This came mainly as a result of the lessons learned during the Spanish Civil War[citation needed] – i.e., military aircraft were now sufficiently advanced to pose a major threat to land and sea targets.
They also continued the practice (first introduced in the Js) of making the leader Laforey almost indistinguishable from the rest of the class, having only more extensive cabin accommodation and better radio (wireless telegraph, W/T) equipment.
The Mark XX mount was fully enclosed and supposedly weatherproof; in service, crews found otherwise.
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.
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 bombers prior to the use of radar proximity fuzed ammunition.
[5] The 4.5-inch (114 mm) guns fitted to HMS Ark Royal 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.
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.
[8] 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.
Experience in the Mediterranean, especially that of Force K which contained two of the 4-inch (102 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 mm) HA gun instead and that is how the 4.7-inch (120 mm) gunned ships eventually went to sea, although some surviving ships, including Matchless and Marne, had the after tubes replaced later in the war.
[9] In the early 1950s, it was proposed to convert the five remaining ships of the M class, together with seven War Emergency Programme destroyers to Type 62 Air Direction Frigates.
In March 1952, the programme was reduced, as the War Emergency Destroyers were too small to accommodate the heavy radars.