First I definitely understand projects and how what you want to do, often gets put on hold.

YES my vote is for a SxS too; but not having a donor, I keep thinking about a Encore or Contender single shot Howdah.

Why rifled barrels? Im picturing ranges measured in feet not yards, certainly no more than room distance. What does rifling add at that distance?

As to the cartridge issue, for something like a Stopping Gun, Im thinking mass is more important than velocity. Obviously there has to be a balance, and penetration is important; but Im leaning to fat vs fast