I'm updating this 1867 RRB pistol thread; maybe my experience can help someone else in the future.
I took my RRB pistol to a local gunsmith in my hometown (Sadly, he's the only one left in the area and he will probably retire sooner rather than later). He was ecstatic to have a chance to work on this pistol, as it was completely new to him, although he knew about them. I brought to him several types of brass (.50-70, .56-50, .500 Linebaugh), plus molds for RRB and similar arms.
After much experimentation, he found the Linebaugh was the best, but he had to machine the rims down to the following:
Diameter: .600
Thickness: .062
Case length: .865
OAL: 1.20
BP: 23gr
Even with a custom-made mold for a 290gr .512 bullet, it turned out the one that worked the best was a Pietta .50 cal. for a Smith carbine.
I guess while my pistol was in at his shop, he had numerous offers on it. His reply? "If it was for sale, you wouldn't be getting it, as it'd be in my collection!" :-)