I suggested it to the Boss because at that "other place" there was a similar topic - "Name That Ship" - and it generated quite a deal of participation. It did not deal with only RAN ships, and had blokes "Googling" furiously".

I know I learned a lot from it - like what American warship still remains in "enemy" hands and where is she.

Now, to answer Seejay's question:

In 1941 work began on the design of a twin 4.5-inch B.D. (between decks) HA/LA gun mounting providing 80 degrees of elevevation. Intended for fleet destroyers of the 1942 Programme the mounting was based on the 4.5-inch BD mounting carried in the (modernised) battleships Queen Elizabeth and Valiant and battlecruiser Renown and the "Illustrious" Class aircraft carriers. The mounting took the form of a small, fully-enclosed turret which extended down through a hole in the weather deck to the deck below. The well below the guns therefore provided more than enough clearance for high angles of elevation. The prototype of this mounting, the Mk.IV carrying two twin 4.5-inch Mk.III guns, was fitted in Savage of the "S" Class destroyers for sea trials. The turret was placed in 'A' position, 'B' gun being omitted to compensate for the additional weight of the twin mounting. The two 4.7-inch guns aft were replaced by two single 4.5-inch mountings in order to keep a uniform calibre armament and avoid complications in ammunition supply.
Some idea of the difficulties involved in providing a satisfactory HA/LA armament for destroyers can be gained from the fact that the twin Mk.IV 4.5-inch mounting weighed almost as much as all four single 4.7-inch mountings on the intermediate destroyer design. The 1942 fleet destroyers, the "Battle" Class, carried two of these mountings together with a complex stabilised fire control system and a sophisticated close-range AA armament.
Although the "Battle" Class carried onlt the same number of main armament guns and torpedo tubes as the Emergency destroyers their designed displacement was over 600 tons greater. Destroyers of this size and cost would have been unthinkable prior to the war. They were certainly the type of destroyer that met the requirements of the time but neither the ships nor the armament could have been produced at the rate demanded by the Emergency Flotilla Programme.
The decision to adopt the twin 4.5-inch mounting led to the general adoption of the 4.5-inch gun for destroyers. A new single mounting was introduced, the Mk.V, carrying the 4.5-inch Mk.IV gun. It was virtually identical to the 4.7-inch Mk.XXII mounting except for some changes in the cradle and loading tray to accommodate the different calibre of gun.
The gun fired a 55-pound (25kg) shell at a muzzle velocity of 2450 feet per second (747 metres per second compared with 50 pounds (22.7kg) and 2650 fps (808mps). The 4.5-inch was considered to have superior ballistic qualities, the loss in muzzle velocity being made up for by the increased weight of the shell.
In May 1942 it was approved to substitute this new 4.5-inch mounting for the four 4.7-inch in the "Z" Class.

The 4.7-inch gun was first fitted in RN destroyers in the flotilla leaders of the "Scott" Class, replacing the 4-inch of the previous "V" and "W" Classes.
Throught the 1930s, nations armed their destroyers with every calibre from 4-inch to 5.1-inch; the Brits and Italians chose the 4.7-inch calibre, the Yanks, Germans and Japanese chose the 5-inch calibre, while the French chose the 5.5-inch for their larger destroyers and 5.1-inch for the smaller ones.
Late in the war the Germans even tried mounting 5.9-inch guns on their latest destroyers.

The purpose of this topic is to have a bit of fun, test the knowledge of individuals, and inform others.
It is not a competition, and the topic can contain any number of questions simultaneously.
Neither is it intended for it to be somewhere for someone to find an answer they previously didn't know to a particular question; such questions should be posed under an existing topic.

Questions will be numbered sequentially, to avoid confusion in answering.
Seejay's question re the 4.7-inch/4.5-guns is therefore Question No.1.

And here is Question No.2:

Who claims to be the first Australian warship to have fired a "shot in anger" in WW2?

Delta

P.S.:

Yes - the "Tribals" were the last RAN destroyers to carry the 4.7-inch gun. The "Q" Class also carried them prior to their conversion to Type 15 frigates.