Kid Jr. was the name of Irving "Kid" Tannen's gun. It was a revolver resembling the Colt Peacemaker that featured a long "Cavalry" barrel. Tannen was seen using the weapon in 1931. He used it to deal with many enemies during his time as a gangster, including an attempt on Marty McFly's life in an alternate 1986. In the timeline when Marty helped bring Kid to justice, he fumbled and lost the pistol after failing to hold a young Emmett Brown hostage.