Although there was a wide variation in armament choice at the beginning of the war, it was followed by a convergence of the arsenals of the major air forces of World War II. By 1943 most air forces had a rifle-calibre, a medium-calibre and a 20mm cannon available. In Germany and Japan there was also a development of 30mm cannon with a modest muzzle velocity but very effective ammunition, suitable for bomber-destroyer applications. Of these the rifle-calibre weapon was becoming less important. There was an universal preference for belt-fed weapons, and their muzzle velocities and rates of fire were quite similar. This was an expression of the “state of the art” in aircraft armament. A comparison can be offered in terms of weight of fire, in kg/sec, of the weapons in these four categories:
|UK||USA||USSR||Germany||Jap. Army||Jap. Navy|
This table shows that in each category, the major combattants had roughly equivalent guns, with the exception of the 30mm category which was not developed by the Allies. (Some 23mm and 37mm cannon were developed in the USSR.) There were of course exceptions; most notably the Japanese Navy did persevere in employing Oerlikon-derived 20mm cannon with a low rate of fire.
There were important variations in the way the available guns were used, however. The aircraft designers had to balance weight, hit probability, and destructive power. A sufficient hit probability could be achieved by installing a sufficient number of guns, so that the total number of projectiles fired was kept high. Most air forces seem to have been of the opinion that a fighter should have four or more guns. The maximum number of guns carried by a fighter was eight. Well-known is the debate in the German Luftwaffe about the armament of the Bf 109F, which carried a single MG 151 cannon and two MG 17 cowl guns. Galland was of the opinion that this armament gave a too low hit probability for pilots of average shooting ability.
For an installation of four to eight guns a balance had to be found between firepower and weight. The bigger guns were heavier and had a larger recoil, which required strengthening of the aircraft structure and imparted an additional weight penalty. During the war fighters became more powerful and were able to carry heavier weapons; at the same time the lightest category, rifle-calibre machineguns, was proven ineffective and gradually replaced.
Generally speaking (with all dangers that are included in generalisations) one can distinguish three phases in the development of World War II fighter armament. They covered one general trend: A shift to heavier and heavier weapons. Mark K. Wells in Courage and Air Warfare cites the following significant statistics: In the last 6 months of 1942, of the hits recorded on B-17 bombers, cannon hits numbered only 40% of the machine gun hits. In the autumn of 1943 this had risen to 80%. In 1944 there were 35% more cannon hits than machinegun hits.
Some WWII fighters were armed with was essentially World War I armament: Two machineguns in the front fuselage. Such armament was carried by Italian and Japanese Army fighters; as a concession to modernity heavy machineguns were substituted for the rifle-calibre weapons. This could be considered a “zeroeth” phase in WWII fighter armament development.
In the first phase the rifle-calibre machinegun was still important. Fighters either carried a homogenous armament of such guns, or they used a mixture of rifle-calibre guns with cannon or medium-calibre machineguns. Examples of the first approach are the eight Browning .303s in the Spitfire and the four MG 17s in the early Fw 190. Examples of the second approach are the MG FF and MG 17 weapons of the Bf 109E, the two .50 and four .303 Brownings of the early P-51, or the two 20mm cannon and two 7.7mm guns in the A6M2. This first phase ended when it was understood that the rifle-calibre machinegun was ineffective against modern combat aircraft.
Light machineguns would put a lot of holes in the skin of an aircraft, but they could not cause it to break up. Therefore one aimed for the vulnerable, critical parts of the aircraft: The pilot, the fuel tanks, and the engines. However, armour and self-sealing fuel tanks were an effective defense. Many fighters entered the war without these items, but by 1941 a fighter without them was no longer considered suitable for combat.
In the second phase there were still two options. Either a homogenous armament of medium machineguns was used, or a mixture of modern 20mm cannon with machineguns. The first approach was chosen by the USAAF, which equipped most of its fighters with six or eight .50 Browning guns. Examples are the P-40 Warhawk, P-47 Thunderbolt, and P-51 Mustang. The US Navy adopted the same armament. The second approach was more common, and used by fighters such as the Spitfire, the Bf 109, or the Ki.84 Hayate. The cannon were now in general belt-fed, high-velocity weapons with a satisfactory rate of fire. The disadvantage of cannon was that their weight and recoil precluded the use of more than one or two. Hence they had to be mixed with machineguns, with different ballistic characteristics, different ammunition and different maintenance requirements.
The disadvantage of an armament of medium-calibre machineguns only was that it lacked the destructive power to be effective against anything but fighters or lightly constructed bombers. And armour that protected reasonably well against .50 projectiles was increasingly fitted to fighters, and self-sealing fuel tanks were being designed to survive hits in this calibre.
The third phase, which lasted well beyond WWII, was characterized by a switch to a homogeneous armament of 20mm cannon. Examples of such armament are the last Spitfire models, the Typhoon and Tempest, the Soviet La-7, and the Japanese N1K-2J. Usually four 20mm cannon were carried. This was also the standard armament for most post-war fighters, except those of the USAAF.
Again, there was a second option: That of heavy “bomber killer” armament. Here the German MK 108 cannon must be mentioned, as installed in the Me 262, and the Japanese Ho-155. This option was mainly chosen by Axis powers, because they were confronted with large numbers of heavy bombers. But these weapons, calibre 30mm or larger, were either low-velocity weapons, or they were extremely heavy. In both cases they reduced the suitability of the fighter for combat against other fighters. Because of this and the introduction of spin-stabilized and folding-fin rockets (and still later, effective guided missiles), such armament was installed in few post-war fighters, but one that must be mentioned is the MiG-15.
The characteristic of this phase is that the goal no longer was to destroy an aircraft by hitting the crew of vulnerable parts of its equipment. Especially the larger cannon were intended to destroy the structure of the aircraft itself: A 30mm hit could cut a fighter in two, and put a large hole in a heavy bomber.
The four horizontal lines in each plot represent the firepower of, starting at the bottom, eight Browning .303 guns (blue), six Browning .50 guns (red), four 20mm Hispano Mk.II cannon (green), and finally two 30mm MK 108 cannon (yellow).