The EOC 12 inch 45 calibre gun were various similar 12-inch wire-wound naval guns designed and manufactured by Elswick Ordnance Company to equip ships that the parent company Armstrong Whitworth built and/or armed for several countries before World War I.
Elswick supplied its 12-inch 45-calibre guns for the s completed by itself and Vickers in 1910 for Brazil.
When World War I began, Elswick were completing the battleship for the Ottoman Empire, originally begun as for Brazil. It was armed with 14 of a slightly later version of Elswick's 12-inch 45-calibre guns. The battleship was completed as and served in the Royal Navy in World War I, with its guns designated BL 12 inch Mk XIII. The gun's performance was similar to the standard Royal Navy equivalent gun, the BL 12 inch Mk X designed by Vickers.
Elswick supplied its 12-inch 45-calibre guns to the Imperial Japanese Navy, and they were also manufactured under licence in Japan. In Japanese service from 1908 they became 12"/45 41st Year Type and later after the navy metricised in 1917, 30 cm/45 41st Year Type. They equipped the following ship classes: