Anthony G Williams

This is a modified and extended version of an article which first appeared in Warship World, Winter 1988 issue

Most naval guns in the Second World War can be divided into three categories. These were: small-calibre weapons of up to 40 mm, which had fully automatic loading and firing cycles and were used almost exclusively as anti-aircraft weapons; large-calibre guns of 6in (152mm) or more for surface action, which had power loading systems as the ammunition was too heavy for easy manhandling; and medium-calibre guns which were manually loaded and often dual purpose, ie designed to be used against both aircraft and surface targets. After the end of the First World War the Royal Navy procured such guns in no fewer than four different major calibres - 4in, 4.5in, 4.7in and 5.25in (102mm, 114mm, 120mm and 133mm) - and in the case of 4in and 4.7in, in more than one version, requiring different ammunition.

These medium-calibre guns can themselves be divided into two types; those with fixed ammunition (ie shell and case handled in one unit) and those with separated ammunition with the shell and case carried separately to the loading tray. The choice made by the weapon designer between fixed and separated ammunition was determined by assumptions about the maximum weight which the loading numbers could be expected to manhandle. This was not quite the simple decision that it might appear as a weight which could easily be managed in favourable conditions was a different matter in North Atlantic winter storms with a cold and tired crew. Considering its vast experience the RN made some surprising errors in its choice of new equipment of this type.

The 4in and 4.7in calibres were traditional ones in the RN, and saw service in various versions in the First World War. Many of these older guns were still in service in World War 2, such as the 4in Mk IV L40 (the L40 referring to the calibre length, i.e. the barrel was 40x the calibre or 160in long) and Mk V L45, and the 4.7in Mk V. The 4in Mk V actually remained in production into WW2 and was installed in both high and low angle mountings.

After WW1 new guns were introduced. The 4in Mk XII and XXII were L40 submarine guns while the 4.7in Mk VIII was an L40 gun on a high-angle AA mounting - the only one in this calibre to use fixed ammunition, with a 45lb (20.4kg) shell - fitted to the new battleships Nelson and Rodney plus four other ships. More significant was the 4.7in Mk IX L45, fitted in single low-angle mounting. Equipped with a gunshield giving partial protection to the crew, this was the standard destroyer weapon throughout the 1930s and early 1940s. The 4.7in Mk XII was ballistically similar but fitted to a twin mounting in the 'Tribal', J', 'K' and 'N' Classes of destroyer. Both guns used separated ammunition with a 50lb (22.7kg) shell and could be fired at around 12 rpm (rounds per minute). They were perfectly satisfactory weapons against surface targets but the size and recoil distance restricted the maximum angle of elevation, at first to only 30, later to 40 and eventually to 55. Even in later versions they were therefore of limited use as anti-aircraft weapons.

Next to arrive were the 4in Mks XVI and XXI L45 (the latter being a lightened version) which were mainly fitted in the Mk XIX twin mounting, which achieved an elevation of 80 for true AA performance. A fixed round of ammunition weighing 65lb (29.5kg) - including a 35lb (15.9kg) shell - was used and a rate of fire of 12 (later 16) rpm per barrel could be attained. The mounting was highly successful and was used as the primary gun armament of a few destroyers, many escort vessels, some AA cruisers and minelayers and as the secondary armament of cruisers. The 4in Mk XIX L40 was also introduced, in a single mounting with 60 elevation, mainly for fitting to frigates and other escorts.

Work commenced in the 1930s on the design of new dual-purpose guns for aircraft carriers and light cruisers and as the secondary armament of new and refitted battleships and battlecruisers. It was at this point that the Navy's judgement went seriously awry. It was estimated that the maximum ammunition weight for fast manhandling was about 90lb (40.8kg) and two new calibres were designed accordingly; a 4.5in L45 using a fixed round of 90lb - the shell weight being 55lb (25kg) and a 5.25in L50 with separated ammunition and a shell weighing 80lb (36.3kg).

The 4.5in (which came in three marks, all virtually identical) was initially produced in twin mountings with an elevation of 80. Three different patterns of twin mounting were used; the Mk II BD (between decks) was fully enclosed by a gunhouse and designed for aircraft carriers and reconstructed battleships and battlecruisers; the Mk III UD (upper deck), open to the rear, was relatively little used while the late-war Mk IV was a fully enclosed turret mounting. Ships so equipped included the refitted old battleships Queen Elizabeth and Valiant and the battlecruiser Renown (all with ten mountings), the Ark Royal and Illustrious class aircraft carriers and the Battle class destroyers (which had the Mk IV). Later in the war the 4.5in was also fitted in single mountings to some destroyer classes in place of the 4.7in but again the elevation could not exceed 55. It is hard to understand why this gun was designed in the first place as its performance was only marginally superior to the well-established 4.7in and its production was obviously costly and caused ammunition supply complications.

The 5.25 in was only produced (for shipboard use) in a twin enclosed mounting offering an elevation of 70. It was used in the Dido Class light AA cruisers (4 or 5 mountings) and the King George V Class and Vanguard battleships although it was also planned for the Hood as a part of the postponed refit. The Vanguard was fitted with modified mountings with a larger gunhouse.

The problems with the new calibres arose as wartime experience revealed that the maximum weight which the loading numbers could handle comfortably was much lower than 80-90lb and the weight of the 4.5in and 5.25in ammunition caused serious difficulties. This was subsequently remedied in the 4.5in by separating the shell from its case and its rate of fire was increased to 15 rpm. However, initially nothing could be done about the 5.25in (not helped by its cramped gunhouse) which could reportedly manage a rate of only 7-8 rpm instead of the designed 10-12, a failing which significantly reduced its AA effectiveness. The problem was not remedied until the introduction of the improved mounting in HMS Vanguard, which achieved the intended rate.

There was one further development early on in the war, the introduction of the 4.7in Mk XI, an L50 gun which fired a 62lb (28kg) shell of more streamlined design than the old 50lb type. This was only fitted, in a huge twin gunhouse, to just twelve destroyers of the 'L' and 'M' Classes. With a maximum elevation of 50 it was mainly intended for surface fire. It is again very hard to imagine that the development of yet another new gun in this class was worth the cost and supply problems.

With the benefit of hindsight it could be argued that the 4.5in, 5.25in and 4.7in L50 should never have been developed and that the Navy should have concentrated on producing more of the 4in and 4.7in L45. The 4in twin was little heavier than the 4.7in or 4.5in single and destroyers could therefore have been fitted with four mountings. The high-angle capability of the 4in Mk XIX, combined with the total rate of fire of 96-128 rpm (compared to 48 rpm of the four-gun 4.7/4.5in ships) would have produced vastly superior AA capabilities. However the RN preferred the heavier 4.7/4.5in shells for surface action despite the AA limitations of this weapon and the fact that the 4in ships could actually have thrown a heavier weight of fire (3,360-4,480lb per minute compared to 2,400-2,640lb) and obviously stood a much better chance of hitting a target through sheer volume of fire. Unfortunately it was not realised before the war that aircraft and even fast torpedo boats posed far greater threats than the enemy destroyers which the guns were intended to counter.

The 4.7in could have been fitted into the same high-angle twin mountings as the 4.5in and, given improved ammunition with the 62lb shell, could have outperformed the newer gun. Indeed, its performance would not have been too far from the 5.25in, yet each mounting would have been significantly lighter, giving the option to save weight or fit more mountings. If mounting weight alone is considered, the Didos could have been fitted with six mountings for the same weight as four 5.25in, giving a rate of fire of 144 rpm instead of 60 and a weight of fire of about 9,000lb per minute rather than 4,800. The new battleships could similarly have been fitted with twelve instead of eight mountings. Even if (taking other factors such as space and manning into consideration) the number of mountings remained the same, the weight as well as rate of fire would have been usefully increased. The AA performance would therefore have been greatly improved and against surface targets the slight loss of individual shell weight would have been more than compensated by the increased volume of fire.

Is it fair to criticise the Navy for decisions made at that time? Certainly the RN was not alone in underestimating the importance of AA fire (although the USN did much better with its 5in L38 DP gun and - of even greater importance - an effective DP director). However, it is difficult to defend the proliferation of calibres and weapons or the misjudgement over optimum ammunition weights.