4.7 inch QF Mark IX & XII

QF 4.7 inch Mark IX & Mark XII
A twin Mk.XII mounting on HMS Javelin. The cylindrical Fuse Setting Pedestal is clearly visible on the right, at waist height.
Type	Naval gun
Place of origin	United Kingdom
Service history
In service	1928–1970?
Used by	Royal Navy Royal Canadian Navy Royal Australian Navy Royal Hellenic Navy Royal Netherlands Navy Polish Navy Royal Norwegian Navy Turkish Navy Dominican Navy Argentine Navy Brazilian Navy
Wars	World War II Korean War
Production history
No. built	742 (Mk IX); 372 (Mk XII)
Specifications
Weight	2.963–2.984 long tons (3,011–3,032 kg) (Mk IX) 3.238–3.245 long tons (3,290–3,297 kg) (Mk XII)
Length	220.62 in (5.60 m) (Mk IX) 224.08 in (5.69 m) (Mk XII)
Barrel length	Bore: 212.58 in (5.40 m) L/45 (cal)
Shell	Separate-loading
Shell weight	50 pounds (22.7 kg) SAP or HE
Calibre	4.724 inches (120 mm)
Breech	Semi-automatic horizontal sliding-block
Recoil	Hydro-pneumatic
Elevation	Varied by mounting
Rate of fire	about 12 rounds per minute
Muzzle velocity	2,650 ft/s (810 m/s)
Maximum firing range	16,970 yards (15,520 m) at 40°

The 4.7 inch QF Mark IX and Mark XII were 45-calibre, 4.7-inch (120 mm) naval guns which armed the majority of Royal Navy and Commonwealth destroyers in World War II, and were exported to many countries after World War II as the destroyers they were mounted on were sold off.

These guns succeeded the similar World War I-era BL 4.7 inch gun, changing the cartridges from BL silk bags to separate QF in brass cases and a new horizontal sliding block breech mechanism.

Mark IX was deployed in single mountings CP Mk XIV on the A-class destroyers of 1930 and on most subsequent destroyer classes up to and including the R class of 1942.

The almost-identical Mk XII gun was deployed in twin mountings CP Mk XIX on the Tribal class destroyers of 1936 and J, K and N classes of 1938. This mounting limited the maximum elevation to 40 degrees, but all twin CP Mk XIX were dual-purpose mountings and were equipped with Fuze Setting Pedestals or Mk V Fuze Setting Trays, to allow the mountings to be fired against aircraft while being controlled by the Fuze Keeping Clock fire control computer. Typical maximum rate of fire was twelve rounds per gun, per minute. During gunnery trials in 1930, HMS Basilisk'' was able to fire "...five rounds in 17 seconds." The Mk XII gun fired a 50 lb shell and used a separate cartridge, with both shell and cartridge being loaded via a loading tray, with power ramming, elevation, and traverse. The maximum range at 40 degrees elevation was 16,970 yards (15,520 m) fired at the new gun muzzle velocity of 2,650 fps (808 m/s). The 40-degree elevation was justified on the grounds that destroyers would be screening the battle-fleet during aerial attack, and 40 degrees elevation was adequate to engage aircraft that were concentrating their attack on other ships.