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.
Continuity mistake: In this scene we see several Government Officials talking in a room. One of them, while talking, is playing a game. The game is to balance the lid of an ink bottle on the end of a ruler and over the ink bottle. When the ruler is flicked rapidly gravity would do the rest and the lid would fall into the ink bottle. Things don't work to plan and the lid falls on its side. In the next wide shot, the lid is right side up and a little distant from where it fell earlier.
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.