Usually the closer they fit the bore, the better, Tom.
We tightly patch balls in our smoothbore guns for shooting the black powder trail.

It doesn't happen often, but there are times when one of the better shooters who is using a 28 or 20 bore smoothbore with patched round balls, will outshoot a rifle over the entire course of 50 targets.

The ranges vary from a card split at about 14 yards and small plates as close as 25 or 30 yards to larger ones to 110yards. One of the most challenging is a snowshoe hare hanging on a chain at 100yards - it's a difficult rifle target, yet I've seen all 4 guys in a smoothbore group, hit it one after the other.

You might try a patched ball inside a shotcup, if it will fit inside a plastic hull and still chamber. different thicknesses of cloth will vary the outside diameter considerably.