For a hunting gun intended for Elk, I'd probably go with the 50-90 as well as it is optimal in the BP ctgs.for really big game. The trouble with comercial barrels for it is the 36" twist, which will work fairly well with 450gr. bullets. Some people have reasonable luck with 500 to 550gr. slugs, but the longer bullets might not be stable after impact with that slow a twist.
: We mustn't snear at the 50-70 as it is a much better round than the .45/70, back int he 1870's and now as well.
: Unlike the present day's 45/70 capacity of 65gr., 76gr. of black powder can be put in the 50/70 case & indeed, 76gr. of Black Mag 3 can be used for almost 1,500fps with 420gr. slugs. With straight BP, the velocities run down around the 1,300fps range, but accuracy is still good.
: There are a flood of good lubricants now-a-days that work well with BP and it's phone replacements like Pyrodex and BlackMag3. Of lubes that work in lube sizers, Lyman has a couple stick BP lubes, OxYoke has a stick bore butter and SPG is one of the best. As well, a mix of 40% Vaseline, 60% Beeswax is also a GREAt BP lube, keeping the fouling soft and maintaining shot after shot accuracy.