Working on old Rolling Block rifles can get very pricy! I own two, One is a Spanish copy in 43 Reformado in very usable condition. The cartridge is similar to 43 Spanish. Reformado uses a.454" dia. 400 grain semi RN lead bullets. Cases are 57mm or 2 1/4" long. Around 70 to 80 grain FFg with wads and filler. Brass and reloading dies are readily available.
The other is a 40 year long project! Started as an Egyptian RB in 43 Egyptian. The poor barrel was too bad to do anything with including unscrew! Sawed off flush with front of action. Took a Dixie provided U.S. Navy surplus 50-50 (aka 50-45) barrel and turned off the threads, and Counterbored the old barrel stub. Silver soldered in place. I found an 1861 Springfield rear sight that fit the predrilled holes. Already had a very short blade front sight. Trim 50-70 brass from 1 3/4" to 1 5/16. I use a as cast 425 grain spitzer solid base with rubbed on SPG lube over 50 FFg Goex. Over the powder card and a 3/16" "cookie" of SPG lube.
First time I shot it I thought the light carbine would kick like my 45-70-400's! The full-size rifle butt plate made recoil similar to 44-40 in a lever carbine. (Surprisingly pleasant!) I'm still missing a forearm and the block & hammer pins retainer, but it works well!
Ivan
