2 different 20 gauge smoothbore guns.
One has 2 "sweet spot," loads.
TVM Early Virginia Smoothrifle:
80 grains 3f, thin "overshot card," lubed felt wad, .610 ball, 2 thin overshot cards.
Or,
67 grains 3f, thin card, .595 ball patched with lubed .010 cotton.
This gun does not like tight loads. A loosely patched .595 ball shoots far better with 67 or 80 grains of powder than a tightly patched .600 ball does with any powder charge.
Smoothbores are not rifles.
I don't consider a load acceptable unless it shoots 4 inch or less groups off the bench at 50 yards. I don't have much occasion to try it further, or reason to for that matter.
The other gun, a Centermark Fusil des Chase, will shoot a patched .600 ball over 85 grains 3f very well, but it is a lighter gun and can get uncomfortable to shoot this load in anything but an offhand position. Also, I'd like a more historic load for it.
Right now 75 grains 2f, unused felt wad, lubed tow/faux-tow wadding near, .610 ball, overshot card seems to be on the right track. This does need to be played with more. Going to try a 1×2" strip of brown paper folded to 1×1" between powder and lubed tow/faux tow. I like to have a barrier between lubed material and powder, and something to help keep gasses from blowing through the tow wadding. I've tried thin overshot cards and nitro wads but this gun reacts poorly to something stiff between powder and ball.