Yaa,sounds like a 62ish trade gun i have,only your rifled. Heck of a slug,right close to 20 gauge?
Working up a load with patched R.B.try a 15 thousandths, if it loads easy enough try a 20 thousandths. Integrity of patch till out muzzle important,if 15 s retrieved and are ok see how well it groups. Lube choice or a backer patch over powder could affect durability of patch too.
Maker would have recommended max charge. No choice i,d start around 60 grains of 2f or equivalent. 3 shot group. Increase charge in 5 grain increments ea 3 shot group till 80 grains(i don,t know max allowed on your barrel) unless group opens up before then.
Dixie gun works may still carry mold. Or try visiting a near by black powder shoot to see if any traders have one. Usually where i pick up stuff.
A conical ,some grooved to accept lube,firing upsets base to seal,not saying a solid base would.
A conical 20 guage solid, yikes. That would get the cat out from under the bed!.
Sounds like a fun piece you have there. Half baked is trying to tell you a specific twist desired for optimum performance with round ball. another for conical. In between compromise, less accurate for either. I.e. 50 cal. Round ball barrel 1 turn in 66 inches. Conical barrel 1 in 35 maybe. A heavy charge behind a roundball in a conical rifled twist barrel hard for ball to stay engaged in rifling thus "skipping" and trashing accuracy.