First large-caliber British gun to use the Welin breech mechanism which considerably shortened the loading time. Serious difficulties arose in this weapon as a result of "steel choke" and inner A tube cracking problems and some rebuilding was required.

During World War I nineteen of these guns were transferred to land service and then used on nine railway mountings. A further four guns from HMS Queen were sent to Italy.

Near the end of the war, three of these guns were used on the "M" class submarines. These were meant to be utilized as anti-shipping weapons, as it was felt that a single hit from a 12" (30.5 cm) projectile would be enough to sink a merchant vessel. This was moderately successful, although if the firing tube leaked it could blow the muzzle off the weapon.

Consisted of A tube, inner A tube, B tube and overlapping jacket. A total of 110 guns were made, including guns reworked to repair "steel choke" problems, being five Mark IXE (Elswick), five Mark IXv (Vickers) and three Mark IXw (Woolwich). A further six Mark IXw guns with tapered inner A tubes and no forward shoulders were manufactured during World War I. The Welin breeches were hand operated in the early ships, but hydraulic operation was fitted in latter ships. Vickers "pure-coupled" breech mechanisms were used on some guns, designated as A Mark IX. Mark IX* was assigned to guns intended for railway mountings. These were to have had their front chamber slopes modified, but it appears that no guns were actually altered.

• ^The rifling grooves ran straight for the first 4 feet (1.2 m) and then spiraled with a gradually increasing twist towards the muzzle.
• ^The Rate of Fire figure given above is found in references for British guns of this caliber, but "Warrior to Dreadnought: Warship Development 1860-1905" quotes Jellicoe's 1906 figures for rates of fire for these guns in gunlayers' tests and in battle practice and notes that the latter figures corresponded well to those actually attained by the Japanese at Tsushima:

  Gunlayers Test: 2 rounds per minute
  Battle Practice: 1 round per minute

• ^The submarine gun could only be reloaded on the surface. The procedure was to submerge and approach the target to within 1,200 yards (1,100 m), drive the submarine to the surface, quickly fire the gun and then dive back down. It is apparently a myth that the gun would normally be fired with only the muzzle out of the water when attacking an enemy, as such a condition would mean that the gun was at its maximum elevation while the sighting periscope would be at a minimum height out of the water. Such a combination would mean an extremely slight chance of hitting anything.

• ^Projectile weights from ADM 186/169. The sources below disagree as to muzzle velocity, propellant charge weights and maximum ranges. I have chosen to use those values given in "Jutland: An Analysis of the Fighting."
• ^Outfit for Duncan class was 64 AP shells, 216 Common steel, 28 practice and 40 AP shot.
• Ships originally carried 2crh shells. 4crh was later carried by the two ships altered to allow 30 degree elevation and by the submarines.

• The sources below disagree as to the maximum ranges and muzzle velocities of these guns. I have chosen to use those values given in "British Naval Guns 1880-1945 No 4" by John Campbell.
• It should be mentioned that these maximum ranges were of little use at the time these ships were built. Fire control systems and rangefinders capable of accurately firing at ranges over 10,000 yards (9,140 m) were nonexistent at the time.

• ^The BVI mounting was a rotating stalk design with a shell chamber below the gunhouse. The ammunition hoist was interrupted at the shell chamber rather than continuous from the magazine to the gunhouse. Shells were delivered horizontally to the shell chamber where a grab picked them up and delivered them to the gunhouse hoist. From a sketch, this gunhouse hoist appears to be a three level cage with the shell on top and half charges in the lower compartments. This mounting was also designed with safeguards against a cordite fire in the gun chamber flashing down the ammunition tube into the magazine. This mount had a secondary bogie used for hand-loading which was fitted at the rear of the gun and aligned at 1 degree elevation.
• The BVI mounting originally had two ready-use rounds per gun in the gunhouse, later increased to five rounds. There were eight more rounds per gun stowed in the working chamber which could be accessed via a winch should the shell hoists suffer a breakdown.
• ^The BVII mounting achieved the long-sought British goal of any angle loading, a featured carried through on subsequent ships until it proved impractical on the 15"/42 (38.1 cm) Mark I twin mounts.
• ^The submarine mount needed to be loaded and fired on the surface, although some trials had just the muzzle of the gun above the water. The muzzle was fitted with an electrically-operated, water-tight tampion. The gun mounting was free-flooding, thus allowing quick access after surfacing.
• ^HMS Commonwealth and HMS Zealandia were altered in 1918 to allow an elevation range of -3 / +30 degrees.

"The M Class Submarine Monitors" article by Thomas A. Adams in "Warship Volume VII"
"Warrior to Dreadnought: Warship Development 1860-1905" and "The Grand Fleet: Warship Design and Development 1906-1922" both by D.K. Brown
"British Battleships 1889-1904" by R.A. Burt
"British Naval Guns 1880-1945 No 4" article by John Campbell in "Warship Volume V"
"The Big Gun: Battleship Main Armament 1860-1945" by Peter Hodges
"British Battleships: 1860 - 1950" by Oscar Parkes
"A Concentrated Effort: Royal Navy Gunnery Exercises at the End of the Great War" article by William Schleihauf in "Warship International" No. 2, 1998
"Navy and Army Illustrated Annuals, Volume VI - 1898"
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ADM 186/169 "Shell Committee: Reports 1917 and 1918"
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Special help from Dave Perkins and Neil Stirling

Images at The Vickers Photographic Archive

Model of M1 Submarine 2660

03 March 2007 - Benchmark
30 January 2009 - Added a comment regarding guns repaired for "steel choke" problems
24 May 2009 - Replaced picture of Prince of Wales with picture of Russell
30 August 2011 - Added to mounting notes
04 January 2014 - Added photograph of railway gun
01 December 2015 - Changed Vickers Photographic Archive links to point at Wayback Archive
21 September 2016 - Converted to HTML 5 format
29 February 2020 - Reorganized notes
02 February 2022 - Added outfit for Duncan class
27 July 2024 - Corrected typographical error