BL 13.5 inch naval gun Mk I - IV

Ordnance BL 13.5 inch gun Mk I - IV
The forward 13.5-inch (343-mm) gun turret of the battleship HMS Hood in the 20th Century
Type	Naval gun
Place of origin	United Kingdom
Service history
Used by	United Kingdom Kingdom of Italy
Production history
Designer	Woolwich
Designed	1880
Variants	Mk I, II, III, IV
Specifications
Weight	67-69 tons barrel & breech
Barrel length	405 inches (10.29 m) bore (30 calibres)
Shell	1,250 pounds (570 kg)
Calibre	13.5-inch (342.9 mm)
Muzzle velocity	2,016 feet per second (614 m/s)
Effective firing range	12,000 yards (11,000 m)

The BL 13.5 inch naval gun Mk I ("67-ton gun") was Britain's first successful large breechloading naval gun, initially designed in the early 1880s and eventually deployed in the late 1880s. Mks I - IV were all of 30 calibres length and of similar construction and performance.

The gun was designed to match the new large guns of the French Amiral Baudin-class battleships. Development and manufacture occurred far slower than intended. The first ships armed with the 13.5 in gun were four of the Admiral-class battleships: Anson, Camperdown, Howe and Rodney, which were laid down in 1882-83 and completed in 1888-89. The Howe and the Rodney were laid down to the same dimensions as the preceding Collingwood, which was designed for 12 in 45 ton guns. The increase in weight of the 13.5 in guns, their mountings and ammunition increased draught from 26 feet 4 ¹⁄₂ inches (8.039 m) to 27 feet 10 inches (8.48 m), and increased displacement by 800 long tons (810 t). The Anson and Camperdown were laid down later, to greater dimensions. All four ships carried their 13.5 in guns in twin barbettes on the centreline at each end of the superstructure.

The guns also equipped the subsequent Trafalgar class laid down in 1886 and Royal Sovereign-class battleships laid down in 1889.

A single Mk III gun was mounted as a disappearing gun for coast defence at Penlee Battery, Plymouth.