BL 6 inch gun Mk I - VI

Ordnance BL 6 inch gun Mks II, III, IV, VI
Mk IV gun on disappearing carriage at Lei Yue Mun Fort, Hong Kong
Type	Naval gun Coast defence gun
Place of origin	United Kingdom of Great Britain and Ireland
Service history
In service	1880 - 1905
Used by	Royal Navy
Production history
Designer	Royal Gun Factory (RGF)
Manufacturer	RGF and EOC
Variants	Mks II, III, IV, VI
Specifications
Weight	Mk II : 81 cwt or 89 cwt (4½ tons) Mks III, IV, VI : 5 tons barrel & breech
Barrel length	Mk III : 153.2 inches (3,891 mm) (25.53 calibres) Mk IV, VI : 156 inches (3,962 mm) (26 calibres)
Shell	100 pounds (45.36 kg)
Calibre	6-inch (152.4 mm)
Breech	3 motion interrupted screw. De Bange obturation.
Muzzle velocity	Mk III, IV, VI : 1,960 feet per second (597 m/s) QFC guns : 1,913 feet per second (583 m/s) BLC guns : 2,166 feet per second (660 m/s)
Maximum firing range	10,000 yards (9,100 m)

The BL 6 inch guns Marks II, III, IV and VI were the second and subsequent generations of British 6-inch rifled breechloading naval guns, designed by the Royal Gun Factory in the 1880s following the first 6-inch breechloader, the relatively unsuccessful BL 6 inch 80 pounder gun designed by Elswick Ordnance. They were originally designed to use the old gunpowder propellants but from the mid-1890s onwards were adapted to use the new cordite propellant. They were superseded on new warships by the QF 6 inch gun from 1891.

These were Royal Gun Factory designs, although they were also manufactured by Elswick Ordnance.

Mk II followed the early weakly made and less powerful Mark I 80-pounder and introduced a 100-pound projectile, which became standard for British 6-inch guns until 1930. It consisted of a much thicker steel barrel with wrought-iron jackets shrunk over it and as originally introduced weighed 81 cwt (9720 pounds). The gun proved to be too weakly constructed, and 5 steel chase hoops were added to strengthen it and the gun was shorted by 12 inches to rebalance it, resulting in a bore length of 144 inches (24 calibres) and final weight of 89 cwt (9968 pounds), or 4½ tons. These guns were relegated to non-firing drill use following a burst gun incident on HMS Cordelia in June 1891.

Mark III finally introduced an all-steel construction, with a steel barrel and steel breech-piece and hoops shrunk over it, weighing 89 cwt (4½ tons). However, as originally introduced Mk III was still limited to weak charges and low muzzle velocity, and most guns were strengthened by being chase-hooped to allow a full powder charge of 48 lb gunpowder and muzzle velocity of 1,960 feet per second. This brought the gun weight up to 100 cwt (5 tons).

Mk IV incorporated the improvements to Mk III. Mk VI differed from Mk IV only in having slightly simplified construction.

Marks III, IV and VI became the most commonly deployed versions, and their widespread adoption would indicate they were considered successful. Marks III, IV and VI were interchangeable and had the same performance. They are generally referred to as "6-in 5-ton B.L.R." in contemporaneous publications such as Brassey's Naval Annual.