Dual purpose gun
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A dual purpose gun is a naval artillery mounting designed to engage both surface and air targets.
[edit] Description
Dual purpose guns are designed as a compromise between the heavy main armament of a surface combatant and dedicated anti-aircraft guns. Usually of a mid-range caliber, the gun is heavy enough to prove useful against surface targets including ships, surfaced submarines, and land targets. However, it is compact enough to fit into a mounting capable of good traverse and high elevation, as well as being capable of a high rate of fire, allowing it to successfully engage air targets at all angles.
Dual purpose guns, often abbreviated to DP guns, were originally designed as a secondary armament for large surface ships such as cruisers and battleships to complement their heavy main armaments. Later, such guns began to be added to smaller vessels as their primary gun armament, and with the progression of ship design away from heavy caliber guns, today nearly all main gun armaments are of dual purpose nature.
Most DP mounted guns have calibers in the range from three to five inches.
In British service the term HA/LA for "High Angle/Low Angle" was used.