Law of the Sixgun (1978)

A young man bent on avenging his father's death accepts the job of town marshal, which immediately puts him in conflict with the Manning brothers who lead a gang of rustlers and murders. Quickly realizing that the new marshal is too good with a gun for any of them to handle, the Mannings send for the notorious gunfighter, the Rio Kid. Rio discovers that the marshal is, in fact, his long-lost brother and sides with the young lawman to clean up the territory.