Along the Oregon Trail (movie, 1947)

The period is the 1840's and Greg Thurston is out to establish his own empire out of a large area of the west. He needs rifles to give to the Indians but Monte Hale breaks up his attack on the supply train. But when they get them by robbing the warehouse, Monte suspects Thuston who had the other key. He follows Thurston only to be caught by him just as Thurston launches his final big attack.