How far can a 308 caliber rifle bullet travel?
About 3,400 yards.
Is there a plastic which can be used on a 22-250 bullet which allows it to be fired from a 308 calibre rifle?
Given the limited information in your question I can only give a general reply. The caliber is the diameter of the rifle groove to groove in thousands of inches. So the difference would be .450" - .308" which equals .142". There are a lot of specified rounds of ammunition that use .308 and .450 bullets. In general the larger caliber bullet would also be more powerful.
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.