Dual purpose gun A dual purpose gun is a naval artillery mounting designed to engage both surface and air targets . Description . 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 automatic cannons ( 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 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 . Dual purpose guns are designed as a compromise between the heavy main armament of a surface combatant and dedicated anti - aircraft gun s . 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 . For example , a Royal Navy battleship of the " King George V " class had sixteen QF Mark I guns that could engage either enemy ships or high level aircraft , as necessary . Not all dual purpose guns have high elevation . The determining factor was whether or not the mounting was provided with an anti - aircraft fire control system and a method for setting the time fuze in the A . A . warhead , fired by the gun . Starting with the Tribal class , the Royal Navy introduced a series of destroyer classes that had dual purpose guns , but in 4 . 7 inch QF Mark XII , twin CP Mk . XIX and later mountings limited to 40 , 50 or 55 degrees elevation , however , the guns were controlled by an A . A . fire control system and the mountings were provided with A . A . shell fuze setters . The USN had developed a similar class of destroyer , the " Porter " class with eight Mk 12 5 inch / 38 caliber ( 127 mm ) guns in four Mark 22 Single Purpose ( surface action only ) twin mounts , limited to 35 degrees elevation , but with no provision for A . A . fire control and no on - mount fuze setters . The 40 degree elevation did limit the ability of the " Tribal " class guns to engage dive and high altitude bombers but they were still effective in engaging low altitude , level and torpedo , bombers and could still provide barrage fire over other ships being attacked by dive bombers . Admiral Sir Philip Vian describes the use of 4 . 7 inch Mark XII guns against aircraft during the 1940 campaign in Norway : " It became clear at once that in an attack from the air in narrow waters flanked by mountains , the cards were held by the aircraft . There was too little sea - room for full freedom of manoeuvre , and the aircraft ' s approach was screened by the rock walls . As often as not , when they did come into view it was at such an angle that our 4 . 7 - inch guns , whose maximum elevation was only forty degrees , could not reach them . . . Aandalsnes is approached through the Romsdal Fiord , and lies forty miles from the entrance , off which we arrived on the 24th April . The daylight passage of the convoy and escort through this waterway , speed five knots , on a steady course and with mountains rising steeply either side , presented an alluring invitation to enemy aircraft . Junkers attacks persisted to the end , but the fire of the destroyers , although limited to an elevation of forty degrees , was enough to keep the enemy just too high for their standard of marksmanship . Not a ship received a direct hit , though some were damaged by the splinters from near misses . " " Dual purpose guns , often abbreviated to DP guns , were originally designed as a secondary armament for large surface ships such as cruiser s and battleship s 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 .