. Each time you make such a ranged attack, your bullet is magically replaced. Any pieces of ammunition created by this spell disintegrate when the spell ends. If the bullet leaves your possession, the spell ends.