Umm, nevermind that. How many pounds of powder would that take to fire, and I could only guess at the cost of each projectile.
The way I understand it, muzzle-loading cannons and field rifles are fine, but as soon as you get to the big breechloaders, it's a no-no.
The Parrot Rifle has been done, I've got pics of a guy with a replica, he casts his projectiles, then swages them to size. Here's the 3" rifle and the website, note the accuracy at 400 yards!
http://www.hatchergun.com/blkpwdrcannon.htm