I made a potatoe cannon once.... :
and lets just say the first time shooting it was a little nerve racking.
When I was stationed in CFB Cold Lake in the mid-80s I was out shooting at the local range (outside of Bonneyville) with a couple RCMP friends, when a couple of guys drove in a half-ton with a home-made muzzle-loading cannon in the back.
My two RCMP buddies looked at it wondering if was legal or not, when one said, " It must be, its single shot and has a barrel length over eighteen and half inches!" Everyone had a good laugh and carried on.
For shells they used pop cans filled with cement. Can't remember what kind of powder they used, but they could blast those pop cans out 5-600 metres or more.
All the good-old days. :'(