F Class ships (1933):
The F class were the only ships to mount the hand loaded and unpowered 4.5" dual purpose gun. This mounting was a stop gap given to the ships because of the German 5" armed destroyers supposedly outclassing the 4" armed CDE's. In practice this was not the case. The 4.5" gun was a good gun and in the twin powered turrets they were a winner. Bag and shell guns, they had a range of 20,000 yards and a 54lb shell. It was the hand loading of the 54lb shell that was awkward on these ships as crew fatigue could slow down the rate of fire. The F class were the first destroyers to be fitted with the quad 2 pounder mounting that provided a much heavier AA capability and also the first to mount the Hispano 20mm cannon that did not last long in Naval service, being of too fragile a mounting for the harsh conditions aboard ship. The F class also changed the dimensions of the ships for the first time, and actually made what looks like one class of ship into two.
The F class through no fault of their own were treated much like the A-B class ships. By the end of 1942, the remaining six ships of the class has been repurposed as long range escorts, with A (in some) and Y guns, the aft set of torpedoes, and the forward funnel, being removed. In their place a larger and better ASW set of depth charges and then Hedgehog were fitted in place of the A/Y guns and torpedoes, while the forward boiler room was replaced with oil fuel tanks.
G-H Class Ships (1934-36):
The disappointment of the single 4.5" mounting meant a return to the 4" dual purpose gun, but this time in the Mk XVI twin mounting, as the twin 4.5" they were meant to be armed with were just not available. All the battleship, and aircraft carrier construction and reconstruction used the twin 4.5" guns in their armaments. With four twin 4" mountings, the sheer volume of fire they produced, compared to the 4.5", was just awesome. These ships proved themselves time after time in the Mediterranean where their AA fire was most needed.
The ships of the G-H types were kept pretty much as completed with the main additions being to the electronic fittings and the side mounted Hedgehog. They made very good escorts to the big carriers.
By the time all of the different Commonwealth classes, and extra ships being built for other countries were completed, over a 100 of the A-I class were in service. From 1934, the DP revolution altered the armaments being fitted with single 4.5", twin 4", then twin 4.5" mountings. During the war, different classes and indeed ships within each class could be armed differently depending on what the Admiralty required at the time the ship was due for refit/rebuilding. The H and I classes featured extra ships to the classes, with an extra four H class and an extra eight I class. On the outbreak of war the Admiralty found eight 'H' type under construction for Brazil, four 'I' type under construction for Turkey, and another six G, H, and I types for four other countries. These were all incorporated into the Royal Navy, who was very glad to get them.