Allow me to put in my oar on this. After reading various comments on SB accuracy, I think accuracy depends on the gun and shooter, e.g., "cheek weld" and steadiness for the latter. Then too, different guns prefer different load arrangements, i.e., bare ball, patched RB, tow wads, etc. They also "like" different RB diameters (within the same caliber) and perhaps powder charges as well. The tight patched RB, while good for rifles, may not be so good (accurate) for SB's.
Years ago, I thought a bore sized RB, .618" in my case, would be especially accurate in my GRF 20ga. NW trade gun. In short, it needed a hard over powder wad + a greased felt wad + a thin over powder wad to get accuracy approaching that of a smaller (~.600") RB. Running the .618" between two coarse files enlarged the RB by .001", but didn't improve accuracy at all. In fact, it made it worse.
I'm posting two photos below of 25 yd. results (kneeling with my right arm on shooting bench) using a variety of .600" patched RB's and the biggest surprise (photo #2), using a .572" RB (although the Lyman mould is stamped "575") + a ~.028" duck cloth patch. Pay particular attention to the groupings on the lower part of the target in photo #2.
I hope these results offer some food for thought.

