Why convert anything at all? Why not just buy a Howdah, a Howdah look-alike or a double barreled flintlock 20ga smoothbore right out of the box?
Why fuss around with fooling around with converting one thing to be another thing?
If it is the 6 shot capacity that is an issue... Well, although I don't know if people do this as a regular type of thing, but I see no reason why someone couldn't come up with a home made paper cartridge shot load setup. By using a shot shell liner, and gluing a strip of flash paper at the bottom, forming a tube, adding in the powder, and twisting or folding the ends over, creating the charge portion of the paper shot cartridge. You could then, also, wrap a layer of flash paper around the plastic shot retainer portion of the shell, fill it with your shot, and again, either twist or fold the paper over to hold in the shot. If you don't want to use the plastic shot retainers, I am sure there are available from various pyrotechnical supply houses online, various sizes of thin cardboard tubing that would either be the right size, or, a little oversized and could be easily trimmed down to the proper size. (Just a thought on this one, I have never made one of these myself.)
Something like that would be very quick loading I would think, negating the need for a six shooter conversion altogether.
Sincerely,
ElvinWarrior... aka... David, "EW"
Howdah Look-Alike, retails for $399.00
20Ga Double Barreled Flintlock Pistol, Retails for $425.00
Single Shot Blunderbuss Pistol with folding knife blade, Retails for $299.0
9 Shot LeMat Revolver with 20 Ga under barrel that serves as the cylinder pin. Retails for about $850.00 to $1,100.00 depending on where you shop.