For black powder 44/40 is the best caliber. Reason being is that the bottle neck case seals the chamber well and all the fouling is in the barrel. With the straight wall cases you get blowback into the action requiring a takedown to clean. A real PIA with a 92 or Marlin.
I'm not going to rule out the 44/40 cartridge. But I'm not leaning towards it either at this time. I understand what you say and I can't argue with you a bit about it, I'm just resistant to adding another caliber to the weekly reload list. I already reload .38spl and .357 for the wife to shoot as well as well as .45Colt for myself. If I can keep with the .45colt I would be happier I think. But I'm not ruling it out.
I'm leaning right now towards Henry's Big boy in .45Colt. I like that it's made in USA. But a '73 would sure be nice too.
Keep it coming, and some experiences with CAS BP rifle would be nice to hear about as well.