One of the things we did in my machining class was we built miniature brass cannons. They were .177 caliber (In the US that is a common size of BB). They were fully functional. You put in some gun powder, a bit of wadding, and a BB, then you could fire them by putting a match to the touch hole.

Somewhat difficult to wield in a gun fight though, not that we didn't occasionally imagine the students holding off an attack by firing a broadside from cannon of various quality :-)