BL 6 inch gun Mk V

Armstrong BL 6-inch 30 calibre gun
Disappearing gun at Fort Chulachomklao, Thailand
Type	Coast defence gun
Place of origin	United Kingdom
Service history
In service	1884 - 1945
Used by	United Kingdom Australian Colonies New Zealand Siam
Production history
Designer	Elswick Ordnance Company (EOC)
Manufacturer	EOC
Unit cost	£3,400
Specifications
Weight	5 tons barrel & breech
Barrel length	183.5 inches (4,661 mm) bore (30.58 calibres); 192 inches (4,877 mm) bore and chamber (32 calibres)
Shell	100 pounds (45.36 kg)
Calibre	6-inch (152.4 mm)
Breech	3 motion interrupted screw. Elswick cup obturation
Muzzle velocity	1,890 feet per second (576 m/s)
Maximum firing range	8,000 yards (7,300 m)

The BL 6 inch gun Mk V was an early Elswick Ordnance Company breech-loading naval gun originally designed to use the old gunpowder propellants. They were used for coast defence around the British Empire.

This was an Elswick Ordnance export design, completely different from and longer (30-calibres, 183.5 inch bore) than the contemporary 26-calibres British naval service 6-inch Mk III, IV and VI guns designed by the Royal Gun Factory, although it fired the same 100-pound projectiles. The gun was of a complex all-steel built-up construction, of a steel A-tube surrounded by multiple steel hoops, breech-piece and jacket.

Several were acquired by the British government for coast defence in the UK and were given the designation 6-inch gun Mark V. The breech fittings and firing mechanism were modified in British service to standardize them with the British service guns, Mark IV and VI. The breech-screw was locked by turning to the left, unlike standard service guns made by the Royal Gun Factory, which all locked to the right. Rifling consisted of 28 grooves of the polygroove "Elswick section" type, increasing from 0 to 1 turn in 30 calibres (i.e. 1 turn in 180 inches) at the muzzle.

They were also exported for use as coast-defence guns in Hong Kong, New Zealand, Australian colonies and Siam (Thailand), typically as disappearing guns.

During the 1890s, when the new "QF" technology of loading propellant charges in brass cases to increase the rate of fire was in favour, 4 guns were returned from New South Wales, Australia to the UK to be converted to QF.

2 of the resulting QFC guns are known to have been still in commission until 1945, in the Princess Royal Fortress defending the port of Albany, Western Australia.