The famous English "pom pom"
The installation, called Vickers QF 2 pounder Mark II, was actively used in the battles of the First World War. They were even used as a ZSU, installing the Pearless armored truck on the chassis. A number of such vehicles were supplied to the Russian imperial army. This automatic anti-aircraft gun was recognized successful and became the prototype for the creation of an improved version - Vickers QF 2 pounder Mark VIII. It was with this air defense system that the British Royal Navy entered World War II.
After the end of the First World War, which in those years was also called the Great War, the high cost, with not the most obvious need, squeezed out "pom-poma" from the arsenal of land units. However, they were very firmly entrenched in the arsenal of warships, the price of which, in turn, could not allow savings on such an important defensive aspect as the air defense system.

The creation of multi-barreled anti-aircraft guns was based on the proven 40-mm Vickers Mk II anti-aircraft gun. The undoubted advantage of these anti-aircraft guns at the time of creation was a sufficiently high rate of fire - 100-115 rounds per minute, which for those years exceeded the rate of fire of all light guns without exception. Another undoubted advantage was that a huge amount of ammunition for these guns had accumulated in England - a legacy of the First World War. During the modernization of the English engineers did not touch the barrel, but improved the automation. Reliable water cooling of the barrel was also left unchanged (hi Maxim machine gun). A prototype of the updated gun was already ready for the 1923 year, and after all the finishing, in the 1927 year, fully combat-ready “helpers” were created.
With all this, the gun was distinguished by a sufficiently small effective range of shooting - 2000 meters. This means that it was possible to conduct an effective, and most importantly, effective fire, only at a distance of a little more than one nautical mile. At the same time, one nautical mile is the 10 cable or the maximum allowable distance so that the seaplane of those years could relieve the torpedo. They still knew nothing about dive bombers. Since the most optimal distance for the discharge of a torpedo was two times smaller, the Vickers anti-aircraft gun could not be attributed to ineffective solutions. In addition, from it the fire was supposed to be carried out on the plane, which lay on the combat course and was not able to actively maneuver in the air. At the same time, the British understood perfectly well that a guaranteed defeat of an aircraft can provide only a high density of fire in this critical segment of the distance. It was possible to achieve high density due to the elementary build-up of the number of barrels in the anti-aircraft installation.
Vickers machines were initially placed in mechanized single-barrel stops, but rather quickly the standard was installed on the 4 (Mark VII) and 8 guns (Mark VI). Thus, the four-mount Pom Pom installations were placed on destroyers and cruisers of the Royal Navy, and eight-barrel installations were put on cruisers, battleships and aircraft carriers. For the years of the Second World War, the Vickers QF 2 anti-aircraft gun pounder became the main means of the short-range air defense system on warships of the British fleet.
The multi-barrel installation, adopted in 1927, bypassed all existing foreign analogues in its characteristics, and behind this success was the fact that this installation had no development prospects. Its main disadvantage was the low initial velocity of the projectiles. With a barrel length of only 40,5 calibers, the initial velocity of the projectile was only 701 m / s. This speed was enough to deal with aircraft "shelves" originally from 1920-s, but for high-speed monoplanes of the end of 1930-s and even more so 1940-s it was not enough. Due to the low initial velocity of the projectiles, the firing range, which was 4,5 km, also suffered, while the targeting range, as mentioned above, was 2 times smaller. The second, as it turned out only in active exploitation, was the fatal disadvantage of the supply of shells.
A solution that was successful in its idea (the use of continuous tape) was absurd in its implementation in practice. Anti-aircraft projectiles were placed in the cells of a special ribbon of tarpaulin, which in practice very often stuck when fired. In this respect, the 37-mm German anti-aircraft guns and 40-mm "borforsy", which have a charger feed, turned out to be significantly more reliable in combat conditions. In addition, the English anti-aircraft projectile, equipped with a contact fuse, was unsatisfactory, as it required a direct hit on the plane. As practice has shown, when shooting at the new Japanese torpedo bombers, the “pom-pom” simply did not have time to organize a curtain of aimed fire until the torpedo was dropped by an airplane.
In addition, the 8-barreled anti-aircraft gun Mk VI was cumbersome, it weighed 16 tons, which was due to the presence of a mechanical drive, which was dependent on energy sources. At the same time, the speed of horizontal and vertical pickup did not exceed 25 degrees per second, which was not enough for enemy enemy dive bombers moving quickly in the field of aiming. The 1800 ammunition from the shells on the barrel was enough for 15-20 minutes of continuous firing, which was clearly not enough when carrying out lengthy operations in the area of enemy aircraft.

Understanding and accepting all these shortcomings, during the war years, part of the “pom-pom” was replaced by the well-established Swiss 20-mm Oerlikon and 40-mm Swedish Bofors machine guns. Such anti-aircraft guns in single installations did not require a power drive, so they could be installed in almost any convenient place of the vessel. At the same time, over time, single-barrel “airlikons” were almost completely superseded in the fleet by a new pairing based on an installation similar to that for 40-mm single-barrel “beaufors”. They had other benefits. Thus, the effective range of fire of “bofors” was twice as high as “pom-pom”, and the Swiss anti-aircraft guns provided 4 times higher rate of fire at approximately the same height reach.
At various times, without exception, all British battleships, as well as battlecruisers and heavy cruisers of the English fleet, armed themselves with "pom pomom". Most of them who entered the war with large-caliber machine guns in the role of near-air defense systems, light British cruisers, also overwhelmingly received Vickers 40-mm anti-aircraft guns no later than 1942 of the year. The destroyers of the Pom-Poma usually went in single-barrel variant, but, starting with the ships of the Tribal series, they began to receive four-barreled installations. At the same time, even at that moment, when the Erlikons and Bofors poured in a wide stream at the English fleet, the British anti-aircraft guns were very rarely dismantled, this was mostly only for single-barreled installations that were mounted on old small destroyers. Other ships of the English fleet did not rearm, but only re-equipped with new anti-aircraft weapons. The fashion for mass de-commissioning of the “pom-pom” began after the end of the Second World War, when they very quickly left the stage.
It should be noted that at the time of the appearance of "pom-pom" were the most effective means of air defense near zone. At the same time, installations of this type or created in their image and likeness were available to many countries of the world, as they did not carry any special know-how in themselves and were perfectly familiar to military experts from many states during World War I.

Of course, by the end of the 1930-s such anti-aircraft installations looked outdated, especially against the background of new developments. The same "Oerlikon" had a significantly higher rate of fire, and "Bofors" - an effective firing range. Not particularly able to catch up with modern high-speed aircraft and multi-barrel guidance drives Vickers QF 2 pounder. However, no one was going to send them to the scrap. And it's not about the conservatism of the English admirals, whose fleet at the time of the end of the war had in 2 times more “helpers” than the Swedish “beaufors”. The fact is that not always having time to hit fast-moving air targets, multi-pom-pom installations created a very dense curtain of barrage fire, attempts to break through which were deadly for the pilot, turning him into a kamikaze.
Information sources:
http://alternathistory.livejournal.com/1072598.html
http://wunderwafe.ru/WeaponBook/KGV/08.htm
http://www.warfleet.ru/zenitnoe/pompom.html
http://ru.wikipedia.org
Information