S1, E4: The 1873 "trapdoor" Springfield is the first focus of episode four. Also noted early is the Gatling gun that, by 1893, was capable of firing some 800 rounds per minute, spurring an advance in ammunition towards the development of the centerfire cartridge and the push towards the modern wave of firearms. Episode four examines the pioneering work of Smith & Wesson in the development of revolvers with rear loading cylinders, including the famed Models One and Three, the latter of which saw large sales to the Russian army. Meanwhile, Colt produced its Single Action Army 1873, or "Peacemaker," revolver. It was dubbed "the gun that won the West" along with the Winchester 1873 repeating rifle, both chambered in .44 caliber ammunition that was interchangeable between the weapons. Also highlighted is the emergence of other manufacturers such as Remington and Marlin and the introduction of smaller Derringers.