Officially designated as the 12-in 'A' to conceal the increase in caliber size from the 12"/50 (30.5 cm) guns used on previous ships, this was the largest dreadnought battleship gun in the world at the time of its introduction. A Vickers design, these guns are sometimes suffixed as Mark V(L) and Mark V(H), which signifies the weight of the projectiles fired. See below for further details.

The failure of the high-velocity 12"/50 (30.5 cm) Mark XI led to a reconsideration of how to achieve an increase in the destructive power of naval guns. As a result, instead of simply raising the muzzle velocity as had been done in the past, the Admiralty now decided to maximize the weight of the projectiles by increasing the gun caliber. In October 1908 the Admiralty requested Vickers to quote on new designs for 13" (33 cm), 13.5" (34.3 cm) and 14" (35.6 cm) guns. Part of the design requirements for these weapons was that their projectiles were to have the same nominal residual velocity of 1,640 fps (500 mps) at 8,000 yards (7,320 m) as did projectiles from the 12"/45 (30.5 cm) Mark X. The 13.5" (34.3 cm) design was selected in January 1909 and the first prototype was fired at Shoeburyness late that same year.

The greater momentum inherent in a heavier projectile means that it does not loose its velocity as quickly as does a lighter projectile. So, although the muzzle velocity of the 13.5" (34.3 cm) gun was equal to that of the 12"/45 (30.5 cm) Mark X and less than that of the 12"/50 (30.5 cm) Mark XI, the striking velocity and penetration power of the new gun at long ranges was much greater than that of either of the two smaller guns.

HMS Orion was the first ship armed with these weapons and was the first British dreadnought with all mountings on the centerline and with superfiring turrets fitted on the bow. These new 13.5" (34.3 cm) guns were found to be very reliable and accurate in service with exceptionally low barrel wear. As the design had been given a considerable safety margin, it was decided that later ships could use a heavier shell to increase their hitting power still further. No changes were made to the guns themselves to fire the heavier projectiles, the changes were limited to the shell handling equipment and hoists in the mountings.

Although these guns had a maximum elevation of 20 degrees, as built the prisms in the director and turret sights were only good to slightly over 15 degrees. This was because prior to World War I it was expected that battles would be fought at relatively short ranges and thus the advantages of firing beyond 15,000 yards (13,700 m) were not appreciated. Auxiliary prisms that could be attached to the existing sights and which gave an extra 6 degrees of elevation were produced during World War I, but many ships still lacked these as late as the Battle of Jutland (Skagerrak).

At Dogger Bank two 13.5" (34.3 cm) APC projectiles hit German armor of 30 cm (12") without penetrating. A third shell of this caliber hit Seydlitz on her 23 cm (9") barbette armor at an angle of 33 degrees to the normal. Although this burst without any splinters entering the ship, flash and spalled armor fragments ignited 62 propellant charges and this fire burned out both stern turrets, killing 165 crewmen. Seydlitz was hit again by a 13.5" (34.3 cm) projectile almost the same way during the Battle of Jutland (Skagerrak) and this again holed the armor but was kept out except for flash which ignited four charges.

The "steel choke" problem first found on the 12"/35 (30.5 cm) Mark VIII was finally fixed in the later models of these guns by the introduction of a slow taper fit between the inner A and A tubes. The locating shoulders, which in the earlier guns were well forward and overly concentrated longitudinal stress towards the muzzle, were moved well back in the new guns.

These guns were of the standard wire-wound construction and the first 67 of the 206 guns produced still had the forward locating shoulders and lacked the taper fit. 54 guns were still in existence in 1939 along with six turrets removed from HMS Tiger and HMS Iron Duke which had been placed in storage at Rosyth. An additional three turrets remained on the demilitarized Iron Duke.

During World War II three of these guns were used as railway artillery in the Dover area and were supplied with super charges. A further three guns were relined to 8" (20.3 cm) as a "not very successful super-velocity gun - a remarkable waste of effort" (John Campbell). Four guns together with their slides and cradles from the scrapped battlecruiser HMS Tiger were sold to Turkey before the start of World War II, but none were ever delivered.

• ^As no spare Mark VI guns had been manufactured, one Mark V(H) gun was carried by HMS Erin for a short period while one of her guns was being relined.
• ^A note on Sources: "Naval Weapons of World War One" mistakenly says that Queen Mary carried "(L)" guns but all other sources say that she carried "(H)" guns. This and all other sources agree that she carried the BII* mountings, which were used only with "(H)" guns.

• ^Projectiles were nominally 4crh. The railway guns used a new 1,250 lbs. (567 kg) AP shell of 8/16crh which achieved a much greater range.
• ^2.12.2There were as many as five different vendors for these munitions - Hadfield, Vickers, Firth's, E.O.C. and Cammell Laird - each producing a slightly different projectile from the others. Their APC projectiles had the same burster and total weights, but differed in internal design and had AP caps of different designs and weights. The projectile lengths given above are average numbers.
• ^Although most secondary sources quote a 1,250 lbs. (567.0 kg) weight for the "Light" APC, both the APC Mark IIa (pre-Greenboy) and the APC Mark IVa (Greenboy) actually weighed more than this value as can be seen in the table above. My supposition for this discrepancy is that, like the earlier 12-inch APC Mark VIa, the 13.5-inch APC Mark IIa was originally designed as a 2crh projectile but was later redesigned and changed the ballistic cap to 4crh. This changeover was apparently performed before any of the ships were commissioned.
• ^"Greenboys" were issued starting in 1918.
• ^5.15.2Super charges were used solely by the railway guns.
• ^The sources below disagree as to the muzzle velocity for these guns. I have chosen to use those values given in "British Naval Guns 1880-1945 No 2."
• ^The muzzle velocities given above are for new guns. When worn and due for replacement, the Mark V(L) gun had a muzzle velocity of 2,100 fps (640 mps).
• ^Ammunition storage given above are the design figures. More rounds were added during the war. Lion was originally given 24 APC, 28 CPC, 28 HE and 6 shrapnel or 86 rounds total per gun. During the early part of the war, this was changed to 110 rounds per gun of 33 APC, 38 CPC and 39 HE. By the time of Jutland (Skagerrak), she carried 66 APC, 22 CPC and 22 HE. The final war outfit was 77 APC (Greenboys) and 33 CPC per gun. The outfits for the other 13.5" (34.3 cm) battlecruisers had similar changes.

• ^Guns in all mountings were individually sleeved. Mountings with a single asterisk (*) after the Mark Number indicate that they were modified to handle the 1,400 lbs. (567 kg) projectiles. Mountings with a double asterisk (**) after the Mark number indicate a further modification to increase the elevation speed by enlarging the elevation cylinder. These mounts were equipped with the newly invented swashplate training engine which gave them smoother training movements. Two 7-cylinder swash plate engines were provided for each mounting.
• ^The Mark II turrets were very similar to the previous BXI mountings used for the 12"/50 (30.5 cm) guns of the St. Vincent class. As the designers of the Mark II mounts kept approximately the same dimensions as those for the BXI, the Mark II mounts were cramped and compromises in shell handling were accepted. For example, the projectiles in the Mark II were hoisted at a 38 degree angle in order to maintain about the same hoist dimensions as in the earlier mounting, although the projectiles entered and exited the cage hoist horizontally. Auxiliary supply propellant was eliminated and in their place were secondary projectile and cage hoists outboard of the guns. Stowage for eight rounds per gun were provided in the Mark II and II* gunhouse, with two powered derricks provided to lift the shells. These derricks also lifted shells from the secondary supply hoists mentioned above. Bins for six rounds per gun were arranged in the working chamber below the gunhouse and an emergency shell and cordite supply route was built into the main trunk. The main hoists combined the projectile and propellant cages such that when at the lowest level, the shell cages were in line with the shell bogies in the shell room and the propellant cage was in line with the cordite hoppers in the magazine. When the cage reached the working chamber, the projectile and propellant cartridges were automatically tipped onto loading trays where powered rammers pushed them into the gunloading cage. The Mark II* had improved chain rammers and breech mechanisms and a new type of shell room bogie was introduced that eased moving shells from storage into the hoists. Previously, shells were simply rolled over to the hoists.
• ^The Mark III turrets were the first large-caliber mountings to be designed by the Coventry Ordnance Works. It is noted in "The Big Gun" that the ammunition supply arrangements reflected the inexperience of the Coventry designers and that this resulted in more hand work being necessary than in the Mark II mountings.
• ^As built the prisms in the director and turret gunsights controlling these mountings allowed ranging only up to an elevation of +15' 21". By the time of Jutland (Skagerrak), some ships had been given removable 6 degree "super elevation" prisms that could be attached to the existing director sights and to the center position turret sights but this process was not completed until 1917.
• ^Superfiring turrets could not fire within 30 degrees of the axis because the blast effects would have penetrated into the lower turrets through the sighting hoods. "A" turret on HMS Tiger was to an experimental design whereby the two sighting hoods for the gunlayers were replaced by armored horizontal periscope sight ports on the turret sides. The center sighting hood used by the turret trainer was left in place which negated any advantage to blast protection. These side ports were not repeated on later ships as it was found that they wooded earlier than the sighting hoods on the roof.
• ^"Q" turret on Tiger was in a more favorable position which increased its firing arc to 60 degrees before the beam and 90 degrees abaft it on either side, although it could not fire directly aft below about 10 degrees of elevation. Firing aft at angles between about 170 to 190 degrees would also affect wireless antennas on any British capital ship of this era.
• ^Although these mountings were designed to be loaded at any elevation angle, it was found that HMS Lion and HMS Princess Royal, which were equipped with two hydraulic run-out pumps to operate all four turrets, "stalled" at high elevations which slowed their reloading cycles. As a result, loading for these ships was normally performed at a low angle. The later Orion class battleships and the battlecruisers Queen Mary and Tiger were given three pumps and later battleships were given four pumps. At the Dogger Bank battle it took the battlecruisers 11 seconds to run their guns out at high elevations and the guncrews adapted by lowering the guns before reloading. This would have "unsighted" the gun layers, which must have caused difficulties in maintaining target lock.
• The gun axes were 90 in (228.6 cm) apart on all mounting types.
• Armor thickness for battleships as given in "Naval Weapons of World War One" by Norman Friedman:
      Face: 10.78 in (27.4 cm) (440 lbs.)
      Sides: 10.78 in (27.4 cm) (440 lbs.)
      Rear: 10.78 in (27.4 cm) (440 lbs.)
      Front Roof: 3.92 in (10.0 cm) (160 lbs.)
      Rear Roof: 2.94 in (7.5 cm) (120 lbs.)
      Floor: 1.96 in (5.0 cm) (80 lbs.) + 1.96in (5.0 cm) (80 lbs.) splinter protection on overhanging floor

  Armor thickness for battlecruisers as given in "British Battleships of World War One" by R.A. Burt:
      Face: 8.82 in (22.4 cm) (360 lbs.)
      Sides: 8.82 in (22.4 cm) (360 lbs.)
      Rear: 7.84 in (19.9 cm) (320 lbs.)
      Front Roof: 3.19 in (8.1 cm) (130 lbs.)
      Rear Roof: 2.45 in (6.2 cm) (100 lbs.)
      Floor: 1.96 in (5.0 cm) (80 lbs.) + 1.96 in (5.0 cm) (80 lbs.) splinter protection on overhanging floor.

  Face, Sides and Rear used KC armor while the roofs used KNC armor.

  Following the Jutland (Skagerrak) battle, surviving battlecruisers and at least some battleships had 1 in (2.54 cm) armor plates added to their turret roofs and magazine crowns. See photograph of HMS Benbow below.

Images at The Vickers Photographic Archive

See Princess Naval Gun

"The Grand Fleet: Warship Design and Development 1906-1922" by D.K. Brown
"British Battleships of World War One" by R.A. Burt
"Naval Weapons of World War Two," "Jutland: An Analysis of the Fighting," "The Battle of Tsu-Shima" article in "Warships Volume II," "Warship Special 1: Battle Cruisers" and "British Naval Guns 1880-1945 No 2" article in "Warships Volume V" all by John Campbell
"Battleship Design and Development 1905-1945" and "Naval Weapons of World War One" both by Norman Friedman
"The Big Gun: Battleship Main Armament 1860-1945" by Peter Hodges
"The Battleship Builders: Constructing and Arming British Capital Ships" by Ian Johnson & Ian Buxton
"The Admiralty Regrets: British Warship Losses of the Twentieth Century" by Paul Kemp
"British Battleships: 1860 - 1950" by Oscar Parkes
"British Battleships of World War Two" by Alan Raven and John Roberts
"Battlecruisers" by John Roberts
"Dreadnoughts in Camera: Building the Dreadnoughts 1905-1920" by Roger D. Thomas and Brian Patterson
"The Royal Navy in Old Photographs" by Wilfrid Pym Trotters
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ADM 186/169 "Shell Committee: Reports 1917 and 1918"
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Special help from Neil Stirling and Anthony Lovell of Dreadnought Project

04 December 2006 - Benchmark
01 May 2009 - Added picture of HMS Monarch
29 December 2011 - Added information about hits on Seydlitz and ammunition outfits on battlecruisers
11 February 2012 - Updated to the latest template
25 November 2012 - Corrected comment about auxiliary prisms
26 November 2015 - Added mounting note about "A" turret on HMS Tiger
02 December 2015 - Changed Vickers Photographic Archive links to point at Wayback Archive
22 February 2016 - Added ammunition details, armor thickness and information about reloading at high elevations
16 September 2016 - Converted to HTML 5 format
02 January 2020 - Added information on remaining guns and turrets in 1939, added note regarding "(H)" guns on HMS Queen Mary, reorganized notes, minor formatting changes
29 February 2020 - Added note regarding weight of Light APC
01 May 2021 - Added photographs of 13.5"/45 Mark V guns being manufactured
22 August 2022 - Added photograph of 13.5"/45 Mark V gun in workshop
27 May 2023 - Added photograph of HMS Iron Duke turret interior
14 November 2024 - Added note regarding extra armor protection added to battlecruisers following the Jutland (Skagerrack) battle.