Cannons were typically smooth bore, and designated in size by the weight of the ball they fired. Rifled guns were designated by bore size, and usually (but not always) shot a projectile of a shape other than round.
As was said before, cannons also typically did not shoot a patched ball, and as was said, could be shot with a wad over the powder charge. The ball was definitely smaller than the bore size, because it was typically made from iron, and that stuff is pretty non-malleable. As you might guess, the "round" cannon balls of the day were not machined perfect.
If an iron ball got stuck on its' way out the bore the rest of the gun would blow up. Not a good thing, so they were made with a fair amount of windage between the bore size and ball size to allow for such things.
To me, the words "precision" and "cannon shooting" really don't go together, particularly when it comes to smooth guns, and, particularly with a piece that is essentially designed as an area denial weapon!