Short of removing the front sight of a smooth bore and moving it over to one side or the other, bending the barrel is the answer to adjusting windage issues. But before you do, shoot the gun a lot to determine if it's really the gun or is it you? Work up a load that satisfies your accuracy requirements, and then shoot many targets with it. If after all that you a re certain that the gun is shooting left, for example by several inches, remove the barrel from the stock and bend it to the right. There are several ways to do the bending, some primitive and some controlled and scientific. I determine which way I want the barrel to bend, seize it by the muzzle and give the barrel a whack against the side of a pole or tree. Replace the barrel to the stock, and try a couple more groups.
As an example of this process, I built a nice fowler for Leatherbelly (RIP my friend) and when we took it out to fire it, his groups were three inches to the left at 25 yards off a bench. He was consistent and disappointed with the accuracy/consistency. So I gave him his money back and promply had myself a brand new Chambers' Pennsylvania Fowling in 20 gauge. On the next outing to the range, I and my brother Daryl both shot the gun, again at 25 yards off the bench, and chewed the centre out of the target. The accuracy issue was the shooter. And in spite of showing Leatherbelly our results, he wouldn't have anything to do with the gun, 'cause it shot 3" left!! I love the piece and have won many matches with it with round ball.
I have also used the other system of correcting windage, ie: remove and replace the front sight. Usually, this is after the client has filed way too much off the front sight in an effort to get it to shoot higher, so I corrected the height issue and the windage issue at the same time by adding a new sight.