I knew a fellow who turned a nice snug 'false shell' with a recess in the head D/Ted for a percussion nipple so it sat right to get popped by the firing pin. The thick walled shell also was a length to fill the specific chamber with a powder chamber to hold a specific amount of black powder, the wads and shot then sitting atop the works. I dont remember if he made an o-ring groove on the OD too. I think he carried a decent sized dowel as a loading rod since you cant make the rod run down through the forearm mount.
He would load it as a muzzle loader and only cap once loaded, and only remove the 'shell' to clean.
