https://en.wikipedia.org/wiki/Black_Codes_(United_States) After the Civil War many Democrat dominated Southern state governments enacted Black Codes that regulated virtually every aspect of freed people’s lives. A common element was restricting possession and carrying of firearms by Black people (or by anyone without a license), often implemented through local ordinances, licensing requirements, or explicit prohibitions. The Black Codes precede the Mulford Act by a hundred years.

Which party did “post-war southern Democrats” eventually join?

Hint: https://en.wikipedia.org/wiki/Dixiecrat