Factual error: The rifle model which the rifleman used was manufactured in 1892, but the film takes place in 1881.
Factual error: Lucas is constantly twirling the rifle after rapid firing a few shots. Since the cocking ring is set on semi-automatic, the rifle should fire as it comes around.
Suggested correction: He twirls the rifle in the opening of the show, after he's just emptied it completely.
The original comment doesn't state the beginning of the show. This does happen throughout the series.