| 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 Canadian NavyFree French Navy  Hellenic Navy Royal Indian Navy Netherlands Navy Royal New Zealand Navy Norwegian NavySouth African Navy |
| Wars: | World War I World War II |
| Number: | 2,382[1] |
| Weight: | 2 tons barrel & breech[2] |
| Part Length: | 180inches bore (45 calibres) |
| Cartridge: | 31lb |
| Caliber: | 4inches |
| Rate: | 10-12 rpm[3] |
| Velocity: | 800m/s |
| Max Range: | 12660m (41,540feet) |
| Breech: | Welin interrupted screw |
| Elevation: | -10 degrees to +30 degrees[4] |
The BL 4-inch Mk IX naval gun[5] was a British medium-velocity naval gun introduced in 1917 as secondary armament on the battlecruisers and "large light cruisers", but which served most notably as the main armament on s throughout World War II.
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[6] 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".[7] Guns were thereafter used in single-gun mountings, typically on smaller ships as the main armament.
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.