RML 9 inch 12 ton gun

Ordnance RML 9 inch 12 ton gun
Restored Mark I, RML 9 inch 12 ton gun being fired at Simon's Town in 2014, with replica ammunition in the foreground
Type	Naval gun Coast defence gun
Place of origin	United Kingdom
Service history
In service	1865–1922 (Mk VI)
Used by	Royal Navy Australian Colonies Spanish Navy
Wars	Bombardment of Alexandria (1882)
Production history
Designed	1865
Manufacturer	Royal Arsenal
Unit cost	£740 (equivalent to £456,200 in 2013)
Variants	Mk I–VI
Specifications
Barrel length	125 inches (3.2 m) (bore)
Shell	Mk I–V : 250 to 256 pounds (113.4 to 116.1 kg) Palliser, Common, Shrapnel Mk VI : 360 pounds (163.3 kg) AP
Calibre	9-inch (228.6 mm)
Muzzle velocity	1,420 feet per second (430 m/s)

The RML 9 inch guns Mark I – Mark VI were large rifled muzzle-loading guns of the 1860s used as primary armament on smaller British ironclad battleships and secondary armament on larger battleships, and also ashore for coast defence.

The rifling was the Woolwich pattern of a relatively small number of broad, rounded shallow grooves : there were 6 grooves, increasing from 0 to 1 turn in 45 calibres (i.e. 405 inches).

Mark I, introduced in 1865, incorporated the strong but expensive Armstrong method of a steel A tube surrounded by multiple thin wrought-iron coils which maintained the central A tube under compression, and a forged steel breech-piece. 190 were made.

Mark II in 1866 incorporated the modified Fraser design. This was an economy measure, intended to reduce the costs incurred in building to the Armstrong design. It incorporated fewer but heavier wrought-iron coils but retained the Armstrong forged breech-piece. Only 26 were made.

Mark III in 1866–1867 eliminated the Armstrong forged breech piece and hence fully implemented the Fraser economy design. It consisted of only 4 parts : steel A tube, cascabel, B tube and breech coil. 136 were made.

Mark IV, introduced 1869, and V incorporated a thinner steel A tube and 2 breech coils. The explanation for separating the heavy breech coil of Mk III into a coiled breech piece covered by a breech coil was "the difficulty of ensuring the soundness of the interior of a large mass of iron".

In the late 1880s and early 1890s a small number of guns were adapted as high-angle coast defence guns around Britain : known battery locations were Tregantle Down Battery at Plymouth, Verne High Angle Battery at Portland and Steynewood Battery at Bembridge on the Isle of Wight.

The idea behind these high-angle guns was that the high elevation gave the shell a steep angle of descent and hence enabled it to penetrate the lightly armoured decks of attacking ships rather than their heavily armoured sides. To increase accuracy the old barrels were relined and given modern polygroove rifling : 27 grooves with a twist increasing from 1 turn in 100 calibres to 1 turn in 35 calibres after 49.5 inches. These guns fired a special 360-pound armour-piercing shell to a range of 10,500 yards using a propellant charge of 14 lb Cordite Mk I size 7½, remained in service through World War I and were not declared obsolete until 1922.