I managed to make a "gun" when I was about 14 with some scrap metal parts, home made explosive and hand tools. Mind you, I only ever test fired it while it was safely held in a vice, but it certainly worked.
Someone I know converted a model cannon to an actual cannon and loaded it with blackpowder from firecrackers and a marble. It worked very well and almost hit an unsuspecting passerby.
My uncle built a similar cannon from some pipe and shot holes through sheet metal with it.
