I'm from southern West Virginia but that particular feud took place two counties to the west from where I grew up. That was also the place where the snake handler ran out of luck (the snake lacked faith) just the other day.
I was going to say as late as WWI. I think later than WWII is a little too late. Such things were collector's items by then. Remember also, the union troubles in southern West Virginia between the wars that resulted in regular pitched battles between miners and first the "detectives" and later the state militia. They weren't using muzzleloaders then. The militia used machine guns against the miners. However, muzzleloading rifles did hang on.
I don't watch television but according to a review in the paper, the series was filmed in Rumania, of all places.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.