do you have buy the gun, how much was it. I would say its a very old one, made for 1880. rifled bore gun in old times where made for "thick" paper cases. if you measure an 20bore paper case believe you will find that your roundball wiil fit it. you have get 20 bore Magtech brass, 2,5" long, with the gun?
I recommend the RCBS shotshell die set 20 bore. you have all you need: case holder, resizing die and crimp die. aneal the brass case every time.I load in my 20 bore cape gun(steel barrels) over hundred grain ffg under a 420grain brennecke slug. cicle fly wads have good wads, load it under the roundball and crimp the case.