Editing L and M-class destroyer (section)

=== Main armament ===
The ships were to have six QF Mark XI {{convert|4.7|in|mm|adj=on}} guns in Mark XX twin mountings in 'A', 'B', and 'X' positions. The 'X' mount gave an estimated [[arc of fire]] of 320 degrees at low elevations and 360 degrees at elevations above around 20 degrees. The Mark XI gun was a big improvement on the previous version in that it threw a {{convert|62|lb|abbr=on}} shell (compared to the {{cvt|50|lb}} shell in the preceding J class). The Mark XX mount was fully enclosed and supposedly weatherproof; in service, crews found otherwise. It also allowed the guns to be elevated independently. The Mark XX was not technically a turret, as the ammunition feed system was distinct from the weapon mounting, and did not train with the revolving mass. This meant that ammunition supply when the guns were at the limit of training was somewhat difficult. This also meant that the ammunition hoists had to be located between the guns just as in the USN [[5"/38 caliber gun|5" guns]].<ref name="ReferenceA">Destroyer Weapons of WW2, Hodges/Friedman, {{ISBN|0-85177-137-8}}</ref> The axes of the guns were very widely spaced, a feature instantly obvious with the Mark XX mounting.

The Mark XX mounting permitted an increased elevation to 50 degrees (compared to 40 for previous marks) but this still limited the engagement time against enemy aircraft, although medium calibre guns posed little threat to [[dive bomber]]s prior to the use of radar proximity fuzed ammunition.<ref>Hodges, ''Tribal Class Destroyers'', p32: Diagram of High Level Bomber Attack: A 240mph target, at 12 thousand feet altitude could expect to be under for fire about 75 seconds, from the time it enters the effective range of the HACS until it flies to within the minimum range of a 5.25 gun elevated to 70 degrees. A Tribal class destroyer with 40-degree elevation guns would be able to engage the same target for about 37 seconds.</ref><ref>Friedman, ''US Destroyers-An Illustrated Design History'', p203:"In theory, the 5in gun could counter either horizontal or torpedo bombers; it could not fire nearly fast enough to present any threat to dive bombers, which, ironically, were probably the most lethal threat to fast manoeuvrable craft such as destroyers."</ref> The [[Imperial Japanese Navy]] had already introduced a 5-inch (127&nbsp;mm) gun with 70-degree elevation into service which had very poor performance as an anti-aircraft weapon, while the [[United States Navy]]'s 5"/38 cal Mark 32 mount could elevate to 85 degrees.<ref>[http://www.navweaps.com/Weapons/WNJAP_5-50_3ns.htm 12.7 cm/50 (5") 3rd Year Type]: "''However, the very slow training speeds and lack of power ramming made these mountings almost useless against the fast-moving aircraft of World War II''"</ref> The 4.5-inch (114&nbsp;mm) guns fitted to {{HMS|Ark Royal|91|6}} were already in service and capable of elevations of 80 degrees, although the mountings were not suitable for a destroyer-sized ship. Coupled with the lack of powered elevation, the Mark XX mounting was compromised in its chosen anti-aircraft role, although it compared favourably with similar Axis weapons.

Another development regarding the main armament was the adoption of a combined high-angle/low-angle director tower, the HA/LA Mk.IV (TP). This was never entirely satisfactory in the HA mode, and was at least a ton overweight.<ref>Whitley 2000, p. 121.</ref> It was later reworked, somewhat unsuccessfully again, as the Mk.I "K tower" of the [[W and Z-class destroyer|Z class]]. These ships used the [[Fuze Keeping Clock]] HA Fire Control Computer.<ref name="ReferenceA"/> Despite its problems, the L and M class' director tower and its [[Type 285 radar]] provided better high-angle fire control than any similar Axis destroyer, the vast majority of which did not have any high-angle fire control system, much less a specialised AA fire control radar.<ref>Campbell, Naval Weapons of WW2. Campbell notes that no German or Italian destroyer had a high-angle (anti-aircraft) fire control system, and that the Japanese system was very rudimentary.</ref>

As originally ordered, the class had no close-range armament as the various departments could not agree on what to fit. Arguments raged as to the effectiveness of mounting one or two four-barrelled [[QF 2 pounder naval gun|2-pounder "pom poms"]], one pom-pom and one of the {{convert|0.661|in|mm|adj=on}} multiple machine guns then in development, one pom-pom and the traditional 0.5-inch (12.7&nbsp;mm) Vickers machine gun. The argument was exacerbated by the manufacturing schedules (a second pom-pom per ship would not be available until 1942), the poor performance of the development models of the 0.661 and a number of younger officers (led by [[Lord Louis Mountbatten]]) dissatisfied with anti-aircraft weaponry. Eventually, development of the 0.661 was dropped as it clearly would not be available quickly enough and this simplified the arguments somewhat.

The outbreak of war focused minds. Apart from the AA armament issue concerns started to be raised about progress generally. By February 1940 the two factors led to a proposal to change the design of four of the 'L's and fit a main armament of 4-inch (102&nbsp;mm) Mark XVI* guns in Mark XIX High Angle/Low Angle (HA/LA) twin mounts as used as secondary armament in the {{sclass|Southampton|cruiser|1}}s already in service and main armament in the {{sclass|Black Swan|sloop|4}} of [[Sloop-of-war|sloops]] under construction. Associated changes were provision of two quadruple 0.5-inch (12.7&nbsp;mm) machine guns. All ships of this class except ''Lightning'' and ''Laforey'' carried a four-barrel 2-pounder pom-pom.<ref>March, British Destroyers, p358.</ref>

The lessons of the [[Norwegian campaign]] and the [[Battle of Dunkirk]] drove home the need for this change and it was agreed in July 1940 that there were also to be four of the twin mounts instead of the originally proposed three. The fourth was to be at the forward end of the after superstructure which cut down on the fire arcs of both mounts but ensured the fourth would still be available for use in heavy weather. Not all senior officers were in favour and some said that it would mean the ships could not fight their foreign equivalents. Experience in the [[Mediterranean]], especially that of [[Force K]] which contained two of the 4-inch (102&nbsp;mm) 'L's, showed that the loss of gun power against surface targets was balanced against a higher rate of fire.

Review of AA armament continued and in October a decision was taken to remove the after bank of torpedo tubes and fit a 4-inch (102&nbsp;mm) HA gun instead and that is how the {{convert|4.7|in|mm|adj=on}} gunned ships eventually went to sea, although some surviving ships, including ''Matchless'' and ''Marne'', had the after tubes replaced later in the war.<ref>March, British Destroyers, p371</ref>