Liked Jerimiah Johnson movie too, but I also liked the Mountain Men. My favorite line was the one Brian Kieth said when asked if he was ever lost "Nope, can't say that I have. I've been confused for a month or so, but never lost."
As for the rifles in the JJ movie, yeah I think the producer/director made an error. They should have probably said a 32 or 36 cal (not 30).
I would assume that the calibers made in those days were 32 (maybe), 36, 45, and 50 in the Hawken. You still had the big bores like the 69s and such.
But most of the mountain men preferred the 50.

Skin this one pilgrim and I'll bring you another.