The RN Gunnery Pocket Book published in 1945 states:
These guns are combined High Angle and Low Angle Guns. The Mark II Mounting is found in all Dido class cruisers. The Mark I Mounting is found in King George V class battleships, where they fulfil the combined functions of H.A. Long Range Armament and Secondary Armament against surface craft. The main differences between the two mountings lie in the arrangements of the shellrooms and magazines, and the supply of ammunition to the guns. In this chapter, only the Mark II Mounting, as found in Dido class cruisers, is discussed. The 5.25 in. calibre with separate ammunition is used for dual High Angle and Low Angle Armament, since it gives the reasonable maximum weight of shell which can be loaded by the average gun's crew for sustained periods at all angles of elevation. The maximum rate of fire should be 10-12 rounds per minute.
A wartime account describes HMS Euryalus firing her 5.25 in guns:
We left Suez and headed for the Gulf, where at 1PM the ship's company closed to action-stations and gave a demonstration of the cruiser's fire power to the army officers. Fire was opened with the 10 5.25" guns in the form of a low angle barrage accompanied by fire from smaller guns. Set to burst at 2000 yds range, a terrific barrage was put up for two minutes and we fired some two hundred rounds of 5.25-inch HE...A wall of bursting shell was thrown up just above sea level and I could see that the army officers were impressed...
The gunhouse was cramped, and the heavy projectile and cartridge cases resulted in a reduced sustained rate of fire from the designed twelve rounds per minute to seven or eight according to postwar publications. However, this does not appear to have reduced HMS Euryalus's rate of fire, at least over a one-minute period, which would be the typical time for a World War II AA engagement. The dual-mount turret could traverse at 10 deg/s, which was too slow to track quickly enough to engage at close ranges the higher-speed aircraft of the Second World War. The elevation and traverse rates were still higher than some other contemporary weapons, such as the 4.1-inch C/31 and C/37 twin mounts carried on the Bismarck and Tirpitz.
These guns performed well on HMS Prince of Wales during Operation Halberd but Prince of Wales was overwhelmed in the loss of Force Z, due to factors unrelated to the 5.25-inch weapon system. No Dido-class cruisers were lost in the Battle of Crete, although the Crown Colony-class cruiser HMS Fiji and the Town-class cruiser HMS Gloucester were both bombed and sunk, after they ran out of AA ammunition. No Dido-class cruiser was lost from air attack, although four were sunk by submarine or surface-launched torpedoes. HMS Spartan, a Bellona-class cruiser, was sunk at anchor in 1944 by a Luftwaffe guided missile.
The gun had a maximum surface range of 24,070 yards, and the 80-pound shell was well-suited for use against destroyers and small cruisers. However, the gun was used on several occasions against heavier ships, most notably against the German battleship Bismarck.
In 1944, VT-fuzed (using radar to detect proximity to a target) shells for the gun became available, making the gun significantly more effective against aircraft.
The RP10 mounting was improved and the fire control upgraded for the installation on the Bellona-class cruisers, and the battleships HMS Anson and HMS Vanguard, the latter of which would prove to be the last battleship ever built for the Royal Navy. However, Vanguard never saw action.