A number of years ago I was in the NSSA while living in Virginia. During the "Nationals" Winchester, Virginia there would be cannon competitions in two catagories. One was smooth boore and the other rifled cannon. They would shoot some plaster paris can projectile in the rifled cannons and quite frankly I don't what in the smooth bore. I tell you what those suckers were accurate hitting flower pots, bull eye targets at over 100 yards +.
Here's a link that i found: