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. The chart below illustrates the evolution from the 1930s to the 1990s. It is obvious that the 1939-1945 period shows the onset of a large increase in fighter firepower. After 1950 the interest in adding guns decreased, because guided missiles became the primary weapon (even if they were not yet fully ready to take that role.)

Fired weight per second is at best a rough approximation of destructive power. It is a reasonable approximation if the destruction is to be caused by the high-explosive or incendiary chemicals contained in the ammunition. The amount of explosive or incendiary material is of course related to the weight of the projectile, but it is not a linear relationship: Rounds of smaller calibre have proportionally thicker walls, and a smaller fraction of their weight is available for chemical loads. Therefore the fired weight per second is usually more relevant for larger calibre guns.

As a second measure of the destructive power, the muzzle power is given, in kilowatt. This is the rate of production of kinetic energy. AP or “ball” rounds that contain no chemical load only have this kinetic energy to cause damage to the target. More is not always better; a round with a too high kinetic energy might pass clean through the target without doing more damage than two neat, round holes. The optimal velocity to do maximal damage a metal plate is just below that required to penetrate it. Of course projectiles lose a lot of the muzzle energy before they hit the target, because of drag. In general larger calibre projectiles retain their kinetic energy for a longer time, because they have a more favourable surface-to-mass ratio.

A disadvantage of AP rounds is that they cause damage in a more limited area than incendiary or explosive rounds. Therefore semi-armour-piercing explosive or incendiary rounds were used more often. These require kinetic energy to penetrate the armour, and have chemical energy to cause destruction afterwards.

A third number given is the number of projectiles fired. If the target is not armoured, the same weight of non-explosive projectiles does more damage when distributed over numerous small projectiles than in a single large one, and the number of projectiles is the most important. But if the target carries armour the smaller projectiles are more likely to be stopped, and that reduces the effectiveness, especially of the rifle-calibre weapons. On the other hand, a larger number of projectiles means that the probability of a single hit increases.

To summarize: Fired weight per second is given as an approximation of the chemical energy that can be transferred to the target, muzzle power as a measure of the kinetic energy, and the number of rounds fired indicates the spreading of this transfer over a number of hits. All three are factors that must be considered in a consideration of the firepower installed in an aircraft. A “firepower formula” that would allow us to actually calculate a single number as a measure of the firepower, would be a nice thing to have. However, too many factors are involved, and the effectiveness of ammunition depends very much on the nature of the target.

Occasionally, firepower effectiveness was measured experimentally. The Germans determined that a large sturdy bomber such as a B-17 or B-24 could be shot down with 20 hits of 20mm ammunition, three hits of 30mm HE ammunition, or one single 55mm hit.

Fiat C.R.42 Falco

The CR.42 is a good candidate for the best biplane fighter ever built. But it was a contemporary of the first generation of monoplane fighters, and completely outclassed.

• Two 12.7mm Breda-SAFAT guns, with 400 rounds per gun. Ammunition for 34 seconds.
• They fired 23 rounds per second, corresponding to 0.86 kg. Total muzzle power was 248 kW. 
• This pathetic armament was very much the standard for the Italian fighters of 1940. Some fighters had two 7.7mm guns added, and that raised their firepower to 1.13 kg/sec. But because they were underpowered they could not carry much more. The Italians finally had to adopt German engines and German guns for their fighters. 

Nakajima Ki.43 Hayabusa

A captured Ki.43 in Chinese markings. The superficial similarity to the A6M “Were” is obvious, and that caused considerable confusion. The Ki.43 was extremely agile, but by the standards of 1942 it was slow, undergunned and woefully vulnerable. Nevertheless the type stayed in production until the end of the war!

• Two 12.7mm Ho-103 guns with 250 rounds per gun, enough ammunition for 17 seconds.
• 30 rounds per second, or 1.14 kg/sec. Total muzzle power was 362 kW. 
• The production of the Ki.43 actually began with two Type 89 7.7mm guns. This was clearly insufficient, and when production of the Ho-103 allowed it, this weapon replaced one or both of the smaller weapons. The Ki.43 was still under-gunned, and remained so until the end of the war. 

Supermarine Spitfire Mk.IA

A Spitfire Mk.I with a three-bladed de Havilland airscrew. The gun ports in the wing have been patched over with fabric, a standard practice at the time to protect the guns from frost.

• Eight Browning .303 guns installed in the wings. 300 rounds per gun, enough for 16 seconds of fire.
• 152 rounds per second, or 1.72 kg/sec output. Total muzzle power 480 kW. 
• The RAF was quick to understand that heavy firepower was needed, but its initial choice was an unfortunate one. The .303 was chosen over the .50 because of its higher rate of fire and better reliability, but the .303 round lacked the power to penetrate armour, and was far too light to do structural damage. Pilots preferred to use incendiary rounds, also because they could see them hit the target. Although some pilots had their guns “synchronized” to converge at a point, it was more common to have some spread, to simplify aiming. 

Yakovlev Yak-3

Yakovlev Yak-3. This was a highly specialized low-altitude interceptor, with brilliant performance and handling at low levels.

• One ShVAK cannon mounted between the cylinder banks of the engine, with 120 rounds. One Berezin UBS in the forward fuselage decking, with 250 rounds. That was enough for 9 and 19 seconds, respectively.
• Both guns fired 13 rounds per second, but the ShVAK cannon put out 1.28 kg and the machinegun 0.64 kg: A total of 1.92 kg/sec. Muzzle power was 473 kW for the ShVAK and 230 kW for the UBS, a total of 703 kW. 
• The Yakovlev fighters had a high performance at low and medium altitude, but their small size and limited engine power restricted their armament. The Lavochkin fighters, although also small, had more engine power and could carry heavier armament. To some extent these disadvantages were compensated by the excellence of the Soviet guns, but their effectiveness was reduced by the primitive gunsights. 

Messerschmitt Bf 109E-3

• Two MG-FF cannon in the wings, with 60 rounds each; two MG 17 machineguns in the engine cowling, with 1000 rounds per gun. The cannon ammunition was enough for 7 seconds, the machineguns had ammunition for 55 seconds.
• Output per second was seventeen 20mm shells and 37 7.92mm bullets. The cannon were responsible for firing a weight of 2 kg/sec. The contribution of the MG 17s was 0.37 kg/sec. The muzzle power of the MG-FF guns was 418 kW, and that of the MG 17 guns 114 kW; a total of 532 kW. 
• The two MG 17 machineguns were too light to be very effective. The MG-FF cannon fired effective, but low-velocity ammunition, with a short range. Ballistic characteristics of the MG-FF and MG 17 were too different. The Bf 109E probably was a better bomber interceptor than the British fighters, but its role in the battle of Britain was that of an escort fighter. 

Mitsubishi A6M2 model 21 Reisen ‘Zeke’

Mitsubishi A6M2 taking off during the Battle of Santa Cruz. The A6M2 had good performance for a carrier-based fighter and a long range, but it was a poor basis for further development and became inadequate in the later war years.

• Two 7.7mm Type 97 machineguns in the front fuselage, with 500 rounds per gun. Two 20mm Type 99-1 cannon in the wing, with 60 rounds per gun. The cannon had ammunition for 7 seconds, the machineguns for 30 seconds.
• The cannon fired 17 shells per second, or 2.24kg. The machineguns fired 33 rounds in the same time, or 0.38kg. The total was 2.62kg/sec. Muzzle power 308 kW for the cannon, and 106 kW for the machineguns; a total of 414 kW. 
• To install cannon in a fighter was an advanced concept, and here the Japanese Navy had a lead on some other services, notably the Japanese Army: The A6M had more than twice the firepower of the contemporary Army fighter, the Ki.43. But the Type 99-1 was too slow-firing, had a too low muzzle velocity, and its ballistic characteristics did not match that of the machineguns. There is a remarkable similarity with the Bf 109E. 

Mitsubishi A6M5b model 52B Reisen ‘Zeke’

Later developments of the A6M remained inferior to their opponents. This is an A6M5c, armed with two Type 99-2 20mm cannon (inboard) and two 13.2mm Type 3 machineguns (outboard) in the wings, and a third Type 3 gun in the engine cowling.

• In the front fuselage one 7.7mm Type 97 with 500 rounds, and one 13.2mm Type 3. Wing cannon two 20mm Type 99-2 with 125 rounds per gun. The cannon had ammunition for 15 seconds, the light machinegun for 30 seconds.
• The cannon 16 fired shells per second, or 2.11kg. The 13.2mm machinegun fired 13 rounds in the same time, a weight of 0.69kg. The light machinegun contributed 17 rounds, or 0.19kg. The total is 2.80 kg/sec. Muzzle power was 53 kW for the light machinegun, 216 kW for the medium-calibre machinegun, and 412 kW for the two cannon. Note that in comparison with the A6M2, the weight per second fired by the cannon is down, but the muzzle power has increased a lot! 
• Upgrading the armament of the A6M gave later models a modestly greater killing power. Much of this was because of the Type 99-2 cannon, still slow-firing but with much better ballistic characteristics, and in its final models equipped with a belt-feed that increased the ammunition supply. This improvement came too late for the A6M, which was already obsolete. The last IJN fighters of the war, such as the N1K2-J Shiden-KAI, had four Type 99-2 cannon. 

North American P-51D Mustang

Compared with previous Mustangs, the P-51D had two more .50 guns, and the armament installation had been redesigned to make it more reliable.

• Six Browning .50 machine guns. The two inboard guns had 400 rounds each, enough for 32 seconds. The four outboard guns had 270 rounds, enough for 22 seconds.
• Output per second was 75 rounds, or a weight of 3.64kg per second. Muzzle power was 1374 kW. 
• Six .50 guns was the armament of most US fighters. Most pilots liked the .50 gun, but it lacked the power to do structural damage to enemy aircraft. Postwar research demonstrated that only armour-piercing incendiary rounds were really effective, by setting fire to ammunition or fuel. This armament was sufficient for the Mustang, because it was an escort fighter, that had to fight mostly against enemy fighters. The guns were usually set to converge at 300 yards, and 2 degrees above the normal flight attitude. The ammunition supply was relatively large, and that was also beneficial for an escort fighter. Last but not least, the Browning was very reliable and had good ballistics. 

Yakovlev Yak-9T

Yakovlev Yak-9T. The Yakovlev family of fighters were small aircraft, very manoeuverable and with good performance below 5000m, where most combat at the Eastern front occurred. The Yak-9T withs it large cannon was a specialised interceptor built in relatively small numbers.

• One NS-37 cannon mounted between the cylinder banks of the engine, with 32 rounds. One Berezin UBS in the forward fuselage decking, with 220 rounds. That was enough for 8 and 17 seconds, respectively.
• The NS-37 put put out 3.06kg per second, and the machinegun 0.64kg: A total of 3.7kg/sec. Muzzle power of the NS-37 was 1240 kW, the machinegun contributed another 230 kW. 
• Compare this with the Yak-3. The powerful 37mm cannon of the Yak-9T had a relatively low rate of fire, but a single hit would destroy an aircraft. The Soviets calculated that on average 31 rounds were fired to down an aircraft, compared with 147 rounds for the 20mm cannon. Normal firing ranges were 100m to 400m against fighters, and 500m to 600m against bombers, but the maximum effective range was about 1200m. On the downside, the recoil was so large that pilots were trained to fire three-round bursts. 

Lockheed P-38 Lightning

The P-38 was the most successful twin-engined single-seat fighter of the war. Some early models had one 37mm cannon, two .50s and two .30s, but production soon standardised on one 20mm cannon and four .50s.

• One 20mm Hispano A/N-M2 cannon with 150 rounds, and four .50 Browning M2 machineguns with 500 rounds each. The cannon had ammunition for 15 seconds of fire, the machineguns for 40 seconds.
• The cannon fired 10 rounds per second, an output of 1.3kg/sec. The machineguns contributed 50 rounds, or 2.43kg/sec. The total is 3.73kg. The muzzle power was 503 kW for the cannon and and 918 kW for the machineguns.
• Because of its twin-engined configuration, the P-38 carried all its armament in the nose, and no synchronisation or harmonisation were necessary. Later models of the P-38 were the equals in combat of any single-engined fighter. Whether this justified the additional cost and size of the P-38 is another debate. A comparison with the P-51 shows that the 20mm Hispano was the equivalent in weight of fire and muzzle power of two .50 Brownings; its more deadly ammunition was another advantage. (The US Navy estimated that the 20mm cannon was the equivalent of three .50s, reducing to 2.5 at long range.) On the other hand the ammunition supply of the cannon was limited. 

Supermarine Spitfire Mk.XIVE

A late-production Spitfire FR Mk.XIVE. Note the modified nose contours for the Griffon engine, five-bladed propeller, bubble cockpit, clipped wingtips, and camera port in the aft fuselage.

• Two Hispano cannon in the wing, with 120 rounds each. Two Browning .50 guns, with 250 rounds each. The cannon ammunition lasted for 12 seconds; the machinegun ammunition for 20 seconds.
• The two cannon fired 20 rounds per second, the two machineguns 25. Total output was 3.81kg per second. Muzzle power was 1006 kW for the two cannon, and 458 kW for the two machineguns. Total 1464 kW. 
• Because the original armament of eight Brownings was too weak, later Spitfires had the “C” or “universal” wing which allowed the installation of eight .303 guns, two 20mm cannon and four .303 machineguns, or four 20mm cannon. But the late production Mk.IX and the Mk.XIV had the “E” wing, with the definitive wartime armament for the Spitfire: Two 20mm cannon and two .50 guns. The Spitfire was a small fighter, and its thin wing complicated armament installations. But the E-wing armament was very effective. An important factor was the use of gyroscopic gunsights by the Allies, because they greatly improved accuracy. The Germans developed similar gunsights, but these were never reliable enough to be useful. 

Kawasaki Ki.61-I-KAI-Hei Hien ‘Tony’

The Nakajima Ki.61 Hien, or Army Type 3 Fighter, Allied codename Tony, was at first believed to be a copy of a German or Italian design. In fact only its Ha-40 engine was a licensed version of the Daimler-Benz DB 601.

• Two 20mm Ho-5 guns with 120 rounds per gun, and two 12.7mm Ho-103 guns with 200 rounds. The machineguns had ammunition for 13 seconds and the cannon for 9 seconds.
• The cannon put out 28 rounds per second, or 2.81kg/sec. The machineguns contributed 30 rounds, or 1.14kg. Total 3.95 g/sec. Muzzle power was 944 kW for the cannon, and 362 kW for the machineguns; a total of 1306 kW. 
• The Ki.61 was the first fighter of the Imperial Japanese Army that was the equal of or better than the Western designs it encountered. The best Japanese Army fighter of the war, the Ki.84, initially carried the same guns, although with more ammunition. Later versions of the Ki.84 had four Ho-5 cannon, or even two Ho-5 and two Ho-155 cannon. This marked a switch by the Army from lightly-built, lightly-armed dogfighters such as the Ki.43, to sturdy, heavily armed all-round fighters. 

Republic P-47D Thunderbolt

With eight .50s, the P-47 carried the heaviest armament of the US single-seat fighters.

• Eight Browning .50 machine guns. Up to 400 rounds could be carried, enough for 32 seconds of fire, but the ammunition load was often reduced to compensate for the carriage of bombs or external fuel tanks.
• Output per second was 100 rounds, or a weight of 4.85kg per second. Muzzle power was 1835 kW. 
• The Thunderbolt was designed around the R-2800 radial and a turbosupercharger installed in the aft fuselage. It was one of the largest fighters of its time, and also one of the most rugged. It carried eight .50 guns in the wings, with ammunition storage in the outer wing panels. The P-47 had a successful career as fighter-bomber after the P-51 replaced it as escort fighter. 

Focke-Wulf Ta 152H-1

Above, a Ta 152H-1. Note the slender long-span wings of this development of the Fw 190, and the Jumo 213 V-12 engine with annular radiator.

• One MK 108 cannon firing through the propeller spinner, and two MG 151/20 cannon in the wing roots. The 30mm cannon had 90 rounds, the 20mm cannon had 175 rounds each. This was ammunition for respectively 9 and 14 seconds of fire.
• Per second, ten 30mm shells and 25 20mm shells were fired. This amounted to a weight of 5.96kg. The muzzle power of the MK 108 was a modest 398 kW, and the 20mm cannon contributed 720 kW. Total 1118 kW. 
• This high-altitude interceptor was armed to fire a short, heavy burst at its target. The MK 108 cannon was a low-velocity weapon, designed to inflict fatal structural damage to the heavy bombers of the 8th AF. 

Hawker Tempest Mk.V

Hawker Tempest Mk.V. This one is armed with Hispano Mk.V cannon. Early Tempests had Mk.II cannon, and the longer barrels extended in front of the wing leading edge.

• Four Hispano Mk.V cannon, installed within the wing, with 200 rounds per gun. That was ammunition for 16 seconds. 
• Per second 50 rounds were fired, with a total weight of 6.5 kg. Total muzzle energy was 2292 kW.
• This was the definitive armament option for British WWII fighters, although a number of designs and prototypes featured six Hispano cannon. It was also retained by the first generation of jet fighters, becoming a de facto standard in the first year after the war. The exceptions were the USAAF, that continued to rely on the .50, and the USSR, that prefered 23mm and 37mm cannon. 

Messerschmitt Me 262A-1a

A Messerschmitt Me 262, photographed in April 1945 in Switzerland. Some Allied observers at first criticised the choice of four 30mm cannon, estimating that the rate of fire could scarcely exceed five per second. But the MK 108 fired two times faster than that.

• Four MK 108 cannon in the nose. Two had 80 rounds, and two had 100 rounds, for respectively eight and ten seconds of fire.
• Forty 30mm shells per second, or 12.5kg per second. Muzzle power 1592 kW.
• The armament of the Me 262 was deadly against heavy bombers. The ammunition for the MK 108 cannon was of the “Minengeschoss” type, thin-walled high-explosive shells. Because of the low muzzle velocity of the MK 108, it was not very suitable for fighter-vs-fighter combat, but in principle the Me 262 could outrun any enemy. In practice the Me 262 were always heavily outnumbered by the escort fighters, and their success was quite limited. 

Comparison Table

Name	Rounds per second	Mass output in kg/sec	Energy in kW
Fiat CR.42 Falco	23	0.86	248
Nakajima Ki.43 Hayabusa	30	1.14	362
Supermarine Spitfire Mk.IA	152	1.72	480
Yakovlev Yak-3	26	1.92	703
Messerschmitt Bf 109E-3	54	2.37	532
Mitsubishi A6M2 Reisen	50	2.62	414
Mitsubishi A6M5b Reisen	46	2.80	681
North American P-51D Mustang	75	3.64	1374
Yakovlev Yak-9T	17	3.70	1470
Lockheed P-38J Lightning	60	3.73	1421
Supermarine Spitfire Mk.XIVE	45	3.81	1464
Kawasaki Ki.61-I-KAI-Hei Hien	58	3.95	1306
Republic P-47 Thunderbolt	100	4.85	1835
Focke-Wulf Ta 152H-1	35	5.96	1118
Hawker Tempest Mk.V	50	6.50	2292
Messerschmitt Me 262A-1a	40	12.50	1592

A longer list of fighter armaments is reproduced elsewhere. It is clear from this table that muzzle energy and weight of fire are related, because a heavier weight of fire usually means a proportionally higher muzzle energy. This simply reflects the fact that the muzzle velocities of the guns used are typically around 825 m/sec, so that the kinetic energy per unit of projectile weight is approximately 340 kJ.

The exceptions are the MG-FF, Type 99-1 and MK 108. These are all low-velocity weapons, and fighters equipped with these weapons have a lower total muzzle power, although the weight of fire might be quite high. The best example is of course the Me 262.

There is no obvious relationship of either weight of fire or muzzle power with the total number of rounds fired per second. This value also does not show any clear trend towards either an increase or a decrease, although there are a few exceptionally low or high values.

Next: Analysis