Dual purpose gun

A dual-purpose gun is a naval artillery mounting designed to engage both surface and air targets.

Second World War-era capital ships had three classes of artillery: the heavy main battery, intended to engage opposing battleships and cruisers (280 mm (11 inch) or greater); a secondary battery for use against enemy destroyers of 127 to 155 mm (5 to 6.1 in); heavy anti-aircraft guns of 76 to 127 mm (3 to 5 in), which could create barrages to knock out airplanes at a distance; finally, light rapid-fire anti-aircraft batteries (A/A) to track and bring down aircraft at close range. The light A/A was dispersed throughout the ship and included both (20–40 mm) and heavy machine guns.

During World War II, the US Navy, Royal Navy, the French Navy and the Imperial Japanese Navy combined the secondary battery with the heavy anti-aircraft guns, creating a dual-purpose secondary battery. They discarded the dedicated, anti-ship secondary batteries altogether, because a battle-line fleet would be screened against cruiser and destroyers most of the time. Also, it was deemed unlikely that a battleship would be simultaneously facing both rushing destroyers and aircraft, and it would take up too much space to have separate types of guns to deal with both threats. Rather, they replaced them with turret-mounted dual-purpose guns that could be used against both aircraft and ships. The space saved from combining the two types of guns added to simplification of supply, increased deck armour coverage, stowage of other equipment, a larger light anti-aircraft battery, and other needs. This arrangement was seen as more efficient, and was deemed adequate to meet anti-surface and anti-aircraft needs under most circumstances.

The Kriegsmarine, in a practice similar to the Italian and Soviet navies, adopted a mixed-calibre secondary battery, with dedicated anti-ship guns, coupled with smaller-calibre heavy anti-aircraft batteries, instead of adopting dual-purpose secondaries like the British or Americans. Both navies were concerned of close-range torpedo attacks from enemy (in particular, French) destroyers and torpedo boats, and considered the more powerful, larger calibre anti-ship secondaries a must. The French Navy used a mixed-calibre system, as well, but their secondary battery was dual-purpose. This tended to complicate ammunition supplies and render certain armament useless in some situations.

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