[5][6] From the BL Mark I gun of 1916 the 4.7-inch (120 mm) calibre was the mid-calibre weapon of the Royal Navy, used particularly on destroyers.

[7] The QF 4.5 inch L/45 was developed originally as a dual-purpose weapon with which to arm aircraft carriers and reconstructed battleships and battlecruisers.

It was later developed as a new dual-purpose weapon with which to arm destroyers, supplanting the ubiquitous 4.7 inch gun.

The guns were fitted with Magslip electrical data transfer from Predictors AA Nos 3, 5 and 10 and were probably used initially with GL radars and UB 10 18 feet base optical height and range finders.

The gun was laid and fuzes set by pointer matching, it is unclear the extent to which advances in 3.7-inch fire control were applied to 4.5-inch.

During the war Machine Fuze Setter No 10 was added, This improved the rate of fire from 8 to 10 rounds per minute and raised the effective ceiling to 34,500 feet.

Initially the standard fuze was an igniferous design, No 199 with a maximum running time of 30 seconds that limited performance.

Deployment included: UK (Royal Artillery) June 1940:[9] (the AA divisions included 3-inch and 3.7-inch regiments in addition to 4.5-inch) Far East January 1942:[10] Mediterranean June 1943:[11] Middle East January 1943:[12] West Africa Dec 1941:[13] India Dec 1941:[14] Colonel Probert of the Armaments Research Department developed rifling with tapered groove depth and with the last few inches of the barrel being smoothbore.