You probably found this, but here is a link to a BP reloading page. Black Powder Shotgun Shells
I have an old Marlin 1898 SG, rated for BP, that has an 2 1/2 inch chamber that I want to load for. It looks like loading for BP in undersized hulls may be easier than loading for a modern SG.
Somewhere I found a suggestion to make shells by cutting down plastic hulls, instead of buying brass ones. I'm thinking about trying this if I ever get this thing to the point where I want to shoot it.