Have done this with fire-formed .30-40 Kraig cases with the rims turned down to .050". Approximately 230 gn of #9 shot and 7 to 8 gn of Bullseye worked well. Like the silicone caulk idea. I have used epoxy or crimped in a .41 cal gas check. Couldn't get the cases completely fire-formed without using a full load. They work well but lead the barrel fiercely.

One unerring mark of the love of the truth is not entertaining any proposition with greater assurance than the proofs it is built upon will warrant. John Locke, 1690