empirevr,
re: your question as to how 12 bore rifles relate to 10s. You need to get your hands on the Double Gun Journel volume 13 issue 3 for Frank Findlow's excellent article Some Thoughts on 12 Bore Rifles. A small article packed with information and charts comparing 12 bores to other guns of their times and modern cartridges. The 12's were loaded with from 3 to 9 drams of blackpowder. One collector here in the US has a Holland 12 bore rifle that was factory regulated with 12 drams of blackpowder!