As he said- you want a muzzle loader that uses loose powder and a cannon ball? Have at it.
You want to build a breech loader? Shoots cartridges? Larger than .50 inch bore diameter, you may be building a Destructive Device, which is regulated by the National Firearms Act much like machine guns.
But if you want to build a cannon that loads from the front end and fire bowling balls, go for it. Just stay away from airports- you do not want to come close to an airplane with a bowling ball.