I appreciate the input. Of course this now means smokeless loads in my ROA have been added to the “to do” list.I'm afraid I don't have any actual muzzle velocity data, but the makers recommended a max load is 4.5 grns of Herco with a .454 round ball. A slow burning, large flake powder is required to prevent powder leaking through the flash hole in the chambers during loading..
The maker claims that Blue Dot or Unique would be suitable alternatives to Herco should this not be available.
I've tried charges of 3.5, 4.0 and 4.3 grns of Herco together with both .454 and .457 round ball with 4.3 grns and a .457 pure lead ball proving the most accurate. Interestingly, the same load works equally well in my Ruger Old Army nitro conversion. Others have tried a conical bullet rather than a lead ball without any improvement in performance or accuracy. I'm sure you are already aware that it's essential to use 209 shotgun primers as standard percussion caps will not reliably ignite smokeless powders. This is really a paper target shooting load , but does still provide a satisfying noise with very manageable recoil making it conducive to good accuracy. As a comparison, it feels about the same and groups within the same size as a load of 13grns (weighed) of 777 in my standard Uberti Remington 1858 BP revolver.
Hope this helps.