The Mark 12 5"/38 caliber gun was a US naval gun. The gun was installed into Single Purpose and Dual Purpose mounts used primarily by the US Navy. 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, while 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. 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. United States naval gun terminology indicates the gun fired a projectile in diameter, and the barrel was 38 calibers long, making the 5"/38 dual purp