Modern Fighter Gun Effectiveness

© Anthony G Williams & Emmanuel Gustin

This is taken from

Flying Guns – the Modern Era: Development of Aircraft Guns, Ammunition and Installations since 1945

The is an attempt to compare the cartridge destructiveness, gun power and gun efficiency of post-WW2 fighter guns.  Fighter armament fits in three key periods are also considered.

Cartridge Destructiveness

There are two types of energy that may be transmitted to the target; kinetic and chemical. The kinetic energy is a function of the projectile weight and the velocity with which it hits the target. This velocity in turn depends on three factors: The muzzle velocity, the ballistic properties of the projectile, and the distance to the target. There are therefore two fixed elements in calculating the destructiveness of a projectile, its weight and chemical (high explosive or incendiary) content, and one variable element, its velocity. The key issue is the relationship between these three factors.

A high muzzle velocity will provide a short flight time, which is advantageous in increasing the hit probability and extending the effective range, and will also improve the penetration of AP rounds. However, it might not add much to destructiveness, as unless an AP projectile hits armour plate (and not much of the volume of an aircraft is protected by this), a higher velocity just ensures that a neater hole is punched through the aircraft; the extra kinetic energy is wasted. Also, if the projectile is primarily relying on HE blast or incendiary effect, the velocity with which it strikes the target is almost immaterial. Provided that it hits with sufficient force to penetrate the skin and activate the fuze, the damage inflicted will remain constant. In contrast, AP projectiles lose effectiveness with increasing distance.

It is sometimes argued that a projectile with a high muzzle velocity and a good ballistic shape (which reduces the rate at which the initial velocity is lost) provides a longer effective range. To some extent this is true, but the greatest limitation on range in air fighting remains the difficulty in hitting the target. The problem of hitting a target moving in three dimensions from another also moving in three dimensions (and probably at a different speed and on a different heading) requires a complex calculation of range, heading and relative speed, while bearing in mind the flight time and trajectory of the projectiles. Today, such a problem can easily be solved by a ballistic computer linked to a radar or laser rangefinder, but in the early years of this period the technical aids were far less sophisticated. And this was without considering the effects of air turbulence, G-forces when manoeuvring, and the stress of combat.

For all of these reasons muzzle energy (one half of the projectile weight multiplied by the square of the velocity) has not been used to calculate kinetic damage as this would overstate the importance of velocity. Instead, momentum (projectile weight multiplied by muzzle velocity) has been used as an estimate of the kinetic damage inflicted by the projectile. It might be argued that even this overstates the importance of velocity in the case of high-capacity HE shells, as noted above, but the effect of velocity in improving hit probability is one measure of effectiveness which needs acknowledging, so it is given equal weighting with projectile weight.

Chemical energy is generated by the high explosive or incendiary material carried by all air-fighting projectiles. First, there is the difference between HE and incendiary material, which are often mixed (in very varying proportions) in the same shell. HE delivers instant destruction by blast effect (plus possibly setting light to inflammable material within its blast radius), incendiaries burn on their passage through the target, setting light to anything inflammable they meet on the way. The relationship between the effectiveness of HE and incendiary material is difficult to assess. Bearing in mind that fire has been the big plane-killer, there appears to be no reason to rate HE as more important, so they have been treated as equal.

The comparison between kinetic and chemical energy is the most difficult and complicated subject to tackle. This complexity is revealed by the example of a strike by a delay-fuzed HEI cannon projectile. This will first inflict kinetic damage on the target as it penetrates the structure. Then it will inflict chemical (blast) damage as the HE detonates. Thirdly, the shell fragments sent flying by the explosion will inflict further kinetic damage (a thin-walled shell will distribute lots of small fragments, a thick-walled shell fewer but larger chunks), and finally the incendiary material distributed by the explosion may cause further chemical (fire) damage.

There will therefore always be a degree of arbitrariness in any attempt to compare kinetic and chemical energy, as it all depends on exactly where the projectile strikes, the detail design of the projectile and its fuze, and on the type of aircraft being attacked. To allow a simple comparison, we will reduce all these factors to an increase in effectiveness directly proportional to the chemical content of the projectile. We assign to projectiles that rely exclusively on kinetic energy an effectiveness factor of 100%. For projectiles with a chemical content, we increase this by the weight fraction of explosive or incendiary material, times ten. This chosen ratio is based on a study of many practical examples of gun and ammunition testing, and we will see below that it at least approximately corresponds with the known results of ammunition testing.

To illustrate how this works: a typical cannon shell consists of 10% HE or incendiary material by weight. Multiplying this by ten gives a chemical contribution of 100%, adding the kinetic contribution of 100% gives a total of 200%. In other words, an HE/I shell of a given weight that contains 10% chemicals will generate twice the destructiveness of a plain steel shot of the same weight and velocity. If the shell is a high-capacity one with 20% chemical content, it will be three times as destructive. If it only has 5% content, the sum will be 150%, so it will be 50% more destructive, and so on.

The following table for the most common cartridges and loadings used in aircraft guns shows the consequences of these assumptions and calculations. The first few columns should be self-explanatory, as these are basic statistics about the ammunition. The 'DAMAGE' column shows the results of the calculations described above. To run through an example, let us look at the case of the 30 x 113 B. The projectile weighs  270 g, (which equals 0.270 kg) and is fired at 720 m/s. Multiplying these gives 0.270 x 720 = 194.4, so you have a momentum factor of 194.4. As the bullet contains 17.2% by weight of incendiary material, the momentum is multiplied by 2.72 to give a destructive power score of 528.768 - rounded to 529.

'In the last column – 'Power' – the 'Damage' result is divided by ten and rounded to the nearest whole number (except for the 12.7 x 99) to simplify later calculations.

Table 1: Cartridge Destructiveness

(Figures in brackets are estimates.)

Comments on Table 1

Clearly, the resulting scores can only be approximate, and in particular will vary depending on the particular mix of types included in an ammunition belt. The power calculation takes a typical mix of ammunition, where known. They also take no account of the fact that some incendiary mixtures, and some types of HE, are more effective than others. However, they do provide a reasonable basis for comparison. There is no point in trying to be too precise, as the random factors involved in the destructive effects were considerable.

12.7x99 (.50 M3), 20x102 (M39/M61), 20x110 USN (Mk 11/Mk 12), 23x115 (NS-23, NR-23, AM-23, GSh-23, GSh-6-23), 25x137 (GAU-12/U), 27x145B (Mauser BK 27), 30x113B (Aden/DEFA 550), 30x150B (GIAT 30M791), 30x155B (NR-30), 30x165 (GSh-301, GSh-30, GSh-6-30), 30x173(2) (GAU-8/A), 30x173(1) (Oerlikon KCA), 37x155 (N-37, NN-37)

Gun Power and Efficiency

The cartridge destructiveness table above only shows the relative effect of one hit. When comparing the guns that fired the cartridges, other factors come into play, namely the rate of fire (RoF) and the gun weight.

To calculate the destructive power of the gun, the 'POWER' factor from the above table has been multiplied by the RoF, expressed in the number of rounds fired per second. This gives the relative 'GUN POWER' figures in the table below.

To judge how efficient the gun was, the 'GUN POWER' result is divided by the weight of the gun in kilograms to provide the 'GUN EFFICIENCY' score in the last column. This is, in effect, a measure of the power-to-weight ratio of the gun and ammunition combination.

Table 2: Gun Power and Efficiency

Comments on Table 2

* The figures for the power-driven rotaries are for the full RoF. In practice, the figures in air combat will be lower because of the time taken to accelerate. For example, the M61A1 only fires 18 rounds in the first 0.5 seconds, and 68 rounds in the first full second of firing. In the first second, the gun power figure will be 1,496 and the efficiency 13.1. If only the first half-second of firing is counted, then the (full-second equivalent) figures become 792 for gun power and 6.9 for efficiency. The GAU-12/U has the same spin-up time as the M61A1, 0.4 sec, so will be affected about as much by this factor, with full-second scores of 2,592 and an efficiency of 21, and half-second scores of 1404 and 11.4. The gas-powered GSh-6-23 and GSh-6-30 will be affected far less, and spin-up time has been reduced to 0.25 sec for the lighter M61A2 for the lighter M61A2, improving its scores accordingly.

Two factors not included are gun reliability and total ammunition weight. The former is simply not available in most cases. The latter involves too many variables. First, the ammunition supply for most guns varied according to the installation. Furthermore, in searching for comparators, there would be the problem of which measures to take: the weight of the number of rounds fired per second, or the weight of the number required to inflict a certain amount of damage? There would be a case for either of these, but they would produce very different results. This issue is however addressed in the next table.

Fighter Firepower

Finally, a consideration of how the firepower of fighters compared with each other. The aircraft have been grouped in early postwar, 1954-1970, 1970-1990 and 1990+ fighters, and have been chosen to be representative of their period.

Table 3: Fighter Firepower