If you want to load round ball, then you will want to have a cylinder bore. In 12 gauge, a cylinder bore measures .729", so you will be using a .715 RB, available from one of the catalogs like Track Of The Wolf or Dixie Gun Works, & a .015" thick patch.
Cabbalas used to sell the Pedersoli 12 gauge shotgun with interchangeable choke tubes for people who are going to be hunting game or shooting skeet & trap. I don't see it listed on their website anymore, just fixed-choke barrels. If you are going to be hunting deer with a round ball, use either a fixed-choke, cylinder bore or a barrel with interchangeable choke tube, in cylinder choke.
If you can find a T/C New Englander .50 caliber rifle with the interchangeable 12 gauge shotgun barrel or a New Englander with just with the shotgun barrel only, that will be a good alternative. Even better, later T/C barrels had interchangeable chokes with the barrel.