One nice thing about muzzleloaders being exempt from federal law is that hobbyists can (and do) make their own from scratch. Muzzleloading cannons fit into the same classification and I've built three (my favorite was turned from 1-5/8" ID x 3-1/4" OD steel machine tubing and it's gotten regular Independence Day usage - with a tomato paste can of black powder). My biggest bore is a mortar that will shoot a 12" long x 9" diameter log to good effect.