Well if you are looking to do that you have two options:
1.Find an original 12-Bore double rifle regulated for 9 drams of black powder. This should be about a 15 lb gun, and will be very hard to find, not to mention expensive.
2.Have someone custom make a 12-Bore black powder double rifle regulated for 9 drams of black powder.
Both of these options are going to be expensive. With modern shotguns, magnum or not, the barrels are too thin for conical bullets or even sperical balls. Plus you do not have express sights. It just won't work. You need a gun that is built from the ground up to be a rifle, not built to be a shotgun turned into a rifle.
