Factual error: The opening montage depicting Books's past exploits mentions a Colt .45 and shows a single action Colt revolver being fired. The date indicated on the screen is 1871. This revolver was not manufactured until 1873.
Factual error: The gatling gun that Josey Wales uses had the options of a 40 round magazine, or a 104 round drum magazine. Neither Josey or the Red Legs ever bothered to reload the weapon. The gun would have been empty after the 10 barrels revolved at most 10 times. Josey alone shoots for a couple of minutes straight.