The problem with using a modern-made rifled 20ga barrel blank(s) may be that most are rifled with twist rates for sabot use, and have enfield-style lands and grooves.

Slow-twist rifling for round-ball or stubby slug would be my preference, along with wide grooves and steep narrow lands in keeping with period profiles for BP and lead-bullet bore rifles of ~120 years past.

For some reason the latter is hard to find off the shelf, impossible to find on this side of the pond unfortunately.
I am on quest for a 16-bore barrel blank myself!
Did somebody say howdah pistol....?

The other option may be to make a button for an oval-bore barrel. Less BP fouling, potentially....