I highly recommend you buy a barrel and breech it properly, you are risking your life if you don't, gun building is serious business.
When I was 13 my best friend and I made a pistol with copper tubing for a barrel. We strapped it to a tree, put a fuse in the touchhole and ran behind a tree while it went off.
One day when I wasn't around my friend held it in his hand an touched it off, it blew up and killed him. 56 years later I still think about this tragedy several times a week.