Thanks, Sharps.
I find it appears to be a 10X47R. C&H has dies, but my 41 mag dies will re-load brass, and I can get 9.5X47R cases. That might be a good way to start out.
I got some .401 cast bullets in 180gr and 220gr to experiment with.
Rest assured I'll be in touch for some tech advice.
I have to get the swing down right hammer spring re-arched; the hammer top flips back up when released, hitting the shotgun striker.