I remember The Big Valley (65-69 - looked it up) and thinking even then that it was cheesy horse opera. Outside of their agents negotiating a lucrative contract I wouldn't expect Hollywood players to get anything right.
TV cast, crew and produc-ahs have no interest in firearms. Accurately portraying historic events and historical time periods or ballistics is definably not what they do. Its unrealistic to expect otherwise (not that anyone actually expects something better). Sometimes, knowing something about a subject brings its own challenges and frustrations. Maybe ignorance really is bliss.
My own favorite screw-up is in Dances With Wolves (a pretty good flick, mostly) where a character begins a scene with an 1860 (good start) but finishes it with a Remington, You can just hear them, if they noticed: But whos to care? Dont be a schlemiel - nobody. A friggin gun's a gun already, what.