RML 9-inch 12-ton gun

The RML 9-inch guns Mark I – Mark VI[note 1] 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.

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
TypeNaval gun
Coast defence gun
Place of originUnited Kingdom
Service history
In service1865–1922 (Mk VI)
Used byRoyal Navy
Australian Colonies
Spanish Navy
WarsBombardment of Alexandria
Production history
Designed1865
ManufacturerRoyal Arsenal
Unit cost£740[1] (equivalent to £456,200 in 2013)[2]
VariantsMk I–VI
Specifications
Barrel length125 inches (3.2 m) (bore)[3]
ShellMk I–V : 250 to 256 pounds (113.4 to 116.1 kg) Palliser, Common, Shrapnel[4]
Mk VI : 360 pounds (163.3 kg) AP[5]
Calibre9-inch (228.6 mm)
Muzzle velocity1,420 feet per second (430 m/s)[6]

Design
Diagrams showing the progressive changes in the gun's construction

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).[3]

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,[7] and a forged steel breech-piece. 190 were made.[8]

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".[8]

Mk VI high-angle gun
Former gun positions at Verne High Angle Battery, Portland, England
Spy Glass Battery, Gibraltar

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.[9]

Some guns were bored out and relined in 10-inch calibre. A battery of six such guns is known to have been mounted at Spy Glass Battery on the Rock of Gibraltar, and six guns at Gharghur, Malta.

Ammunition

The projectiles of RML 9-inch guns Marks I-V (the Woolwich rifled guns) had several rows of "studs" which engaged with the gun's rifling to impart spin. Sometime after 1878, "attached gas-checks" were fitted to the bases of the studded shells, reducing wear on the guns and improving their range and accuracy. Subsequently, "automatic gas-checks" were developed which could rotate shells, allowing the deployment of a new range of studless ammunition. Thus, any particular gun potentially operated with a mix of studded and studless ammunition. The Mark VI high-angle gun had polygroove rifling, and was only able to fire studless ammunition, using a different automatic gas-check from the one used with Marks I-V.

The gun's primary projectile was Palliser shot or shell, an early armour-piercing projectile for attacking armoured warships. A large battering charge of 50 pounds P (pebble) or 43 pounds R.L.G. (rifle large grain) gunpowder[10] was used for the Palliser projectile to achieve maximum velocity and hence penetrating capability.

Common (i.e. ordinary explosive) shells and shrapnel shells were fired with the standard full service charge of 30 pounds R.L.G. gunpowder or 33 pounds P (pebble) gunpowder,[10] as for these velocity was not as important.

  • Studded Common shell without gas-check, 1872
  • Studded common shell with gas-check
  • Studless common shell with Rotating gas-check
  • Studless Palliser shell with rotating gas-check
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See also

Surviving examples
A severely corroded Mk III gun at Hurst Castle, UK
A Mark I, 9 Inch MLR Gun at the Middle North Battery, Simonstown. Photograph taken just after gun was fired. Firing sand still in the barrel.

Notes
  1. Mark I – Mark VI = Mark 1 through to Mark 6. Britain used Roman numerals to denote Marks (models) of ordnance until after World War II. Hence this article describes the six models of RML 9-inch guns.

References
  1. Unit cost of £739 17 shillings 8 pence is quoted in The British Navy Volume II, 1882, by Sir Thomas Brassey. Page 38
  2. "Relative worth in 2013: labour cost". Measuring Worth. Archived from the original on 26 August 2014. Retrieved 1 October 2014.
  3. Treatise on Construction of Service Ordnance 1877, page 292
  4. 250 lb projectile is quoted in 1877 Treatise on Ammunition; 253 lb 5 oz in Text Book of Gunnery 1887; 256 lb in Text Book of Gunnery 1902
  5. Hogg & Thurston 1972; Text Book of Gunnery 1902
  6. 1,420 feet/second firing 250-pound projectile with battering charge of 50 pound P (gunpowder). Treatise on Construction of Service Ordnance 1877, page 348
  7. Holley states that Daniel Treadwell first patented the concept of a central steel tube kept under compression by wrought-iron coils.. and that Armstrong's assertion that he (Armstrong) first used a wrought-iron A-tube and hence did not infringe the patent, was disingenuous, as the main point in Treadwell's patent was the tension exerted by the wrought-iron coils, which Armstrong used in exactly the same fashion. Holley, Treatise on Ordnance and Armour, 1865, pages 863–870
  8. Treatise on Construction of Service Ordnance 1877, pages 92–93 and 277–280
  9. Hogg & Thurston 1972, page 158-159
  10. Treatise on Ammunition 1877, page 220

Bibliography

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