A local gunsmith around here built a cannon back in the late 70's and I'm guessing from memory it had about an 18-20 barrel. Smooth bore and was made to shoot golf balls. Didn't think without rifling it would be a joke, but the dimples on the golf balls made it a lot more accurate than you would think. We had a hoot shooting it. Bored it out of a piece of solid stainless bar stock. always planned on rounding it with taper to appear like a cannon but don't think he ever did. Might have had the only square cannon ever made but I doubt it.