.32 shot

I recently aquired a Stevens Favorite smoothbore chambered in the obsolete .32 shot rimfire chambering. (shot not short) The original is a black powder cartridge and these actions are not the strongest. I started this thread over in the "other" section and it went from a quest to gain info on the piece I had found to a quest to recreate an obsolete cartridge.
I have been on a quest to bring her back to life with a proper cartridge. So far I have located some brass with an offset primer pocket that uses a .22 starter pistol blank. The brass I purchased from Dixie Gun Works (hasn't arrived yet is for a .32 rimfire extra long (.755" with an ID of .310). I'm hoping it will chamber and the extra length will provide shot capacity somewhere near the original. I'm also hoping I dont have to ream out the chamber any to accept the extra length. It is designed for use with a .310 round ball but that's simply not a proper shotshell. I plan on using #12 shot. I have received the suggestion to use a gas check as a shot cup but haven't found a source for one in .310. any suggestions on solutions to this problem are welcome, also any suggestions on a good non corrosive black powder substitute would be appreciated. So far the data I have obtained for the .32 rimfire shows a load of 13gr of black powder and a 90gr bullet and I found another load showing a 9gr black powder charge under an 80 gr bullet. Ill probably start with the lighter load. I know there are a lot of guys with creative solutions around here and I look forward to any imput you all may have.
Thanks BHJ