I don’t remember what I used to expand the mouth, some tapered piece of metal I had laying around that did the job perfectly. Then I stole the .44 ball from my brother’s black powder stuff and glued it in.
I keep in with my ammunition display, and it’s convincing enough that people often ask me “what the hell shoots that?!” I usually come up with some line of BS about a hinged chamber that clamps around it. I’ll tell them that the neck acts as a venturi that increases the velocity of the expanding gasses like a rocket engine’s nozzle, while keeping chamber pressures down.