My approach
Hammond Game Getter, comes in many different chamberings
Hammond Game Getter He includes a swaging die so you can tap through buckshot to size to the appropriate diameter. That said, I used .310 diameter Hornady Muzzleloading Roundballs (.310 diameter). I didn't size them at all and they fit nicely in the case neck.
I tried initially on another target and a center hold put them at the bottom ring (about 1" low). New target with sight almost at top ring. 3 shots at 20 yards using Brown (power level 2)
Completely penetrated a pressure treated 1x4.
Wife said 'You didn't shoot the eye out', which I didn't with the 4th shot on target, but good enough.
It was a HOOT to play with.