RML 7-inch gun

The RML 7-inch guns were various designs of medium-sized rifled muzzle-loading guns used to arm small to medium-sized British warships in the late 19th century, and some were used ashore for coast defence.

Design and history
These guns were the first to incorporate the new "Woolwich" rifling system, a modification of the French system, of from 3 – 9 broad shallow grooves after Britain abandoned the Armstrong "shunt" rifling system in May 1865 : "...M.L. 7 in guns in course of manufacture were rifled on this principle, upon which all of our heavy pieces since have been rifled. The 7 in referred to, and introduced into the service in 1865, were the first of the so-called Woolwich guns, which then meant "wrought iron M.L. guns built up on Sir W. Armstrong's principle, improved upon by hooking the coils over one another, and having solid ended steel barrels, rifled on the system shown above, for studded projectiles".

All versions were constructed of a steel A tube surrounded by various numbers and thicknesses of wrought-iron coils. Rifling was 3 grooves with a uniform 1 turn in 35 calibres i.e. in 245 in.

The diagrams below show the progression from the original expensive Armstrong construction in Mk I of multiple relatively thin coils, through to the simplified and cheaper Woolwich design of Mk III.

RML 7-inch 7 ton gun
This was a coast defence gun introduced in 1865 to replace the failed RBL 7-inch Armstrong gun.Three marks were produced.

RML 7-inch 6½ ton gun
This was a naval gun introduced in 1865 "...as a broadside or pivot gun for frigates, to replace the 7-inch B.L. and 68-pr S.B. guns, and is now very extensively used, 331 having been made... These guns are in total length 18 in shorter than the land service [i.e. 7-ton] 7 in gun, being a length more suited to the requirements of the Navy". Some sources credit these weapons with the ability to pierce up to 7.7 in of armour.

The following warships were armed with the gun :
 * Amazon-class sloops in commission from 1865
 * HMS Bellerophon (1865) commissioned in 1866
 * HMS Vixen (1865) commissioned 1866
 * HMS Viper (1865) commissioned 1866
 * Minotaur-class ironclads commissioned 1867
 * Eclipse-class sloops in commission from 1867
 * Defence-class ironclads (as re-gunned in 1867)
 * HMS Enterprise (1864) & HMS Research (1863) (as re-gunned in 1868)
 * Warrior-class ironclads (as re-gunned in 1867–1868)
 * Hector-class ironclads (from 1868)
 * HMS Achilles (1863) (as re-gunned in 1868)
 * HMS Inconstant (1868) in commission from 1869
 * HMS Shah (1873) in commission from 1876
 * Briton-class screw corvettes in commission from 1871

RML 7-inch 90 cwt gun
This was a lighter (90 cwt = 4½ tons) low-powered naval gun introduced in 1874 as a broadside gun on unarmoured vessels, and not intended for attacking armour plate. Early models were made by simply turning off some of the jacket around 7 in 6½ ton guns, as firing with reduced charges placed less strain on the coils. Some new guns were made to similar design.

The following warships were armed with the gun :
 * Osprey-class sloops in commission from 1874
 * HMS Raleigh (1873) in commission from 1874
 * Fantome-class sloops in commission from 1874
 * Bacchante-class corvettes in commission from 1877
 * Condor-class gunvessels in commission from 1877

Ammunition
The primary projectile for 7 ton and 6½ ton guns was Palliser shot or shell for attacking armoured warships, fired with a large "battering" charge for maximum velocity. All guns were also equipped with shrapnel shells for anti-personnel use and explosive common shells for attacking unarmoured targets. The "double" common shell was much longer than the standard common shell, and hence contained approximately twice as much gunpowder. It was unstable in flight and hence inaccurate beyond 2000 yd but was considered useful for attacking wooden warships at ranges below 2000 yd.

This was the only RML heavy gun not to be issued with gas-checks.

Surviving examples

 * Restored 6½ ton Mk I Numbers 148 & 163 at Garden Island Rockingham, Western Australia (Fleet Base West). For restoration story see http://www.defence.gov.au/news/navynews/editions/4719/feature/feature06.htm
 * Several 6½ ton Mk I guns on Ascension Island
 * 6½ ton Mk I at Fort Siloso, Singapore
 * 6½ ton Mk III of 1869, at Elizabeth Castle, Jersey, UK
 * Remains of several 7 ton Mk III guns on Flat Holm island, UK
 * Nine 7 ton Mk III guns on Steep Holm island, UK
 * A 7-ton Mk I gun at The Citadel, Nova Scotia, Canada
 * A 6½ ton Mk I gun on Signal Hill (Cape Town), South Africa.