Slightly reamed inside, 20 or even 24 bore rifled liners could be installed. These could be aquired from many of the barrel makers in the States - Moody, Ed Rayle etc. They will make any bore size and any twist desired. I can see a superb round ball double rifle in the making.
; The double 12 bore round ball gun as suggested by Jabalihunter also holds promise.
; Some time ago, Les Bauska used to make .50 cal liners for 10 bore barrels. They were turned to fit the existing chambers and were a 'drop-in' and glass or solder arranglement. The same could be done in 20 bore for this 12. The added weight would be about perfect. LYman's shotshell loading books from #2 through #5 have data, however in deferance to the age and posibly BP action, black powder loads could easily (more easily than smokeless) be developed for the double 20 rifle.
; 320gr. round balls in 20 bore, on up to 450gr., .54 cal. bullets that fit normal plastic shot-cups could be used.
: Oh yes, this gun shows promise for the future. It all depends on what is desired. The 20 bore rifle for this shotgun kinda holds dear to me at this time.