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. It should not be confused with the RML 9-inch Armstrong Gun, used by the Dutch navy, the Spanish Navy, and other navies.
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 in (1,030 cm)).[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]
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 in (126 cm). These guns fired a special 360 lb (160 kg) armour-piercing shell to a range of 10,500 yd (9.6 km) using a propellant charge of 14 lb (6.4 kg) 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 in (25 cm) 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 in (23 cm) 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 lb (23 kg) P (pebble) or 43 lb (20 kg) 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 lb (14 kg) R.L.G. gunpowder or 33 lb (15 kg) P (pebble) gunpowder,[10] as for these velocity was not as important.
A severely corroded Mk III gun at Hurst Castle, UKA Mark I, 9 in (23 cm) MLR Gun at the Middle North Battery, Simonstown. Photograph taken just after gun was fired. Firing sand still in the barrel.
^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.
^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
^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
^ abTreatise on Construction of Service Ordnance 1877, pages 92–93 and 277–280
