Factual error: The rifles that Jake Cutter has Paul Regret dig up are Henry repeaters, or possibly 1866 Winchesters. Either way weapons unavailable in the period depicted in the movie.
Other mistake: When they're roping the mustangs, the "stallion" doesn't have the genitals of a stallion.
Other mistake: Brian Keith sneaks into an Indian encampment at night while they are sleeping. In order to stampede their horses, he rapidly fires the one and only six-shooter revolver he has into the air...eight times.