Following the end of the First World War, Britain undertook an investigation into new battleship and battlecruiser designs which were to incorporate all of the lessons learned during the war. These ships would have been larger and more powerful than HMS Hood, which at the time and for the next twenty years was the largest warship in the world. Five different main gun sizes, 15"/50 (38.1 cm), 16"/45 (40.6 cm), 16.5"/45 (41.9 cm), 16.5"/50 (41.9 cm) and 18"/45 (45.7 cm), were investigated as possible weapons with which to arm these ships. After much detail design work and weighing of alternatives, the "G3" battlecruiser design armed with nine 16"/45 (40.6 cm) guns and the "N3" battleship design armed with nine 18"/45 (45.7 cm) guns were selected late in 1920.

During the war, British designers had detailed access to USN gun designs and the end of the war brought even more access to German gun designs. These examinations led to a reconsideration of British gun construction techniques with the result that the traditional full-length wire-wound construction method was abandoned. As it was desired to expedite construction of the battlecruisers, the design for the 16"/45 (40.6 cm) guns was only slightly modified from previous practices, but the more leisurely schedule for the battleships allowed time for a through inquiry into alternative gun construction methods.

Elswick, Vickers and the Royal Gun Factory were asked to submit proposals for the 18"/45 (45.7 cm) guns based upon three new construction methods. The first was for a full-length wire wound gun using the new taper winding process, the second was for a partial-length wire wound gun, and the third was for a "no wire" gun using relatively long tubes. These firms submitted a total of more than thirty designs to the Ordnance Committee, which selected three of the proposals from the Royal Gun Factory, one for each construction method, for further development. Orders were placed for these designs with the partially wired gun (known as No. 5) going to Vickers and the "no wire" gun (known as No. 6) going to Elswick on 22 December 1920. The Royal Gun Factory at Woolwich received the order for the full-length taper wound wire gun (known as No. 4) on 20 January 1921 (Serial Numbers 1, 2 and 3 had been assigned to the older 18"/40 (45.7 cm) Mark I guns). A proposal was made for an additional gun based upon Krupp manufacturing techniques, which involved the use of short tubes and a precision shrinking process, but no order was placed for this version. For secrecy purposes, all of these guns were referred to as being "16-in/50 (40.6 cm)" weapons.

No. 4 gun would have been similar in construction to the 18"/40 (45.7 cm) Mark I, but utilizing taper winding. No. 5 gun would have had an unwired chase with inner A, A and B tubes and the wire over the rest of the gun would not have been taper wound. No. 6 gun would have had a chase, comprising of inner A, A and B tubes, with the B tube continued by a C tube towards the rear where there would have been a jacket. All three guns were to have had Elswick short arm breech mechanisms.

On 30 January 1922, long before delivery and shortly before the formal signing of the Washington Naval Limitation Treaty, all of these 18" (45.7 cm) guns were cancelled. Britain did secure the right under the treaty to build two new capital ships, so the designs for the 16"/45 (40.6 cm) guns and turrets already ordered for the "G3" battlecruisers were modified and then used to arm what became the HMS Nelson class battleships.

The 18"/45 (45.7 cm) Mark II was the most powerful gun ever seriously proposed for a British warship. However, in view of the problems actually encountered with the 16"/45 (40.6 cm) Mark I with reliability, blast effect, too-light projectiles and excessive barrel wear, it is somewhat likely that the theoretical value of these larger weapons would have greatly exceeded their actual value. The British had no serious plans to build weapons larger than these 18" (45.7 cm) guns, and Sir Robert Hadfield of the famous steel company of that name was asked to stop talking about making APC shells of 20 or 21 inches (50.8 or 61 cm) as there was no interest in them by the British Admiralty. This may have been an effort by the British prior to the start of the Washington Naval Limitation Conference to keep other nations from starting a caliber race.

Since information on these never-built and never-completed guns is quite sketchy, the data for them is combined on this data page. The 16" (40.6 cm) guns that were intended for the "G3" class battlecruisers, but which instead wound up being used on the Nelson class battleships, are described separately under their own data page.

• ^Only the 18"/45 (45.7 cm) Mark II was ever given a formal designation.
• ^These Vickers weights are based upon proposals submitted by Vickers to the Ordnance Committee. It should be noted that these proposals were all for guns firing the "heavy" APC shells, which for the 18"/45 (45.7 cm) gun was to have weighed 3,320 lbs. (1,506 kg). This required a relatively robust construction and possibly accounts for a portion of the greater weight of this Vickers 18"/45 (45.7 cm) design when compared to the weight of the three guns actually started, as these latter guns were intended to fire the lighter 2,916 lbs. (1,323 kg) APC projectile.
• ^Rifling details were to be finalized following comparative trials with three 15"/42 (38.1 cm) guns with the first having the standard rifling of uniform RH 1 in 30 with 76 grooves, a second gun with uniform RH 1 in 40 with 76 grooves, similar to Italian 15" (38.1 cm) guns, and a third gun with uniform RH 1 in 30 with 100 grooves of equal width to lands. The rifling for this last gun was based upon modified German practice.
• ^The rate of fire is shown "as expected." However, the problems encountered by the 16"/45 (40.6 cm) Mark I guns used on the Nelson class battleships would suggest that these larger guns would not have been able to achieve these values, as they shared the same design concepts in regards to "flash tightness" and triple turret design as in those ships.
• Droop with the three 18"/45 (45.7 cm) guns was estimated to be as follows:
      No. 4 Full-Wire: 0.55 in (1.4 cm)
      No. 5 Part-Wire: 0.51 in (1.3 cm)
      No. 6 No-Wire: 0.48 in (1.2 cm)
• British comments on the differences between their gun designs and German gun designs of the World War I period may be found on the 15"/42 (38.1 cm) Mark I datapage.

Data is for 18"/45 (45.7 cm) guns unless otherwise noted. ^{1a 2a}

• ^As none of these guns were actually prototyped and proof tested, the above values must be considered as very preliminary, especially in light of the difficulties found during the testing of the 16"/45 (40.6 cm) Mark I.
• ^The 15"/50 (38.1 cm) guns would have fired the same projectiles as the famous 15"/42 (38.1 cm) Mark I. The 1920 proposal for the 16.5"/45 (41.9 cm) and 16.5"/50 (41.9 cm) guns was for an APC projectile of 2,552 lbs. (1,158 kg). This weight would have almost certainly been reduced had this caliber been chosen for the new ships.
• ^3.13.2The original 18"/45 (45.7 cm) APC shell weight was decided upon in August 1920. Shortly afterwards, from a series of gunnery trials, a mistaken theory was conceived that held that a high-velocity, low-weight projectile would have superior oblique impact armor penetration characteristics, which was the opposite of previous findings. As a result of this new theory, the APC projectiles were redesigned in 1921 for a significantly lighter shell weight. This theory was not substantiated by later trials, but these took place too late to affect the decision to use light weight APC projectiles. See below and the 16"/45 (40.6 cm) Mark I data page for further details.
• ^4.14.2The 1920 shells had ogival 8crh heads, the 1921 shells had conical (6/∞ crh) heads.
• ^The final design of the 1921 APC round was not firmly established prior to cancellation. The two weights given above are from different sources and are probably for slightly different proposals.

The following excerpt is from "British Battleships of World War Two" by Alan Raven and John Roberts
© Lionel Leventhal Limited, published by United States Naval Institute Press, 1976

The general requirement was for a gun capable of defeating a ship protected by a 15-inch belt and a 7-inch deck. The design for the 18-inch gun was regarded most favourably as the one least likely to be outclassed by foreign designs. . . .

All the heavy gun designs considered early in 1920 and listed above, followed the practice that had begun with the 13.5-inch gun of having a heavy shell/low velocity combination, but in March 1920, the DNO issued a memorandum in which he cast serious doubts upon the advisability of this system. His arguments were based on the results of the proof tests of the 13.5-inch, 1,250-pound and 1,400-pound projectiles, which demonstrated that it was by no means certain that, at oblique attack, the heavier shell was the more efficient armour piercer. Both the light and the heavy 13.5-inch shell had to pass the same proof tests at the same striking-velocities, and while there were no failures with the 1,250-pound projectile, there were many, at first, with the 1,400-pound projectile. The failed shells usually passed through the plate but not in a fit state to burst, probably - it was thought - because of the greater length of the heavy shell. When penetrating armour at oblique angles of attack, the projectile was deflected from its line of flight, which produced a whip in the base portion. The stresses set up by this whip were greater in the long-bodied shell than in the short one, and its chances of breaking up during penetration were, therefore, greater.

It was also pointed out by the DNO that the existing 15-inch armour-piercing capped shell was not, as was commonly believed, capable of penetrating whole, any armour then carried afloat. Under proof conditions, only two such shells had successfully penetrated, in an unbroken condition, a 12-inch plate at 20° to the normal. The first was fired with a striking-velocity of 1,690 feet per second, equal to a range of 12,300 yards, at a standard armour plate, and the second, with 1,347 feet per second, equal to 25,000-yards, at a Vickers trial plate.

Trials against turret roofs showed that 5-inch and 6-inch armour when struck at 60° to the normal, could keep out a 15-inch shell, though a hole was punched in the armour. From these results, it was deducted that a 7-inch armour deck would defeat a 15-inch armour-piercing capped shell at 60°, that is up to a range of 25,000 yards. Only one trial at a steeper angle had been held, in which a 15-inch APC shell penetrated a 7-inch plate, at 45°, with a striking-velocity of 1,465 feet per second. Again, the DNO intimated that a shorter-bodied 15-inch shell would probably have better powers of penetration, and for future designs, he proposed the adoption of a high-velocity gun, firing a light shell. The theoretical advantages of a gun of this type were, a high striking-velocity at short range, a steep angle of descent at long range and a greater danger-space up to certain ranges. The loss in striking-energy resulting from the lower weight of a short shell compared to a long one of the same calibre and type, were considered by the DNO to be far more important in small and medium calibre guns than in heavy guns. Calculations showed, that for an equal maximum chamber pressure, a 1,690-pound 15-inch shell would have a muzzle velocity of 2,650 feet per second compared to 2,450 feet per second in the existing 1,920-pound shell. This gave the lighter shell a greater danger-space up to 22,000 yards, or 25,000 yards with a modified ballistic cap (6 crh instead of 8 crh). In the opinion of the DNO, the greater striking-energy of the heavy projectile would have little effect on the ability of the shell to perforate in a fit state to burst. Calculations were also made for the 18-inch gun with 8 crh projectiles of 3,353 pounds and 2,837 pounds and muzzle velocities of 2,500 feet per second and 2,700 feet per second respectively. The remaining velocities were found to be the same at 23,000 yards and only differed by 25 feet per second at 30,000 yards.

To test these theories, experiments were conducted with short-bodied 15-inch armour-piercing capped shells. The detailed results of these trials are not known, but they must have vindicated the DNO's arguments, for the 16-inch Mk I gun mounted in Nelson and Rodney used the high-velocity/light shell combination; in the event it was to prove a mistake.

It was considered that the best distribution of the main armament was in four twin turrets, arranged as in the Queen Elizabeth class. This system was ideal for efficient fire-control, and twin mountings allowed for turret designs of maximum simplicity and reliability. It was realised, however, that it might be necessary to adopt triple turrets in order to keep the displacement of the new ships within reasonable limits. Designs for both twin and triple mountings were obtained from Vickers and the Elswick Ordnance Company, for inclusion in the sketch designs of the 1921 to 1922 capital ships. These turret designs were given a maximum gun-elevation of 40° or 45°, which, it was thought, was necessary in order to obtain the maximum possible range.

• ^The design of these mountings would have been similar to those for the 16"/45 (40.6 cm) Mark I except that power handling of the charges in the magazine was provided.
• The distance between the gun axes was 102 inches (260 cm).

"Washington's Cherrytrees: The Evolution of the British 1921-22 Capital Ships" article in "Warship Volume I" and "British Super-Heavy Guns Part 3" article in "Warship Volume III" both by John Campbell
"British Battleships of World War Two" by Alan Raven and John Roberts
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Special help from Daniel Muir and Terry Duncan

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