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.

RML 7-inch gun
7-inch 6½ ton Mk I gun and crew on HMS Minotaur
TypeNaval gun
Coast defence gun
Place of originUnited Kingdom
Service history
Used byRoyal Navy
Production history
DesignerRoyal Gun Factory
Designed1865 (7 & 6½ ton)
1874 (90 cwt)
ManufacturerRoyal Arsenal
Unit cost£425[1]
Variants7 ton Mks I – IV
6½ ton Mks I – III
90 cwt
Specifications
Barrel length7 ton : 126 inches (3,200 mm)
6½ ton & 90 cwt : 111 inches (2,800 mm)[2]
Shell112 to 115 pounds (51 to 52 kg) Palliser, Common, Shrapnel[3]
160 pounds (73 kg) double common shell
Calibre7-inch (177.8 mm)
Muzzle velocity7 ton : 1,561 feet per second (476 m/s)
6½ ton : 1,525 feet per second (465 m/s)
90 cwt : 1,325 feet per second (404 m/s)
Maximum firing range5,500 yards (5,000 m)

Design and history
"Woolwich" rifling introduced in 1865

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 (18 cm) guns in course of manufacture were rifled on this principle, upon which all of our heavy pieces since have been rifled. The 7 in (18 cm) 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".[4]

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 (620 cm).[2]

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.

  • The 4 Marks of 7 ton gun
    The 4 Marks of 7 ton gun
  • The 3 Marks of 6½ ton gun
    The 3 Marks of 6½ ton gun
  • 90 cwt gun
    90 cwt gun

RML 7-inch 7 ton gun
Remains of a 7-ton Mk III gun on Flat Holm island, UK

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 (46 cm) shorter than the land service [i.e. 7-ton] 7 in (18 cm) gun, being a length more suited to the requirements of the Navy".[5] Some sources credit these weapons with the ability to pierce up to 7.7 inches (196 mm) of armour.[6]

The following warships were armed with the gun :

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 (18 cm) 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 :

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 2,000 yd (1.8 km) but was considered useful for attacking wooden warships at ranges below 2,000 yd (1.8 km).[7]

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

Surviving examples

See also

Notes and references
  1. Unit cost of £424 11 shillings 10 pence is quoted for the 7-inch 6½ ton gun in "The British Navy" Volume II, 1882, by Sir Thomas Brassey. Page 38
  2. Treatise on construction of ordnance in the British service, 1877, Table XXIX
  3. Treatise on Ammunition 1877 and Treatise on construction of ordnance in the British service, 1877 describe a 115-pound projectile; Text Book of Gunnery 1887 and Brassey's Naval Annual 1888 describe a 112-pound 1 oz projectile. 112 lb 1 oz appears to be the weight of the empty Palliser shell i.e. without gunpowder filling which was typically 2 lb 10 oz. By 1887 palliser shell had been discontinued and existing shell converted to shot by removing the gunpowder – hence old Palliser ammunition would then be 112 lb 1 oz.
  4. Treatise on construction of ordnance in the British service, 1877, Page 91
  5. Treatise on construction of ordnance in the British service, 1877, Page 269
  6. Conway's All the World's Fighting Ships 1860-1905, 1979, Page 9
  7. Treatise on Ammunition 1877, Page 189

Bibliography
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