Halloween, welcome to the boards and the addiction.
First off I personally have not seen a cannon in a bowling ball size (16 lb?
). Unless it was for a candle stick or duck pin bowling ball.
The piece you are after is called a mortar. A cannon is a straight line of site piece while a mortar lobs the projectile of choice high in the air.
As for how long it take to make one. I would assume it depends on your skills, equipment available and how much free time you got.
Hang around. You will get plenty of answers from those far more knowledgable then I.
