BL 18 inch Mk I naval gun

18 inch Mark I
An 18-inch gun fitted to Furious in a single-gun turret (1917)
Type	Naval gun
Place of origin	United Kingdom
Service history
In service	1918
Used by	Royal Navy
Wars	World War I
Production history
Designer	Elswick Ordnance Company
Designed	1915–16
Manufacturer	Elswick Ordnance Company
Produced	1916–17
No. built	3
Specifications
Weight	149 long tons (151 t)
Length	62 feet (18.9 m)
Barrel length	60 feet (18.3 m)L/39
Shell weight	3,320 pounds (1,510 kg)
Caliber	18 inches (460 mm)
Breech	Welin breech block
Recoil	hydro-pneumatic
Elevation	+22° to +45°
Traverse	10°
Muzzle velocity	2,420 ft/s (740 m/s)
Effective firing range	31,400 yards (28,700 m)
Maximum firing range	40,500 yards (37,000 m)
Filling weight	243 pounds (110 kg)

The BL 18-inch Mk I naval gun was a breech-loading naval gun used by the Royal Navy during World War I. It was the largest and heaviest gun ever used by the British. Only the Second-World-War Japanese 40 cm/45 Type 94 had a larger calibre, 18.1 inches (46 cm), but the British shell was heavier. The gun was a scaled-up version of the BL 15 inch Mk I naval gun and was developed to equip the "large light cruiser" (a form of battlecruiser) Furious. Three guns were built, but they did not see combat with Furious, before they were removed from her and transferred to the Lord Clive-class monitors General Wolfe and Lord Clive for coast bombardment duties. Only 85 rounds were fired in anger before the war ended. All three were removed from service in 1920 and served as proving guns for cordite tests. Two were scrapped in 1933 and the last one survived until it was scrapped in 1947.

The 18-inch gun had its genesis in the insistence of the First Lord of the Admiralty, Admiral Fisher, for the biggest possible gun mounted on the fastest possible ship. He conceived of what he called "large light cruisers" carrying four 15-inch (380 mm) guns, which became the Courageous class, but he wanted their half-sister Furious to carry an even bigger gun. The Elswick Ordnance Company was the only company capable of manufacturing such a large gun and began design work in 1915. It was designated as the "15-inch B" to conceal its real size and was derived from the design of the 15-inch Mk I already in service.