United States naval gun terminology indicates the gun fired a projectile 5 inches (127 mm) in diameter, and the barrel was 38 calibers long. The 38 caliber barrel was a mid-length compromise between the previous United States standard 5"/51 low-angle gun and 5"/25 anti-aircraft gun. Dual purpose (DP) means that it is designed to be effective against both surface and aircraft targets because it can elevate to 85° and has on mount AA shell fuze setters. On these 5" mounts, single purpose (SP) means that the mount is limited to 35° elevation with no provision for AA shell fuze setters, and is designed to fire at surface targets only. The gun was installed into Single Purpose and dual purpose mounts used primarily by the US Navy. The Mark 12 5"/38 caliber gun was a United States naval gun. World War II, Korean War, Vietnam War, Gulf War, Falklands War, and wars that involved navies who bought surplus World War II, US Navy warships United States Navy, United States Coast Guard, Royal Navy, Danish Navy, Italian Navy, Japan Maritime Self-Defense Force, South Vietnamese Navy, and every navy that bought surplus World War II, US Navy warships
Two Mk 30 single enclosed base ring mounts on USS David W.