What diameter grinder best for smoothing out 870 shotgun tube?

[terrierkeeper]

I am planning to add a magazine extension to my Remington 870 and after drilling out the moles in the tube what diameter grindstone will I be able to use on my drill? At the same time I got me a bayonet for my shotgun but the Chinese made thing does not fit so I will have to grind that out as well.