It was Britain's first attempt to match the large guns being installed in rival European navies, particularly France, after Britain transitioned from rifled muzzle-loading guns to the modern rifled breech-loaders somewhat later than the European powers.

Their barrels were 328.5 inches (8.344 m) (L/27.5) long and the guns used the same charge as the later all-steel models up to Mark VII.

After the British government returned to using muzzle-loading guns in 1864/5, it was quite content with its muzzleloaders made by the Royal Arsenal at Woolwich.

The long guns that Krupp tested in Meppen in 1878 and 1879 then proved that muzzle loading had become a dead end.

When EOC became aware of Krupp's 24 cm MRK L/25.5, tested in Meppen in 1879, it started to develop a 9-inch gun of 18 tons.

[7] Meanwhile Vavasseur of the smaller London Ordnance Works was busy manufacturing all steel breechloaders.

[9] It meant that the arsenal did not follow the usual practice of first testing new developments (breechloading, longer barrels) with smaller calibers.

By July 1880, a 12-inch rifled breech-loading gun of 43 ton was in advanced state of construction.

The yoke was bent horizontally so that it could move on rollers with vertical axes in grooves in the roof and floor of the casemate.

He wrote that: Consequently, the longitudinal strain on the Woolwich gun is much greater for the same pressure, it is obvious however, that that breech is specially strong.

George Trevelyan, Parliamentary and Financial Secretary to the Admiralty assured British Parliament that the new battleships Conqueror (1881), Colossus (1882), Collingwood (1882), and Edinburgh (1882) (ex-Majestic) would all carry the 43 ton gun.

[15] In May 1881, the new 'Ordnance Committee' endorsed a suggestion by Colonel Maitland, Superintendent of the Royal Gun Factory, for more extensive use of steel in future designs for higher calibers.

Maitland also suggested that the breech openings of several guns should be enlarged to admit cartridges of slow burning powder that had the same diameter as the chamber.

[23] In May 1882, Sir William Armstrong of the Elswick Ordnance Company objected to the design of the gun when he was invited to make an offer to manufacture them by contract.

Furthermore, the admiralty urgently wanted to complete the above ships, and therefore liked to use the Mark II pattern for six guns which where on order.

A big problem was that the Woolwich arsenal could not cast ingots greater than those required for a 12-inch gun.

[30] In August 1882, the government admitted that due to questions regarding the gun and its carriage, this planning would not be met.

It was also lined with a steel cylinder extending from the seat of the obturator to a length of 189 inches (480 cm).

The Mark II, intended for Sea Service, had ribs, which allowed to mount the guns closer together on a ship.

Mk II guns failed in service and were quickly replaced by Mks III.

The later marks were also mounted on Hero (1885), sistership of Conqueror, and on Edinburgh (1882), sister ship of Colossus.

[21] Collingwood (1882) had four Mark II guns mounted in barbette (i.e. open) towers.

On 4 May 1886, she sailed from Spithead to test the Vavasseur mountings of her aft 12-inch 43-ton Mark II guns.

It also thought that the large scale introduction of steel probably contributed to the guns being too weak near the muzzle.

The accident on board Collingwood led to the special committee limiting the charge of the Mark I and II guns to 295 pounds (134 kg) of cocoa powder.

[44] The special committee also ordered that future heavy guns (i.e. Mks III and IV) be hooped to the muzzle.

However, it was later pointed out that the particular weakness of the front part of the muzzle of the gun would only lead to accidents like that on Collingwood, i.e. if it was mounted en barbette.

[46] Mks I, VI and VII were coast defence versions with trunnions for mounting on recoil slides.

[48] The common ballistic tables for Mk I - IV, that were made before the 1886 incident on Collingwood also indicated that the internal construction differences were a mere detail.

[49] After the incident on board Collingwood, the Mk I/II's were chase hooped, which increased their weight to 47 tons.