For me, the 20 is a way more 'civilized' load, with a plentiful selection and availability of loads everywhere. If I 'need' more payload, some 3" 20s will carry about as much as a 12. FWIW, I dont hunt ducks or geese, so the 20 has done everything Ive asked of it to satisfaction.
If you want to shoot big round balls, even patched, it isnt recommended in a choked barrel, unless you want to chance splitting the muzzle. The best accuracy will be likely from a snug fit in a cylinder bore, so if you can get that from a RB, patched or not, that fits into a case long enough to fill the chamber and not let it jump a gap (so maybe use 3" hulls that you verify chamber length with). I made a loading block and soaked some patching in a lube/beeswax mix, placed it on the block, pressed in a RB, folded the patch over and snipped it off. The snipped end was an easy press fit down over the ball and stuck on well. Ths let me use a RB size I had and a patch thickness to get the bore fit I wanted.
If the proper RB or patched RB fit cant fit into a case, you COULD (aint sayin you Should) drop a ball into the forcing cone (slightly oversize to the bore, so it doesnt move downbore thus creating an obstruction) and follow it with a charged and wadded hull, shortened if needed to fit behind the ball. The obvious danger in this is if you decide not to shoot it and remove the charged hull without removing the RB, then load a shotshell as normal (if it would fit, so CHECK IT). Sure dont want somebody blowing up a gun/themselves for a lapse of common sense.
