Factual error: Throughout the film Doc Shabbit is carrying a rifle that is supposed to be a Henry repeater. If you look closely it is a Model 1892 Winchester with the forearm removed and a piece of metal welded on the magazine tube to simulate the magazine plunger. The plunger is up against the front of the receiver, which indicates the magazine is empty. Most weapons carried by other characters are pretty much period correct, but a main character has a cobbled up weapon.
Factual error: When Red prepares to sneak into the camp, Doc Shabbit says if they hear a shot it will mean that he "bought the farm". However, this phrase didn't come into use until well into the 20th Century, when the US Military began paying death benefits. That insurance was enough to pay the mortgage or "buy the farm".