RML 7 inch 90 cwt gun

Ordnance RML 7 inch gun
7 inch 6½ ton Mk I gun and crew on HMS Minotaur
Type	Naval gun Coast defence gun
Place of origin	United Kingdom
Service history
Used by	Royal Navy
Production history
Designer	Royal Gun Factory
Designed	1865 (7 & 6½ ton) 1874 (90 cwt)
Manufacturer	Royal Arsenal
Unit cost	£425
Variants	7 ton Mks I - IV 6½ ton Mks I - III 90 cwt
Specifications
Barrel length	7 ton : 126 inches (3,200 mm) 6½ ton & 90 cwt : 111 inches (2,800 mm)
Shell	112 to 115 pounds (51 to 52 kg) Palliser, Common, Shrapnel 160 pounds (73 kg) double common shell
Calibre	7-inch (177.8 mm)
Muzzle velocity	7 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 range	5,500 yards (5,000 m)

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.

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-inch guns in course of manufacture were rifled on this principle, upon which all of our heavy pieces since have been rifled. The 7-inch 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 inches.

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.

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

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 inches shorter than the land service [i.e. 7-ton] 7-inch gun, being a length more suited to the requirements of the Navy".

The following warships were armed with the 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-inch 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 :