A black powder muzzle loader avoids all restrictions using the "readily available ammuntion" language. Under Federal law, muzzleloaders are not legally firearms (which is why you can buy them mail order, still today)
HOWEVER, some states DO consider muzzleloaders to be firearms, so check your state's laws.
The Federal rules covering cannon are generally not about bore size but about the projectiles. Not certain about today, haven't researched it lately, but at one time you could buy a tank with its cannon intact and functional, and it was not a restricted item. The AMMUNITION for that tank, WAS restricted.
Exploding ammunition, is a destructive device under the law, so explosive tank rounds, like hand grenades, mortar rounds and such are each, individually registered, taxed and tracked under ATF rules.
SOLID SHOT projectiles are not. and muzzle loading cannon firing solid shot are not, as far as I know. IF you're going to fire exploding shot (cannonballs or other) from your muzzle loading cannon, better talk to the Feds BEFORE you do it. If you don't and the Feds find out you're firing explosive shot, they're DEFINATELY going to want to talk to you, and, they won't be happy...
This is not legal advice, its the situation as best I remember it, and is intended for discussion only! I may be wrong. Or things might have changed since I learned the rules. For valid legal advice PAY a Lawyer.
Good luck, have fun! And BE CAREFUL.
I knew of a guy, (a high school shop teacher) who was a homemade muzzle loading cannon enthusiast. He was also missing 3 fingers from one hand.
There's a good reason why the sponge is on the rammer and there's always a bucket of water handy when operating BP cannon! learn the proper procedures and (since no one is shooting cannon back at you) there's no reason to rush things...
Good Luck, have fun BE SAFE!!!
