A classic!

[Mr. 16 gauge]

First, he gets a .36 or .40 Hawken (?) early on and Redford says, "Well at least it is a Hawken." Later he gets a .50 Hawken off a frozen corpse.

Hello Adam....right you are! The narrator in the beginning says "He settled for a .30, but damn....it was a genuine Hawken!"

I don't even think the Hawken brothers made a .30 cal rifle.....this has been bandied about on other websites, and the best explanation I have heard is that it was a typo in the script that wasn't caught.
I'll have to watch the elk hunt scene again for the priming the caplock with a powder horn.
There is also a scene somewhere in the movie where he gets two shots out of the gun w/o reloading.....don't remember where it is, though.

I know, it will make no difference to the untrained eye. LOL.

Nope....no difference to the 'untrained' eye, but it's fun for folks such as ourselves to find stuff like that.....like when lugers turn into P-38s and then back again in WWII movies. Just gotta remember that it's for entertainment and not a historical record, that's all!