I posted on another 1873 thread, but wanted to post an update here too.
I went out shooting with a friend yesterday who had some 125 grain cast bullets loaded in 38 special brass. I shot a group in my rifle, and it seemed to shoot pretty good. He also let me fire his 66, and my 158 RNFP Rimrock bullet with Trailboss in 357 brass seemed to shoot pretty poorly.
When I got home, as luck would have it, some 125gr Rimrock truncated cone bullets I ordered showed up. I decided to try them with the same brass and load as a direct comparison. I was really shocked when I fired them off of a field rest.
The one flier in the 5 shot group on the right I'm sure was me. It would be interesting to fire this load from a proper bench, especially with a peep sight.
For whatever reason my rifle seems to shoot 158 grain cast RNFP or SWC's into huge groups, IE I can out group it with my handguns.
It seems to do fine with 125 cast bullets...