The BL 13.5-inch Mk VI gun was a British heavy naval gun, originally ordered by the Ottoman Navy to equip its Reşadiye-class dreadnoughts around 1911. The one ship completed was seized by the British Government when World War I began in August 1914 and became HMS Erin. Only 10 guns were built, to a design similar to that of their 45-calibre BL 13.5-inch Mk V naval gun. The smaller chamber in the Mk VI gun meant that less propellant could be used which reduced muzzle velocity by 55 ft/s (17 m/s) and range by 630 yards (580 m). Designed by Vickers, they manufactured the guns of X and Y turrets, while Elswick Ordnance Company manufactured the guns of A, B, and Q turrets.

• 340mm/45 Modèle 1912 gun French equivalent

• Campbell, N.J.M. (1981). "British Naval Guns 1880–1945, Number Two". In Roberts, John (ed.). Warship V. London: Conway Maritime Press. ISBN 0-85177-244-7.
• Friedman, Norman (2011). Naval Weapons of World War One. Barnsley, UK: Seaforth Publishing. ISBN 978-1-84832-100-7
• Johnson and Buxton (2013). The Battleship Builders. Barnsley, UK: Seaforth Publishing. ISBN 978-1-84832-093-2

13.5-inch Mk VI on Navweaps