The 4″/50 caliber gun (spoken "four-inch-fifty-caliber") was the standard low-angle, quick-firing gun for United States, first appearing on the monitor Arkansas and then used on "Flush Deck" destroyers through World War I and the 1920s. It was also the standard deck gun on S-class submarines, and was used to rearm numerous submarines built with 3-inch (76 mm) guns early in World War II. United States naval gun terminology indicates the gun fired a projectile 4 inches (100 mm) in diameter, and the barrel was 50 calibers long.