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The WWII Fighter Gun Debate

The Question

One of the recurring questions about the fighter aircraft of the Second World War is the choice of armament. There was considerable change in fighter armament during the war. Options ranged from rifle-calibre machine guns, with a calibre of about .30 (7.7mm) to 30mm cannon and heavier. People have endlessly defended or criticised the firepower of WWII fighters. The most controversial issue is probably the choice of the .50 machine gun as standard armament of US fighter aircraft. Quite often, the arguments used in this debate are pulled from the air.


The Guns

The parameters give for each gun are:

Rifle-calibre machineguns

Rifle-calibre machine guns ranged from 7.5mm to 7.92mm. It is should be remembered that for similar calibres, there can still be a large difference in the weight of the projectile. Another important factor is the size of the cartridge case, which determines the amount of propellant and therefore is an important factor in muzzle velocity.

Name Ammunition Rate of FireMuzzle velocityWeight
Browning .30 M27.62 x 63 ( 9.9 g)1200 rpm 835 m/s 10.4 kg
Browning .303 7.7 x 56R (11.3 g)1140 rpm 745 m/s 10.9 kg
Darne 7.5 x 54 ( 9.1 g)1700 rpm 790 m/s 8.4 kg
MG 17 7.92 x 57 (10.0 g)1100 rpm 790 m/s 12.5 kg
MG 81 7.92 x 57 (10.0 g)1500 rpm c. 730 m/s 6.3 kg
Breda-SAFAT 7.7 x 56R (10.1 g) 800 rpm 712 m/s 12.5 kg
Type 89 mod 1 7.7 x 58SR (10.5 g) 750 rpm 750 m/s 9.0 kg
Type 89 mod 2 7.7 x 58SR (10.5 g) 900 rpm 750 m/s 11.8 kg
Type 98 7.92 x 57 (11.5 g)1500 rpm c. 750 m/s 7.0 kg
Type 92 7.7 x 56R (11.3 g) 600 rpm 750 m/s 8.4 kg
Type 97 7.7 x 56R (11.3 g)1000 rpm 750 m/s 11.8 kg
Type 1 7.92 x 57 (11.5 g)1000 rpm c. 750 m/s 6.8 kg
ShKAS 7.62 x 54R ( 9.6 g)1800 rpm 825 m/s 10.6 kg
Ultra ShKAS 7.62 x 54R ( 9.6 g)2700 rpm 830 m/s 10.0 kg

The best known gun of this list is the Browning M2. The origins of this weapon go back to a gun designed in 1917 for use by the infantry. The M2 version was developed for installation in aircraft in the early 1920s, and compared with earlier models its rate of fire was increased. The gun is still in use today. The British version fired rimmed .303 ammunition instead of rimless .30 ammunition.

The gas-operated French 7.5mm Darne was used in both fixed and movable installations, and its good characteristic was its high rate of fire. But because of the early French defeat its impact on the war was minimal, and after the war it quickly disappeared.

The German MG17 was derived from the Swiss Solothurn design. It was often in synchronized installations, on the engine cowlings of German fighters, and this reduced rate of fire to 1000rpm. In 1939 the Luftwaffe introduced the superior Mauser MG81, much lighter and with a high rate of fire. But this was used almost exclusively in defensive installations, because the Luftwaffe recognized that the 7.92mm calibre was obsolete as fighter armament.

The Italian Breda-SAFAT 7.7mm gun is reported to have had about the same rate of fire as the 12.7mm version, which does not seem to make much sense. If it is true, it was a rather poor weapon. Nevertheless it was fairly important in the early years of the war, used in combination with the 12.7mm version.

The Japanese Army and Navy both used uprated copies of the British Vickers, of WWI vintage, and of the the German MG 15 observer gun. Their independent production of nearly identical weapons is almost symbolic for the lack of cooperation between the two services. No country had so many different guns in use as Japan, with so many different types of ammunition. They even managed to produce three types of .303 ammunition, with rimmed, semi-rimmed and rimless cartridges!

The Army guns were the Type 89 model 1, an indigeneous drum-fed design used in flexible installations; the Type 89 model 2, a copy of the Vickers, and the Type 98, a copy of the MG15. The Navy used the Type 92, a copy of the Lewis and again used in flexible installations, the Type 97, an improved Vickers, and the Type 1, based on the MG15.

The Russian ShKAS was probably the best of the lot, with a high muzzle velocity and a high rate of fire, although the bullet was light. The upgraded Ultra ShKAS had an extremely high rate of fire for its time, but it seems to have seen only limited use. Russian fighters were quick to adopt heavy machineguns and cannon.

Most of these guns were whitdrawn from service rapidly, because they were ineffective against the new generation of combat aircraft. Those carried armour plate and self-sealing fuel tanks. The Browning gun survived in the gun turrets of British bombers, but was increasingly ineffective: At the end of the war only incendiary ammunition was used, because the AP round was not able to penetrate the armour of modern fighters.

Heavy Machineguns

Heavy machineguns have a calibre of about 12.7mm (0.50 inch), in practice that ranges from 11mm to 15mm. The bullet is up to four times heavier than that of a rifle calibre machinegun, and can be fired at a high muzzle velocity. Hence the heavy machinegun usually had very good ballistic characteristics. The rate of fire was reduced, however.

Name Ammunition Rate of FireMuzzle velocityWeight
Browning .50 M212.7 x 99 (48.5 g) 750 rpm 870 m/s 30 kg
Ho-103 (Type 1)12.7 x 81SR (38 g) 900 rpm 796 m/s 22 kg
Type 2 13 x 64B (34 g) 900 rpm 720 m/s 17 kg
Type 3 13.2 x 99 (52 g) 800 rpm 790 m/s 30 kg
MG 131 13 x 64B (34.6 g) 900 rpm 730 m/s 17 kg
MG 151 15 x 94 (57 g) 700 rpm 960 m/s 42 kg
Breda-SAFAT 12.7 x 81SR (36.7 g) 700 rpm 760 m/s 29 kg
UBK 12.7 x 108 (48 g) 1050 rpm 850 m/s 21 kg

The Browning M2 was the standard armament of US fighter aircraft during WWII. Its development began during WWI, primarily as a weapon for fighters, but it did not reach maturity until the M2 model was introduced in 1932. By the standards of WWII it was rather heavy and its rate of fire was unremarkable, but muzzle velocity was high and the ballistic characteristics very good. The gun was also easy to manufacture and extremely reliable. In 1948 the M3 was introduced, with the rate of fire increased to 1200 rpm. Both the M2 and M3 are still in use today.

The Japanese Army Ho-103 was a copy of the Browning .50. The copy was lighter and had a faster rate of fire, but it also fired a smaller round, with a cartridge case 81mm long instead of the 99mm of the Browning. The Japanese Navy also copied the Browning, to create the Type 3, but in typical style it chose to use different ammunition. The 13.2mm calibre was the same as used by Hotchkiss anti-aircraft guns, although the 99mm cartridge case was almost identical to that of the Browning. The Navy also used the Type 2, which was a copy of the MG131, retaining the dimensions of its 13 x 64B ammunition, but with percussion firing instead of electrical ignition.

The German MG 131 with its light 13mm ammunition was developed for synchronized installations, typically in the engine cowling of German fighters. It was a light weapon, and it had electrical firing to simplify the synchronisation. It was used in German fighters until the end of the war, because --- despite rumours suggesting otherwise --- it was not possible to install the MG151 in the engine cowling of small fighters such as the Bf 109 or Fw 190. The MG151 was a much heavier, much more powerful weapon, and it replaced the 20mm MG FF as centreline armament on the Bf 109F. The gun was later enlarged to 20mm calibre as the MG 151/20, and this replaced the original 15mm version in fighter applications. In 1943 a few thousand guns based on the MG151 were manufactured in the USA, redesigned to fire a very powerful 0.6 inch (15.2 x 114, 76.5g) round. This redesign had created new problems, and this gun was was never used in combat.

The Italian Breda-SAFAT was the main weapon of Italian fighters in the early years of the war, and most (CR.42, G.50, Re.2000, and M.C.200) carried only two. Unfortunately for them, it was not a very good gun. It fired a Vickers 12.7 x 81SR cartridge, the same as adopted by the Japanese Army for the Ho-103. But the Japanese gun was lighter and fired faster.

Again, the Soviet Berezin UB was probably the best gun, with a ballistic performance similar to that of the Browning gun, but a considerably higher rate of fire. The UBK was the version for normal, fixed installations. In synchronized UBS installations the rate of fire was reduced to 800 rpm. This was significant, for most Soviet fighters had their guns on the engine cowling or in the wing roots.

The 20mm cannon

It is usually considered that 20mm is the smallest practical calibre in which explosive projectiles can be used. Smaller ones have been made, but usually light and heavy machineguns were loaded with a mixture of incendiary and armour-piercing rounds. Such ammunition also existed for 20mm cannon, so the projectile was not necessarily high-explosive.

Name Ammunition Rate of Fire Muzzle velocityWeight
Oerlikon F 20 x 72RB (144 g)500 rpm 550 m/s 24 kg
Oerlikon L 20 x 101RB (144 g)350 rpm 670-700m/s 43 kg
Oerlikon FFL 20 x 101RB (144 g)500 rpm 750 m/s 30 kg
Oerlikon S 20 x 110RB (122 g)280 rpm 830 m/s 62 kg
Oerlikon FFS 20 x 110RB (122 g)470 rpm 830 m/s 39 kg
MG c/30L 20 x 138B (119 g)350 rpm 900 m/s 64 kg
MG FF 20 x 80RB (115 g)520 rpm 570 m/s 28 kg
MG FF/M 20 x 80RB ( 92 g)520 rpm 690 m/s 28 kg
MG 151/20 20 x 82 (115 g)740 rpm 710 m/s 42 kg
MG 151/20 20 x 82 ( 92 g)740 rpm 800 m/s 42 kg
ShVAK 20 x 99R ( 96 g)800 rpm 800 m/s 42 kg
B-20 20 x 99R ( 96 g)800 rpm 800 m/s 25 kg
Hispano Mk.II 20 x 110 (130 g)600 rpm 880 m/s 50 kg
Hispano Mk.V 20 x 110 (130 g)750 rpm 840 m/s 42 kg
Type 97 (Ho-3) 20 x 125 (164 g)400 rpm 820 m/s 43 kg
Type 1 (Ho-5) 20 x 94 850 rpm 750 m/s 33 kg
Type 99 mod 1 20 x 72RB (142 g)490 rpm 555 m/s 23 kg
Type 99 mod 2 20 x 101RB (142 g)490 / 750 rpm 750 m/s 36 kg

The Swiss Oerlikon guns provided the inspiration for many 20mm guns, so they are included here for reference. The Oerlikon F was the basis for the German MG FF and the Japanese Type 99 model 1. The Type 99 model 2 was more similar to the L. And the Japanese Hispano-Suiza S.9 and S.7 were also versions of an Oerlikon gun, probably the S.

The Luftwaffe at first experimented with the MG c/30L, a weapon derived from a Flak gun. It was very powerful, but heavy and slow-firing. A lighter version, the Lb 204 which fired different ammunition (20 x 105B or 20 x 105, 134g) at 400 rpm, later appeared as defensive armament on Do 18E and Bv 138 flying boats. But the weight and slow rate of fire argued against the use of the MG C/30L on fighters, and instead the Germans adopted the Ikaria MG FF, a weapon with a low muzzle velocity, but lighter and with a modestly higher rate of fire. It was a derivative of the Oerlikon F cannon. The MG FF was usually fed from a 60-round drum. The MG FF/M version fired a smaller round with a higher muzzle velocity, but it was still a short-range weapon, and its ballistic characteristics were very different from those of the 7.92mm machineguns installed on the same fighters (mainly Bf 109E).

From the end of 1940 onwards the MG FF was replaced by the excellent Mauser MG 151/20 cannon, derived from the 15mm MG151. Because the overall length of the cartridge remained the same, the 20mm version fired a heavier projectile with less propellant, and had a lower muzzle velocity than the 15mm. Some weight was saved by reducing the length of the barrel. Again, both 115g and 92g projectiles could be fired. The MG151/20 was an excellent weapon against fighters, but the Bf 109F carried only a single cannon. Later fighter began carrying more guns, but against sturdy bombers such as the B-17 even the MG 151/20 was insufficient. The MG151/20 was also used by the Italians, and some were shipped to Japan by submarine. Recently, a development of the MG151 was made in South Africa.

The USSR had an excellent cannon in the ShVAK, a compact, fast-firing and powerful weapon. The ShVAK was basically an enlarged ShKAS. It was fitted to Soviet fighters throughout the war, but in 1945 its replacement by the equally performant but much lighter B-20 began.

For French fighters such as the Dewoitine D.501 and Morane-Saulnier Hispano-Suiza had manufactured a licence-built versions of the Oerlikon cannon, as the S.7 and S.9. These weapons were regarded as unsatisfactory, especially for installation between the cylinder banks of Hispano-Suiza V-12 engines. Engine designer Mark Birkigt decided to develop a new 20mm cannon, the HS.404. It had a different action and a much higher muzzle velocity. By 1939 it was in production also in Britain, but not until the autumn of 1941 was a reliable belt-feed mechanism for it produced. This cannon was mostly used by British and by some American fighters.

The Hispano was a rather big gun (2.36m long, compared to 1.76m for the ShVAK), and its rate of fire was lower than that of other 20mm cannon. However, it fired a heavy projectile with a high muzzle velocity. The Mk.V was a lighter version, without in-flight cocking mechanism and with a shorter barrel. It could be installed within the wing of fighters such as the Hawker Tempest. This model is still in use.

Again, the Japanese Army and Navy used different weapons. The Army used the 20mm Ho-5, that was derived from the Browning .50, and was probably the best Japanese fighter gun of the war. It replaced the older Type 97, which had been derived from an anti-tank gun. The Ho-5 was lighter than the older gun, but the cartridge case was reduced from 125mm to 94mm, and muzzle velocity suffered accordingly. While the Ho-5 was under development, some Army fighters were equipped with the German MG151/20. In August 1943 a submarine had brought 800 MG151/20 cannon from Germany, and they were installed in Ki.61s.

The Navy used mainly the Type 99. The model 1, another derivative of the Oerlikon F gun, fired a heavy projectile, but its low muzzle velocity gave it poor penetration and poor ballistic characteristics, and its rate of fire was equally unimpressive. For the model 2 the Japanese used the Oerlikon L as starting point. By using a bigger cartridge case, 101mm long instead of 72mm, and a longer barrel, the muzzle velocity was raised to an acceptable value. Later the mechanism was refined, and the rate of fire significantly increased.

Heavier Cannon

Big cannon were designed for two different roles: Bomber interception and ground attack. The ground attack guns were given a high muzzle velocity for better armour penetration. Bomber killer guns could have a lower muzzle velocity, to achieve a weight reduction. Here we deal with fighter armament, but this includes some weapons that were designed for ground attack.

Name Ammunition Rate of FireMuzzle velocityWeight
M4 37 x 145R (608 g)140 rpm 610 m/s 96 kg
M9 37 x 223SR (608 g)140 rpm 792 m/s 184 kg
MK 101 30 x 184B (330 g)250 rpm 860 m/s 180 kg
MK 103 30 x 184B (330 g)420 rpm 860 m/s 141 kg
MK 108 30 x 90RB (312 g)600 rpm 505 m/s 60 kg
BK 5 50 x 419R (1540 g) 50 rpm 917 m/s 513 kg
NS-37 37 x 195 (735 g)250 rpm 900 m/s 150 kg
NS-23 23 x 115 (200 g)550 rpm 690 m/s 37 kg
VYa 23 x 152 (200 g)500 rpm 905 m/s 69 kg
Type 98 37 x 112R (476 g)hand-loaded 610 m/s 122 kg
Ho-203 37 x 112R (476 g)130 rpm 575 m/s 89 kg
Ho-105 30 x 113 450 rpm 750 m/s 44 kg
Ho-301 40 450 rpm 230 m/s 132 kg
Ho-401 57 90 rpm 518 m/s 160 kg
Type 88 75 (6600 g)hand-loaded 720 m/s

The Browning M4 was installed in US fighters such as the P-39 Airacobra. This was a heavy, slow-firing cannon, with a small number of rounds in an ammunition drum. Such heavy, slow-firing guns were intended to destroy bombers, but their ballistic characteristics were not up to the task. And the fighters that used it, such as the P-39 and early models of the P-38, carried only one. American designers later sought to compensate for the low rate of fire of the M4 by installing four or six of them in experimental fighters, but these plans never matured. Because of their low rate of fire these guns were more useful against ground targets, although it did not have sufficient muzzle velocity to penetrate armour.

A later development, the M10, had the feed mechanism modified for a disintegrating belt, a change that allowed ammunition to be increased from 30 to 58 rounds. It was sliightly heavier, 109kg, but its performance was identical. This weapon was installed in later models of the P-63 Kingcobra.

The M9 was a very different weapon. The large cartridge case gave the same HE round as the M4 a considerably higher muzzle velocity. One of the types of ammunition available was a 752g armour-piercing projectile with a muzzle velocity of 930m/s, and at a distance of 460m this penetrated 6cm of armour plate. At the same distance the M4 could penetrate only 2cm of armour. It is obvious that the M9 was much better suited for ground attack; but it seems that the only aircraft in which it was ever fitted was the one-off P-63D.

The Germans initially developed the MK101 for air-to-air use, the intention being to attack bombers from a safe distance. But its weight and low rate of fire seem to have excluded this use; instead it was used by ground attack aircraft. It was useful against (lightly) armoured ground targets. The MK103 was lighter than the MK101, had electric firing instead of percussion firing, and fired faster. It was an excellent, powerful weapon, but again it could not be carried by a fighter without considerable loss of performance. Only at the very end of the war did some fighters, such as subtypes of the Bf 109K, carry a MK103 gun on the centreline.

Because the fight against heavy allied bombers required a 30mm cannon, the lighter MK 108 was developed, with a much lower ballistic performance, but less than half the weight and bulk. Fighters could carry two or even four MK 108s. This gun had a heavy punch, but because it was a short-range weapon fighter pilots had to get really close to their targets. Its use required strong nerves and better training than German pilots received during these last years of the war.

An alternative was the BK 5, originally an anti-tank gun. The advantage of this weapon was that it could be fired at bomber formations from beyond the range of their defensive armament: A return to the concept behind the MK101 and MG c/30L. Its disadvantages are obvious. It was installed in some Me 410 fighters, and experimentally fitted in a Me 262.

Big Russian cannon such as the NS-37 were mostly intended for use against ground targets, although they were occasionally fired in air-air combat. Usually only a single cannon of this type was carried, typically between the cylinder banks of the Klimov engines of the Yakovlev fighters. These small fighters successfully carried even the large NS-45, but the recoil of the NS-57 was too much for them.

The first gun in service, in 1940, was the powerful VYa, with a 152mm long cartridge case. It was installed mainly in Il-2 ground attack aircraft, but also in some fighters. In 1942 the NS-37 appeared; this gun could penetrate 40mm of armour at an angle of up to 40 degrees. And from 1945 onwards, the NS-23 was introduced, a version of the NS-37 scaled down to 23mm. It replaced the much heavier VYa in fighters, but because the cartridge was only 115mm long the muzzle velocity was reduced. The NS-23 was a more typical fighter weapon, less suitable for ground support missions. It would stay around for a long time.

The Japanese Army installed big cannon in ground attack aircraft, such as the developments of the Ki.45 Toryu, but these were unimpressive weapons. The Type 98 was a hand-loaded 37mm cannon, derived from a French army weapon. The achievable rate of fire was about 15 rpm. The semi-automatic Ho-203, with a 25-round drum, was a much-needed improvement. The Ho-203 was regularly found on fighters, such as subtypes of the Ki.44, despite its low rate of fire and its low muzzle velocity. The big Ho-401 was also intended for anti-armour and anti-shipping attacks, but occasionally it was used against B-29 bombers as well. Not because it was effective, but because the Japanese did not have any better defense against the B-29. A desperate measure was the installation of the 75mm Type 88 anti-aircraft gun in a bomber, to create the Ki.109 "fighter". It seems obvious that this was a bad idea, especially as the gun itself was rather mediocre, but without the planned turbosuperchargers the Ki.109 could not even get close to the B-29s.

The Ho-301 was one of the most unusual cannon used during the war. It fired caseless rounds, which had the propellant charge in the back of the projectile. It had an effective range of only 150m and was effective only against ground targets.


The Fighters

We can now have a look at the armament of some WWII fighters. For convenience, I sorted them by hitting power, in terms of fired weight per second. That is not the only measure of armament effectiveness, but it is one.

Fiat C.R.42 Falco

Nakajima Ki.43 Hayabusa

Supermarine Spitfire Mk.IA

Yakovlev Yak-3

Messerschmitt Bf 109E-3

Mitsubishi A6M2 model 21 Reisen 'Zeke'

North American P-51D Mustang

Supermarine Spitfire Mk.XIVE

Kawasaki Ki.61-I-KAI-Hei Hien 'Tony'

Mitsubishi A6M5b model 52B Reisen 'Zeke'

Focke-Wulf Ta 152H-1

Messerschmitt Me 262A-1a


Evolution

Generally speaking (with all dangers that are included in generalisations) one can distinguish three phases in the development of World War II fighter armament.

Some WWII fighters were armed with was essentially World War I armament: Two machineguns, typically installed in the upper decking of the front fuselage, and with the breeches within reach of the pilot so that he could clear stoppages. Such armament was carried by Italian and Japanese Army fighters; as a concession to modernity heavy machineguns were substituted for the rifle-calibre weapons.

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 heavy machineguns. Examples of the first approach are the eight Browning .303s in the Spitfire and the four MG17s 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. The first phase ended when it was generally understood that the light machinegun was ineffective against modern combat aircraft.

In the second phase there were still two options. Either a homogenous armament of heavy machineguns was used, or a mixture of more modern 20mm cannon with machineguns. The first approach was chosen almost exclusively by the USAAF, which equipped its fighters with six or eight .50 Browning guns. The second approach was far more common, and used by fighters such the Spitfire, the Bf 109, or Ki.84. 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 heavy machineguns only was that it lacked the destructive power to be effective against anything but small fighters or lightly constructed bombers.

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. Such cannon were either low- velocity, low-rpm weapons, or they were extremely heavy; in either case 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, such armament was installed in few post-war fighters, but one that must be mentioned is the MiG-15.

The use of WWII fighter armament ended with the introduction of revolver cannon (developed in Germany during the war), Gatling-type rotary cannon, or Gast-derived dual-barrel cannon. These weapons with their much higher rate of fire revolutionized fighter armament. The introduction of guided missiles came later.

Acknowledgements

Comments by and contributions of Tony Williams are gratefully acknowledged.


Emmanuel Gustin
[email protected]

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