BL 6 inch naval guns Mk XIII – XVIII

Type	Naval gun
Place of origin	United Kingdom
Service history
In service	1914 - 1940s
Wars	World War I World War II
Production history
Designer	Mk XIII, XVII, XVIII : EOC Mk XIV, XV, XVI : Vickers
Variants	Mk XIII, XIV, XV, XVI, XVII, XVIII
Specifications
Barrel length	300 inches (7.620 m) bore (50 cal)
Shell	100 pounds (45.36 kg)
Calibre	6 inches (152.4 mm)
Muzzle velocity	Mk XIII : 2,770 feet per second (840 m/s) Mk XIV & XV : 2,900 feet per second (880 m/s) Mk XVI : 3,000 feet per second (910 m/s) Mk XVII : 2,905 feet per second (885 m/s) Mk XVIII : 2,874 feet per second (876 m/s)

This article covers a number of fairly similar British-built 6-inch 50-calibre naval guns that found their way into temporary British service in World War I. Their specifications and performance differed from standard Royal Navy 6-inch guns but in British service they fired standard service 100-pound projectiles.

When World War I began, Armstrong-Whitworth were completing the battleship Sultan Osman I for the Ottoman Empire, originally begun as Rio de Janeiro for Brazil. Its secondary armament consisted of 20 of their 6-inch 50 calibre guns, which were similar in design and characteristics to the British naval service Mk XI gun and used the same ammunition. The battleship was completed as HMS Agincourt and served in the Royal Navy in World War I, with its guns designated BL 6 inch Mk XIII.

After the decommissioning of Agincourt in 1921 her 6-inch guns were used for coastal defence, and in 1939 the British gunboats Aphis and Ladybird each had their 2 6-inch Mk VII guns replaced with 2 MK XIII guns, in which capacity they served in World War II.

When World War I began, the Royal Navy had just taken possession of 3 river monitors originally built by Vickers for Brazil but which Brazil could not pay for. These were each equipped with 2 Vickers 6-inch 50-calibre guns in a twin turret, and became the Humber-class monitors. These guns were very similar to the British service Mk XI gun, and in UK service they were designated BL 6 inch Mk XIV (right-hand gun, with breech opening to the right) and BL 6 inch Mk XV (left-hand gun, with breech opening to the left). The guns of HMS Severn and HMS Mersey wore out early in the war from continuous use bombarding the Belgian coast and in East Africa, and were replaced by standard 6-inch Mk VII guns, while HMS Humber continued to use various refurbished Mk XIV and XV guns throughout the war for coastal bombardment.

At the beginning of World War I the British Government forcibly acquired the battleship Reshadieh which Vickers had just completed for the Ottoman Government. The ship entered service with the Royal Navy as HMS Erin together with its armament which while of similar calibre to British service ordnance was of differing specification. Erin had sixteen 6-inch 50 calibre guns made by Vickers, with a muzzle velocity of 3,000 feet per second (910 m/s) which was considerably greater than existing standard Royal Navy 6-inch guns. These guns were designated 6-inch BL Mark XVI in British service. Some were redeployed in World War II for coast defence.

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Wikipedia