I've been searching for legal data on black powder cannons, but haven't found the info I need yet:

I've got a barrel from a 20mm GE Minigun (surplus) that'd make a WONDERFUL black powder
cannon, and I can get the M55A3 target practice 20mm projectiles for 87.5 cents each in lots of 200.  That's big-bore cheap shooting!

But I don't know if it can be legally made into a BREECH loading cannon.  Muzzle loading, yes, but this barrel has gain twist on it (!!) and I think I'd wear myself out trying to seat the bullet through that monstrous rifling. I'd need a sledgehammer and a starting punch for the bullet's nose just to get it started!

I'd want to use premeasured bagged powder charges, and would NOT be using cartridges!  No way!  I know that'd be illegal!

So, does anyone know where to point me so I can get the legal aspects of breechloading black powder cannons?

CJ

Read the statutory definition of what constitutes a firearm at the ATF website.

I believe it must be a muzzleloader, or unable to take fixed ammunition of any kind available in the normal channels of commerce.

The muzzleloader aspect would be hard to handle given that it's a heavily rifled barrel with gain twist and the rotating bands on the bullets are made of copper.   It could, though, be designed into a breech assembly which would preclude the use of any form of cartridge.  

More research is going to be needed.  I've got access to a fully equipped machine shop, and I might just build the cannon,  but I've got to be sure it'd be a legal project before starting.

CJ