The 3 inch 50 caliber Mk 21 (76mm) dual purpose cannon deck gun was standard issue on board many United States Navy Submarines, Destroyers and Destroyer Escorts (DE) during the early stages of World War II. By US Navy standards, it was generally considered to be a defensive weapon designed to be used against aircraft that were far away or very high up, although on a submarine it was primarily employed against enemy surface ships. Mounted on a pedestal either forward or aft of the conning tower, this weapon could fire a 13 lb high explosive projectile with a muzzle velocity of 2,700 feet per second and a maximum range of just over 16,000 yards with a ceiling of 21,500 feet. This gun could elevate to 85° and depress to 10°. Cartridges were packed four to a box, the weight of a full box being about 125 lbs with cartridges weighing approximately 32lbs each. The 3 inch 50 had a watertight tampion for the muzzle and a watertight cover for the breech with stainless steel mechanisms and bore. This weapon could be used with equally deadly effect against both surface and aerial targets.