 These were built-up guns with a tube, partial-length jacket, hoop and vertical sliding breech block.Dual-purpose 3″/50 caliber guns (Marks 10, 17, 18, and 20) first entered service in 1915 as a refit to , and were subsequently mounted on many types of ships as the need for anti-aircraft protection was recognized.During World War II, they were the primary gun armament on destroyer escorts, patrol frigates, submarine chasers, minesweepers, some fleet submarines, and other auxiliary vessels, and were used as a secondary dual-purpose battery on some other types of ships, including some older battleships.