A dual purpose gun is a naval artillery

Naval artillery

Naval artillery, or naval riflery, is artillery mounted on a warship for use in naval warfare. Naval artillery has historically been used to engage either other ships, or targets on land; in the latter role it is currently termed naval gunfire fire support...

 mounting

Weapon mount

A weapon mount is a weapon component used to secure an armament. Weapon mounts can be broken down into two categories: static mounts and non-static mounts.-Static mount:...

 designed to engage both surface and air targets.

Description

Second World War-era capital ships had four classes of artillery: the heavy main battery, intended to engage opposing battleships and cruisers (356 mm (14 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 90 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

Royal Navy

The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...

, the French Navy

French Navy

The French Navy, officially the Marine nationale and often called La Royale is the maritime arm of the French military. It includes a full range of fighting vessels, from patrol boats to a nuclear powered aircraft carrier and 10 nuclear-powered submarines, four of which are capable of launching...

 and the Imperial Japanese Navy

Imperial Japanese Navy

The Imperial Japanese Navy was the navy of the Empire of Japan from 1869 until 1947, when it was dissolved following Japan's constitutional renunciation of the use of force as a means of settling international disputes...

 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

Kriegsmarine

The Kriegsmarine was the name of the German Navy during the Nazi regime . It superseded the Kaiserliche Marine of World War I and the post-war Reichsmarine. The Kriegsmarine was one of three official branches of the Wehrmacht, the unified armed forces of Nazi Germany.The Kriegsmarine grew rapidly...

, 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

Surface combatant

Surface combatants are a subset of Naval Warships which are designed to engage in combat on the surface of the water, with their own weapons. They are generally ships built to fight other ships, submarines or aircraft, and can carry out several other missions including counter-narcotics operations...

 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. For example, a Royal Navy battleship of the King George V

King George V class battleship (1939)

The King George V-class battleships were the most modern British battleships used during World War II. Five ships of this class were built and commissioned: King George V , Prince of Wales , Duke of York , Howe , and Anson .The Washington Naval Treaty of 1922 limiting all of the number,...

 class had sixteen QF Mark I 5.25 inches (133 mm) guns that could engage either enemy ships or high level aircraft, as necessary, but had lower potential in the AA role due to its 70 degree maximum elevation, and slower than expected rate of fire due to the larger shells it fired. In service these guns scored a number of A.A. kills. French experience on the Richelieu class battleship

Richelieu class battleship

The Richelieu class battleships were the last and largest battleships of the French Navy, staying in service into the 1960s. They still remain to this day the largest warships ever built by France...

s with large calibre dual-purpose artillery was disappointing. On the other hand, the US Navy's 5" DP guns were considered highly successful, although they still required about 1000 rounds of ammunition fired for every aircraft downed.

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

Tribal class destroyer (1936)

The Tribal class, or Afridi class, were a class of destroyers built for the Royal Navy, Royal Canadian Navy and Royal Australian Navy that saw service in World War II...

 class, the Royal Navy introduced a series of destroyer classes that had dual purpose guns, but in 4.7 inch QF Mark XII

4.7 inch QF Mark XII

The 4.7 inch QF Mark IX and Mark XII were 120-mm 45-calibre naval guns which armed the majority of Royal Navy and Commonwealth destroyers in World War II, and were exported to many countries after World War II as the destroyers they were mounted on were sold off.-Description and history:These guns...

, 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

Fuze Keeping Clock

The Fuze Keeping Clock was a simplified version of the Royal Navy's High Angle Control System analogue fire control computer. It first appeared as the FKC Mk1 in destroyers of the 1938 Tribal class, while later variants were used on sloops, frigates, destroyers, aircraft carriers and several...

 and the mountings were provided with A.A. shell fuze setters. The USN had developed a similar class of destroyer, the Porter

Porter class destroyer

The Porter-class destroyers were a class of eight 1,850-ton destroyers in the United States Navy.The first four ships were laid down in 1933 by New York Shipbuilding and the next four in 1934 at Bethlehem Steel Corporation in Quincy, Massachusetts...

 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

Norwegian Campaign

The Norwegian Campaign was a military campaign that was fought in Norway during the Second World War between the Allies and Germany, after the latter's invasion of the country. In April 1940, the United Kingdom and France came to Norway's aid with an expeditionary force...

:

"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

Romsdalsfjord

Romsdalsfjord is the ninth longest fjord in Norway. It is long and located in the Romsdal district of Møre og Romsdal county. It flows through the municipalities of Midsund, Haram, Vestnes, Molde, Nesset, and Rauma...

, 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

Battleship secondary armament

The secondary armament of a capital ship are smaller, faster-firing weapons that are typically effective at a shorter range than the main weapons...

 for large surface ships such as cruiser

Cruiser

A cruiser is a type of warship. The term has been in use for several hundreds of years, and has had different meanings throughout this period...

s and battleship

Battleship

A battleship is a large armored warship with a main battery consisting of heavy caliber guns. Battleships were larger, better armed and armored than cruisers and destroyers. As the largest armed ships in a fleet, battleships were used to attain command of the sea and represented the apex of a...

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.