5&quot;/54 caliber Mark 42 gun5&quot;/54 caliber Mark 42 gun 5"/54 caliber Mark 42 gun The Mark 42 5"/54 caliber gun (127mm) is a naval gun (naval artillery) mount used by the United States Navy and other countries. It consisted of the Mark 18 gun and Mark 42 gun mount. United States naval gun terminology indicates the gun fires a projectile in diameter, and the barrel is 54 calibers long (barrel length is 5" × 54 = 270" or 6.9 meters.) In the 1950s a gun with more range and a faster rate of fire than the 5"/38 caliber gun used in World War II was needed, therefore, the  gun was created concurrently with the 3"/70 Mark 26 gun for different usages. The 5"/54 Mk 42 is an automatic, dual-purpose (air / surface target) gun mount. It is usually controlled remotely from the Mk 68 Gun Fire Control System, or locally from the mount at the One Man Control (OMC) station. The self-loading gun mount weighs about including two drums under the mount holding 40 rounds of semi-fixed case type ammunition. The gun fires projectiles at a velocity of . Maximum rate of fire is 40 rounds per minute. Magazine capacity is 599 rounds per mount. The Mark  42 mount originally was equipped for two on-mount gunners, one surface and one antiaircraft, but the antiaircraft gunner position was scrapped later on when the increasing speed of naval aircraft made manual aiming of anti aircraft weapons impractical. The Mark 45 lightweight () gun mount began replacing the Mk 42 mount in 1971 for easier maintenance and improved reliability in new naval construction for the United States Navy. 5"/54 caliber Mark 42 gun The Mark 42 5"/54 caliber gun (127mm) is a naval gun (naval artillery) mount used by the United States Navy and other countries. It consisted of