The BL 8 inch gun Mark VIII was the main battery gun used on the Royal Navy's cruisers, in compliance with the Washington Naval Treaty of 1922. This treaty allowed ships of not more than 10,000 tons standard displacement and with guns no larger than to be excluded from total tonnage limitations on a nation's capital ships. The 10,000 ton limit was a major factor in design decisions such as turrets and gun mountings. A similar gun formed the main battery of Spanish cruisers. In 1930, the Royal Navy adopted the BL 6 inch Mk XXIII naval gun as the standard cruiser main battery in preference to this 8-inch gun.
These guns, 50 calibres long, were built-up guns which consisted of a wire-wound tube encased within a second tube and jacket with a Welin breech block and hydraulic or hand-operated Asbury mechanism. Two cloth bags each containing of cordite were used to fire a projectile. Mark I turrets allowed gun elevation to 70 degrees to fire high-explosive shells against aircraft. Hydraulic pumps proved incapable of providing sufficient train and elevation speed to follow contemporary aircraft; so simplified version of the Mark II turrets with a maximum elevation of 50 degrees were installed in . Each gun could fire approximately five rounds per minute. Useful life expectancy was 550 effective full charges (EFC) per barrel.
The following ships mounted Mk VIII guns in 188-tonne twin turrets. The standard main battery was four turrets, but Exeter and York carried only three to reduce weight and formed the separate York class.
Six single guns capable of elevating to 70 degrees were installed as coastal artillery in the Folkestone-Dover area during the Second World War.
Range with 256 lbs. (116.1) SAPC with MV = 2,725 fps (831 mps)