Lyman lists loads for the 20 bore, that have round balls and conical .54 cal bullets set inside normal shot wads. The conicals would have to be shot from rifled barrels and were in 450gr. weigths at around 1,200fps to 1,400fps range.
: If driven from a twist of around 32" or a bit faster, maybe 28, they'd be quite accurate and very powerful. I suppose a 35" twist would be ok as well. I've no idea what the rifled 20 bore shotgun tubes carry for rifling.
; In 12 bore, a 16 bore ball should be useable inside normal shot wads, like trap wads with a filler of a 20 bore wad inside as well. Of course, the nice thing about using this system, is the legs of the wad collaps and give the correct length for crimping.
; My friends use coth patched undersized round balls in their choked guns. They are currently using .680" balls.