What is the difference between a 30-30 and a 308 bullet?

None... it is the same sized bullet (.308 diameter). You have to choose the selection of bullet when reloading 30-30 ammo for the type of bullet. A pointed bullet in a tubular magazine can present a problem when the rifle recoils. The ammo is not interchangable, you must shoot the ammo that the rifle was chambered for.