Factual error: John Blair rides for the pony express, which operated until October, 1861. However, all of the handguns in the movie are cartridge Colts, which did not come into existence until 1871.
Factual error: When McNelly dictates his last will and testament, he leaves, among other items, his Navy Colts to Dunnison. He may well have a couple of Navies stashed in his belongings somewhere, but the pistols he wore and used were .44 peacemakers.