I found this bullet in a paddock that had been ploughed up for planting  in south Taranaki , there was a small skirmish there during the land wars ( 1860's) and about 6 shots were fired , I guess this was one of them




As you can see it a Minie with grease grooves and quite a blunt nose , note the rifling grooves and the large hollow base .

I think you need a .550 bullet for a paper cartridge .
I don't know of any one making Pritchett bullets , either for themselves or for sale . You can buy moulds on line , but the problem is they may cast bullets that will not work in your rifle .

Will you be tearing off the base and loading the whole thing .?
Paper cartridges use thicker paper than paper patched bullets .
and in my experience  cartridge paper cartridges are not always totally consumed , including nitrated ones, and need a clean out between shots . My mate was loading with paper cartridges when a spark ,from the previous load ,ignited the charge , burnt his hand , fired the loading rod down range and gave everyone a fright  , luckily the rifle was ok  .

If you just want to paper patch your bullets use "rag " paper , I have some you can try .