I once made one out of a cheap souvenir cannon bought at a battlefield gift store and stainless steel tubing from the hardware store. It used the fuse and gun powder from a firecracker to shoot a BB. Fun at the time.
How old are you? I did the same thing with some friends in 1965 with a souvenir Alamo cannon.
Dixie gun works has been doing cannons for decades. I was heavy into them, myself. My advice? Go small. The big cannons are very expensive to shoot. 50 caliber is perfect!
Flash