These were the last large-caliber gun designs of the Royal Navy and were intended for the never-built Lion class battleships. They would have fired a new, heavier APC shell than did the 16"/45 (40.6 cm) Mark I guns carried by Nelson and Rodney.

The Mark II and Mark III were almost identical, differing only in structural details of the forward shoulders where the jacket was located on the A tube. A total of five guns were actually built - one source says three Mark II and two Mark III, another source reverses these numbers. In most respects, these guns resembled a scaled-up version of the 14"/45 (35.6 cm) Mark VII guns used on the King George V class. These new guns were designed primarily for accuracy and regularity of the muzzle velocity, not for ballistic performance. Both designs consisted of tapered inner A tube, A tube, jacket, breech ring of rectangular external shape, breech bush located in the A tube and a shrunk-on collar located on the A tube. The A tube forging was the heaviest ever made in Britain, weighing 64 tons (65 mt), while the finished A tube weighed 45 tons (46 mt). An Asbury roller cam breech mechanism was used and it was planned to use a balance weight to bring the center of gravity closer to the breech end. The design was modified in 1939 to use a loose liner type of inner A tube.

The redesign of the Lion class battleships during 1942 and 1943 prompted the development of the last British large-caliber naval gun, the 16"/45 (40.6 cm) Mark IV. This design was to use flashless powder and heavier projectiles than the Mark II and III. However, work never progressed much beyond the design stage and none were actually built. A prototype was constructed using one of the Mark III guns, but this conversion could not have been used at anywhere near the intended pressure. This prototype was relined to allow heavier and longer shells and on 13 November 1947 some 47 rounds were fired from this gun using flashless cordite propellant. These firing trials showed good wear results but the ignition of the propellant was unsatisfactory. No further trials were conducted and in "1948 approval was sought to stop development of the 16in and its ammunition and this was approved in the following year" - John Roberts in "Penultimate Battleships."  Here ended the Royal Navy's long history of big gun development.

In April 1945, the Admiralty set up a "Committee on the Size of Battleships" to evaluate designs for new battleships. This committee issued a report on 1 May 1945 which recommended that new battleships of about 45,000 tons (46,000 mt) should be built. In regards to main armament, the committee evaluated main battery designs of 9 x 14" (35.6 cm), 9 x 15" (38.1 cm) and 9 x 16" (40.6 cm). The committee noted that the 15" (38.1 cm) design would save not more than 3,000 tons (3,050 mt) over the 16" (40.6 cm) design and that the 14" (35.6 cm) design would save an additional 2,000 tons (2,030 mt). In its evaluation of the effectiveness of each caliber in terms of inflicting damage, the committee determined that the 16" (40.6 cm) gun was 27% more effective while the 14" (35.6 cm) gun was 25% less effective than the 15" (38.1 cm) gun. As a result of these evaluations, the committee concluded that the 16" (40.6 cm) caliber should be used for arming any future battleship.

It is notable that the Admiralty put serious effort into designing new battleships so late in the war. It has been suggested by John Roberts that this was the result of having almost all of the "air minded" senior officers located in the Far East during the final year of the war and thus unable to bring their considerable experience to the late-war design conferences. However, considering that work on designing new battleships and new heavy guns went on well past the end of the war and into 1949, it must be concluded that these obsolete warships still ranked highly in the thinking of the post-war Royal Navy.

The data that follows is specifically for the Mark II and Mark III unless otherwise noted.

• ^When the first two ships of this class, Lion and Temeraire, were laid down in mid-1939, it was estimated that they would complete in July and September of 1942. They were instead repeatedly postponed starting in October 1939, with the few hundreds of tons of material actually assembled being scrapped in 1942-1943, but they were not formally cancelled until 1946.
• ^The balance weight would have added a little less than 13 tons (14 mt) to each gun.

• ^Data in this table is primarily for the Mark II and Mark III guns except where otherwise noted.
• ^APC was 6/12crh.
• ^The HE Mark IIC(?) was the replacement for the HE Mark IB which was withdrawn from service after HMS Rodney had suffered many duds during her bombardments at Oran during the North Africa landings. This later HE projectile was issued about 1944, had a true nose fuze and was of better ballistic shape - 6/12 crh. Not certain of the actual designation for this projectile, but it was based upon Ordnance design D.D./L/ 9145. The details listed here are taken from "Proceedings of the Ordnance Board 1938 MEMO. 1,679."
• ^Although the HE IIC projectile was the same weight as the HE Mark IIB projectiles used for the older 16" (40.6 cm) Mark I guns on the Nelson class battleships, they were to a shorter design, as the older projectiles were too long for the new mountings intended for the Mark II and Mark III guns.
• ^Projectiles for the Mark II and Mark III guns were limited in length to 73 inches (185 cm).
• ^Propellant was to be in six bags and flashless.
• ^The Mark IV guns were intended to be able to operate at pressures up to 24 tons/in² (3,780 kg/cm²). This high pressure would not have been necessary for the planned naval projectiles, but it would have allowed special, lighter projectiles to be fired across the Straits of Dover. This would have required the use of super-charges and hence the need for higher barrel pressures.
• ^For Treaty compliance purposes, the "standard tonnage" ammunition stowage was listed as being as low as 60 rounds per gun, but the actual stowage capacity was at least 100 rounds in all pre-war designs. Some of the late war design studies showed as much as 120 rounds per gun.
• The redesigned mountings for the Mark IV guns could have handled projectiles up to 78 inches (198 cm) in length. Although projectile designs were not finalized at the time of cancellation, the plan was to design the best possible CP shell (base fuzed HE) that could be made to fit in this length. This shell was expected to be able to inflict heavy damage against light armor up to 3 inches (7.62 cm) thick. The APC shell would have been designed to be the same weight as this CP shell, but optimized for performance against deck armor.

• ^The Mark II mounting was similar in design to the 14" (35.6 cm) quadruple mountings used on the King George V class. It is stated in some of the references below that propellant was to be rammed two bags at a time, the same as the King George V class, but this would have meant three propellant rams per shot. It would seem more likely that the propellant was to be rammed three bags at a time.
• ^In 1944, a new mounting design using the Mark IV guns was begun. Designated as the Mark III, this design was never finalized. The Mark III had a number of improvements over the Mark II, including greater flash protection and single-stroke ramming of the propellant. It has been speculated that these mountings would have achieved a ROF of about 20 seconds, but there is little evidence to support this figure, as no detailed design of the turrets was ever completed. At the time the design was halted in 1949, the sketches for it were in a very preliminary state and some fundamental problems such as finding enough space to rotate the projectiles from a vertical to a horizontal position were unresolved.
• ^Many of the preliminary designs and the 1937 draft Staff Requirement TD 60A/38 showed a maximum elevation of 30 degrees. This elevation was adopted as part of the formal Staff Requirements approved by the Admiralty Board in December 1938. The maximum elevation was not raised to 40 degrees until the 1939 Staff Requirements were adopted in February 1939.
• The gun axes were 102 in (259 cm) apart for both the Mark II and Mark III designs.
• Even though construction of Lion and Temeraire were halted soon after the war began, work on their six turrets continued well into the summer of 1943.
• Armor thickness for the Lion class (1939) as given in "Naval Weapons of World War Two" by John Campbell and "British Battleships of World War Two" by Raven and Roberts:
      Face: 14.7 in (37.3 cm) (600 lbs.)
      Fore Sides: 9.8 in (24.9 cm) (400 lbs.)
      Rear Sides: 6.86 in (17.4 cm) (280 lbs.)
      Rear: 6.86 in (17.4 cm) (280 lbs.)
      Roof: 5.88 in (14.9 cm) (240 lbs.)

"Naval Weapons of World War Two" and "British Super-Heavy Guns Part 4" article in "Warships Volume III" both by John Campbell
"Battleships: Allied Battleships in World War II" by W.H. Garzke, Jr. and R.O. Dulin, Jr.
"Penultimate Battleships: The Lion Class 1937-1946 Part 1 and Part 2" articles in "Warships Volume V" by John Roberts
"British Battleships of World War Two" by John Roberts and Alan Raven
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Proceedings of the Ordnance Board 1938 MEMO. 1,679
Proceedings of the Ordnance Board 1939 NO. 915
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Special help from Zhu Shipeng

28 December 2008 - Benchmark
24 August 2014 - Added Turret Armor note
12 September 2016 - Converted to HTML 5 format
17 December 2020 - Reorganized notes
17 February 2022 - Added HE burster weight
12 December 2024 - Added information about HE Mark IIC