Last barrel I had made by Bauska(no longer around). It is a 20ga with a 60-70 twist. I think that it was the minimum his machine would cut. I had the bore and groove cut in relation to what was posted above.
I think that the tightest ball that would fit in a 20ga plastic case was .725 and it was bored .715 with the full size groove.
I was hoping that it would shoot round balls adequately and never got around to them.
Full bore slugs shot best with the 7/8oz 3" Brenekee shooting into 2.5" or so at 100 and the similar lightfield into an inch less.
If the slugs weren't available I would certainly shoot the rb ammo as it is probably better in nearly every respect.

Good shooting
Tom