The Flower's 4-inch/45 gun was intended to engage submarines on the surface. Its mounting could depress to ten degrees below horizontal but elevate only to thirty degrees, limiting its value against high-flying aircraft. A firing rate of ten to twelve rounds per minute, muzzle velocity of about 800 metres per second, and maximum range of 12.6 kilometres gave it a useful surface-fighting role. The design dated from the First World War and belonged to the British BL category. Later four-inch guns used charges in metal cartridges and carried the QF designation.
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Why the Flower's four-inch gun targeted surfaced submarines
Limited elevation kept the four-inch gun focused on surface action, while its firing rate and range made it useful against an exposed U-boat.