Back in the heyday of the TV Western, Steve McQueen, the star of “Wanted: Dead or Alive,” rode across black-and-white screens toting a sidearm like nothing any lawman or outlaw had carried before. A cut-down, modified version of repeating rifle, this recognizable shootin’ iron could be holstered like a pistol but fired with the speed of lever-action rifle. Nicknamed the Mare’s Leg,