The bore on my Bess seems to mic out right at .750", and I've been shooting a .735" ball with a thin (.010") patch.
With that combo, Swiss powder, (110 grains) a wonder wad and Wonder lube 1000+, I can get about six shots without wiping, but shots #5 and #6 go down pretty hard. I can also load the first shot into the clean barrel with a .018" patch, which I always do.
However, after shooting that combo for a while now, I think a .725" ball, with the .018" patch would be ideal, or more better, in my bore, which again seems to mic out right at .750".
I guess when fouling is "out of control" is when you can't get the ball down the bore eaisly, or at all. Smoothbores are a little more difficult in this respect than a rifle, generally speaking.
Yes, when you shoot "X" number of shots, and it becomes very difficult to ram a ball down the barrel, you just wipe the bore a couple of times with your favorite cleaning solution, (mine is Hoppe's #9+) wipe the bore dry, and go back to shooting. Some people will wipe between each shot, some guns can be fired many more times, depends on how tight your ball/patch combo is, how dirty your powder burns, and what you use for lube.
Rat