i found an old jackhammer bit at my grandfather's garage and made a cannon. the hole in the middle is about 32 cal. i threaded one end to put a screw in and and drilled a little hole on top for fuse. its neato
I know of the excitement. When I was your age I was making my own blackpowder and using 3/4" pipe (genuinely stupid - eh?).
Did you cut the bore using a lathe?
You have at hand some real power - learn well the practices of safey.