Try the Old West Scrounger.
I make them from 3.5" casings and load them on my MEC (you need the 2&7/8 spacer). Made up a simple jig to cut them down with a Dremel tool. Works a lot better than the cutter Ballistics Products sells. After cutting you need to "skivvy" (chamfer) the necks to get a good star crimp.
There are some smokeless loads that are perfectly safe in twist steel guns with mint bores.(See Doublegun Journal)
I use 1&1/8 oz of shot with a 12 YES 12 gauge plastic wads with IMR 4756 and WW primers (the hottest) in my 10 bore Cannon Breech Ithaca Hammer gun. (It is choked so tight a 12 gauge was will NOT drop out the muzzle!)
The boys laughed when I stepped up to the 16 yard line with this 10 pound behemoth but I never break less than 20 with it, usually more.
lotsa fun to shoot!
