The BL 9.2-inch Mk IX and Mk X guns were British breech loading guns of 46.7 calibre, in service from 1899 to the 1950s as naval and coast defence guns. They had possibly the longest, most varied and successful service history of any British heavy ordnance. History The Mk IX/X guns succeeded the BL 9.2-inch Mk VIII naval gun and increased the bore length from 40 to 46.7 calibres, increasing the muzzle velocity from . The Mk IX was designed as a coast defence gun, with a three-motion breech. Only fourteen were built and the Mk X, introduced in 1900 and incorporating a single-motion breech and changed rifling, succeeded them. As coastal artillery, the Mk X remained in service in Britain until 1956, and in Portugal until 1998. A total of 284 of the Mark X version were built by Vickers, of which 25 examples are known to survive today, all except one fitted on barbette mounts. The exception is a gun in Cape Town, South Africa on a disappearing mount. Another four guns manufactured by Elswick Ordnance Company survive, mounted in turrets. The 9.2-inch Mk XI gun introduced in 1908 increased the bore length to 50 calibres in an attempt to increase the velocity still further, but proved unsuccessful in service and was phased out by 1920. The Mk X was hence the final Mark of 9.2-inch guns in widespread British Commonwealth service. The BL 9.2 also affected the development of radar. Immediately before the Second World War, an army team developed a surface-scanning radar to detect enemy ships in the English Channel. By chance, in July 1939 one of these radars was being tested while the 9.2-inch battery at Brackenbury Battery outside Harwich was firing. The radar team noticed odd returns on their displays, and realised that these were due to the waterspouts caused by the shells exploding in the water creating a vertical surface off which the signals reflected. The use of radar to accurately spot the fall of shells was rapidly developed, and was widespread by the war's end. Design These were medium-velocity wire-wound guns with Welin interrupted screw breeches. Naval service Royal Navy Mark X guns were mounted on : Cressy-class armoured cruisers commissioned from 1901 Drake-class armoured cruisers commissioned from 1902 Duke of Edinburgh-class armoured cruisers commissioned from 1906 Warrior-class armoured cruisers commissioned from 1907 King Edward VII-class battleships commissioned from 1905 M15-class monitors