I worked up loads for a 1900 era 12 bore using round balls. I was able to get it shooting about 8" to 10" groups at 100 yards, with 500gr. round balls running 1,550fps with both black powder (7 drams) and smokeless loads.

The smokeless loads kicked about 1/3rd of the BP loads, however they both shot to the sights.

A rifled version would have been super!!!!!