Original aft triple mounts on HMS Repulse circa. 1916 - 1917
The gun was based on the barrel of the QF 4 inch Mk V and the breech mechanism of the BL 4 inch Mk VIII and was first introduced in World War I on capital ships as secondary armament in triple-gun mountings, intended to provide rapid concentrated fire. This turned out to be unworkable in practice : Jane's Fighting Ships of 1919 commented : "4-inch triples are clumsy and not liked. They are not mounted in one sleeve ; have separate breech mechanism : gun crew of 23 to each triple". Guns were thereafter used in single-gun mountings, typically on smaller ships as primary armament.