BL 4 inch Mk IX gun

• Name — Ordnance BL 4-inch Mk IX gun • Type — Naval gun • Is Ranged — yes • Is Artillery — yes • Is Uk — yes • Service — 1917–1945 • Used By — * Royal Navy • Royal Canadian Navy • Free French Navy • Hellenic Navy • Royal Indian Navy • Netherlands Navy • Royal New Zealand Navy • Norwegian Navy • South African Navy • Wars — World War I World War II • Number — 2,382 • Weight — 2 tons barrel & breech • Part Length — 180 in bore (45 calibres) • Cartridge — 31 lb • Caliber — 4 in • Rate — 10-12 rpm • Velocity — 800 m/s • Max Range — 12660 m • Breech — Welin interrupted screw • Elevation — -10 degrees to +30 degrees

• Illustration — Original aft triple mounts on HMS Repulse c. 1916–1917 The gun was based on the barrel of the QF 4-inch Mk V and the breech mechanism of the BL 4-inch Mk VIII and was first introduced in World War I on capital ships as secondary armament in triple-gun mountings, intended to provide rapid concentrated fire. This turned out to be unworkable in practice. Jane's Fighting Ships of 1919 commented, "4-inch triples are clumsy and not liked. They are not mounted in one sleeve; have separate breech mechanism, a gun crew of 23 to each triple". Guns were thereafter used in single-gun mountings, typically on smaller ships as the main armament.

• Illustration — Coast defence gun and crew at Fort Crosby near Liverpool, UK, August 1940 • Illustration — Cleaning the breech on transport St Essylt, Suez 1942 In World War II, the gun was employed on many small warships such as s and minesweepers, primarily for action against surfaced submarines. This was the last BL 4 inch gun in British service: all subsequent guns have used charges in metal cartridges "QF". It was succeeded on new small warships built in World War II by the QF 4-inch Mk XIX gun which fired a slightly heavier shell at much lower velocity and had a high-angle mounting which added anti-aircraft capability.

• On board Sackville, the last surviving, at Halifax, Nova Scotia, Canada • A gun at the entrance to the marina in Hull, UK • A gun at Port Isaac, Cornwall, UK • Leith Harbour, South Georgia.