I cannot answer your question either...first 'cause I don't have a 1873, and second, because I haven't tried to get perfection out of the old 44 WCF.
But I have a Uberti 1860 Henry and here's what I've tried so far, and like a lot.
200 gr. cast bullets lubed with SPG
Fed LP Primers
Winchester brass - OAL 1.592"
5.6 gr. IMR 700 X for velocity 1055 fps
Accurate for gongs out to 200 m offhand
200 gr. cast bullets lubed with SPG
Fed LP Primers
Winchester brass - OAL 1.592"
6.1 gr. Trail Boss for velocity 984 fps
Feels like a light load but vel. says otherwise
Shoots higher than previous load because of longer barrel time (lower velocity)
200 gr. cast lead bullets lubed with SPG
compressed load of FFFg GOEX Holy Black...30 gr. I think
Fed LP Primers
have not Chrono'd...love to shoot them - use 'em for all CAS work