BP cartridge loading, using Brass Casings, requires you use a wad that is one gauge LARGER than the bore of the barrels. I am using 11 gauge wads in my brass hulls for my 12 gauge loads. I ordered them from Circle Fly, which makes the wads for everyone else, it seems. Many suppliers carry their wads. You may have to use several of the 1/2" thick cushion wads to fill the space in the casings, in you want to seat the round ball out at the end, like some old time rifle rounds look. However, the brass hulls are originally designed to shoot shot, and using 2 OS cards( poke off-center holes in them to let air out and aligned them so the holes are at 12 and 6 o'clock in the casing) to hold the Round ball in the casing. As to what size ball works best, tht will depend on your particular gun. As with MLers, you have to measure both bores, with calipers, and then work up loads for each barrel individually. That is part of the fun of owning these guns. Don't expect the barrels to be " regulated " to shoot round balls to the same POA, or to have the same POI. The guns are made to shoot shot patterns, and not RB loads. you may find one or both barrels shoot RBs well. Or you may find one barrel does well and the other doesn't.
Most shotgun chambers have a pronounced shoulder before the throat of the barrel begins, and the throat is pretty short- often only half an inch. When working up loads in a deer slug barrel years ago, I relieved the throat, and got rid of almost all the shoulder before the throat, using a tapered Throating reamer. You can buy them from Brownell's. Many gunsmiths that work on competition shotguns have them.
By getting rid of the shoulder, and lengthening the forcing cone into a more gently sloped bottle " neck ", I damaged the slubs less, and my groups are outstanding! I shot 3 slugs( winchester, 2 3/4" Foster style) into the center "X" of a human silhouette target, off-hand, using iron sights, with all three holes touching in a clover leaf, at 50 yards. I handed the gun to my friend, and Joe fired the last 2 rounds at the same target, and hit just above my three slugs, leaving a group that was only about 2.5 inches across, and easily covered by the palm of the hand. Since he was not used to firing slug barrels, much less one with rifle sights on it, I did not expect his two rounds to go to the same point of Impact as mine did. However, He was closer and tighter than even I expected.
For that reason, I don't hesitate to suggest such a modification be considered by you for your DB shotgun, if you plan to shoot RBs out of the barrels on a steady basis.
