As for fodder for the 12 bore, we've found that round balls, held centre using the base cups from spent trap shells (pickups) worked better than any of the comercial slugs on the market, from our smoothbores, whether double or single. Fopr ful choked guns, my firends worked up loads with patched balls in the case, such that the patched ball could be pushed out through the choke. They used the .684" balls I aprovided. In my cylinder bore, I used .715 and .725 balls, held by the base cup from shotshell plastic wads. The .725's were more accurate and of course, hit slightly harder. The penetration is incredible compared to the 1 or 1 1/4oz slugs available.
: Groups in the 3" to 4" range at 50yds. were comon, yet the penetration of such loads far superior to any soft lead slug. We used WW metal with a brinel of 12 to 13. With a .725" ball in a tight .727" bore, accuracy with 1,500fps was very reasonable from a double 12 shortened to 26". SR7625 seemed to give the best results, and had much les recoil than the BP loads required to get the barels shooting together.
: BTW- the 1,500fps load duplicates the era 1890 HEAVY load for 12 bore elephant double guns, so should handle a deer quite handily.