Ten bore slugs are the way to go...My loading is 768grains hollow based slug, that I make in a mold I had made, 45-50 grains of Blue dot, in RMC made brass shells with enough filler to take up the room. I have also used Alliant Black MZ 135 grains... Nice powder no BP cleanup, but lots of push/boom. As Daryl s indicates you must determine the bore size(s). I wrap the slugs in plumbers tape to assure both will fit tightly enough for barrel differences.