The BL 13.5 inch Mk V gun was a British heavy naval gun, introduced in 1912 as the main armament for the new super-dreadnought battleships of the Orion class. The calibre was 13.5 inches (343 mm) and the barrels were 45 calibres long i.e. 607.5 inches (15.43 m). The guns were greatly superior to the earlier 13.5-inch (30-calibre) Mk I to Mk IV guns used on the Admiral, Trafalgar and Royal Sovereign classes completed between 1888 and 1896.