Intermediate Caliber Aircraft Cannon
— What Are They?

by Louis Bruchiss

Nearly every type of warplane now used by the belligerent powers carries a small or medium caliber cannon. In this article the author describes the part played by machine guns and the various sizes of aircraft cannon in combat airplanes

More than a year and a half of warfare in Europe has shown the world what was obvious many years ago: that those who have the most airplanes will achieve the most advantages. The United States has taken the lesson to heart with a vengeance. An aircraft production schedule based on a fleet of 50,000 airplanes is being worked out, and it is being revised continually to accommodate increases.

The lack of sufficient military planes in our country has been recognized by many officials and private citizens as incompatible with our size, our position in international affairs, and our vital defense needs. But airplanes, particularly military airplanes, are worthless unless they are designed and built to carry out specific military purposes. As part of both defensive and offensive operations they must be equipped to carry and drop bombs of all calibers; they must be able to drive off or shoot down enemy aircraft; their own gun power may be presumably used against enemy ground objects, besides the numerous other operations for which they can be employed.

Against opposing aircraft, an airplane, regardless of whether it is the lightest single-seat fighter or the heaviest bomber, can use only its own gun power. As a prerequisite to the eventual effectiveness of that gun power, the plane itself is important, of course, but planes are changing constantly insofar as the changes include speed, maneuverability, range of action and carrying capacity. All of these factors have an important influence on a plane's usefulness and have a direct connection to the armament or guns with which an airplane may be equipped.

Machine guns, both light and heavy, have been used for aircraft ever since the early days of the first World War until today. The only refinements we have today are novel and improved installations of the machine guns in the wings, fuselage, or landing gear of the airplane. Free or flexible guns for turret mounting located in various parts of the plane, such as the nose, tail, bottom or top have become more effective through multiple mounting of the guns up to the number of four, in motorized or power driven turrets, thereby taking the strain from the gunner, of swinging heavy guns around on a Scarff-type ring. But the machine gun ┬Ěhas been gradually losing favor, because, with increasing speeds of military planes, the percentage of hits scored as against errors decreases in proportion to the speed of the plane, whereas at best, machine gun bullets would not always be effective, even if they did hit. Modern planes are largely made of metal and can sometimes sustain hundreds of hits from machine gun bullets without being put out of action. Of course, there are always vital parts, such as the gas tank, certain sections of the engine and controls, and the pilot himself. But gas tanks are now made self-sealing after being punctured, by having rubber inner tanks and other devices attached. So unless the pilot is hit, a machine gun must score heavily against a plane to bring it down or put it out of action.

Practically every type of warplane now used by the belligerent powers therefore carries small or intermediate caliber automatic cannon firing explosive shells weighing around ½ to 1 lb. A hit by one of these shells will almost surely damage a plane to such an extent that it will either be brought down immediately or consider itself lucky if it can sneak back to its base, badly crippled. The shells from these automatic cannon explode on contact, but they are also equipped with time fuses which cause self-explosion after about 10 sec so that they will not inflict damage to civilians and friendly ground objects in case they miss their target.

The fire power or firing rate of the various intermediate-caliber automatic cannon compares very favorably with that of machine guns and much more so than the heaviest aircraft cannon used. But even the high firing rates of aircraft machine guns, up to 1,500 rounds/min, probably give a false impression to the layman. With the apparent stream of fire available from a machine gun, it would appear that aircraft targets must surely always be hit. Nothing is further from the truth. The actual chances of scoring hits from plane to plane are ridiculously low, and this is due to the fact that a plane has a three-dimensional sphere of action. When firing from another plane, which is also in movement and therefore provides an unstable base, the gunner is lucky if he can score more than a few hits with a machine gun. Assume, for example a speed of 360 miles an hour for a plane. If we reduce this to 6 miles per min, we have 1 mile in 10 sec or 528 ft per sec. On the other hand, with the average light machine gun firing say, 1,200 rounds per min, of which one-fifth would be tracers to indicate to the gunner his line of fire with which to correct his timing, we have the number of rounds available per second at 16. This means that a plane flying 500 ft will have 16 bullets to contend with, from each gun, providing aiming and lead is correct! Under the conditions where the target is flying directly along the line of fire, ie, coming directly at, or flying directly away from, a steadily aimed stream of fire, the chances are greater for retaining the target within this line of fire for longer periods. This situation is more prevalent with pilot fixed gunnery. But, reciprocally, the enemy guns also constitute a menace in this position. Even when pursuing enemy fighters it is possible to receive a surprise hail of bullets from a remote controlled gun mounted in the tail which automatically covers the "blind cone" around the tail structure. It is quite apparent that every hit that is made must be effective if it is to accomplish its purpose and for this reason war pilots favor small caliber cannon.

Besides being able to hold an elusive enemy target within the line of fire for longer periods, pilot gunnery is made more effective through the added firepower given by the eight to twelve guns mounted in wings and fuselage, arranged to converge at a point anywhere from 150 to 250 yds, the most common combat ranges. Actually, the guns, mounted in this way, have a practical range of 300 to 500 yds; beyond these distances the lines of fire of the individual guns would diverge too much for effective scoring. In some foreign installations in the past, particularly in Poland, and in the case of a Dutch fighter by Fokker, provision was made for mechanical variance of the convergence point. While there is reason to believe that such convergence control is advantageous, the extra thought and motion that must be given to the fire control by the pilot, and the installation difficulties of a variance gear, probably offset the advantage.

As a result of increased aircraft speeds, fighter and pursuit planes are now designed and equipped to fight with a heavy load of armament, sometimes up to ten or more machine guns and automatic cannon in combination, with which they attempt to dive down on enemy craft to get in but a few short bursts of fire. If they fail to register a hit the first time, they may not get another chance, either because of rapidly decreasing ammunition supplies, or for fear of flying too far afield, considering the speed with which opposing planes can get away, and also considering opposing gunfire. Airplanes can carry but a limited amount of ammunition, too, as the cartridges and cartridge belt boxes or ammunition drums occupy needed space, and, in the case of cannon, a supply for half a minute's firing of each cannon is all that can be carried due to both weight and space required. However, when 200 rounds of explosive shells can be carried, this supply is sufficient for a number of bursts of fire, and every hit that is made will be positively effective.

Even during first World War days, it was recognized that something better than a machine gun was desirable on aircraft. At that time nothing was available except old-type slow-firing, cumbersome 37-mm cannon, which were experimentally mounted to fire through the propeller hub of the water-cooled engines, but were found impractical for many reasons, among them excessive weight and low fire power.

At this writing the picture has changed considerably. Practically all types of planes now require intermediate caliber automatic cannon of the 20- and 23-mm type, as a vital necessity. Machine guns, both .30 and .50 caliber, still have their field of usefulness and will undoubtedly continue to make up part of military plane equipment. At the other end of the aircraft armament range we have the 37-mm cannon, of which one type in particular is made by American Armament Corp. These 37-mm cannon have a comparatively slow firing rate, while the ammunition is fed into them in clips of five shells each, manually by the gunner.

From the table showing the comparative characteristics of the three classes of aircraft weapons, the light and heavy machine guns in the first, the 20- and 23-mm automatic cannon in the second, and the 37-mm in the third, it will be readily seen that the intermediate class is highly desirable and that more of them are urgently needed for our present and future air force. The writer understands that the United States Government is interested in this type weapon; actually our ordnance experts have always been aware of armament developments all over the world, but they have not been in a position where they could advocate and procure such arms as they felt were desirable for our services. Our present defense program will doubtless permit them to make use of the best known types of arms known anywhere.

Probably the most advanced intermediate caliber automatic cannon are the Danish Madsen 20- and 23-mm aircraft arms. The 20-mm cannon can fire at the rate of 500 rounds/minute while the 23-mm can fire at about 400 rounds. The 20-mm armor-piercing shell can create havoc to any target which it hits, even penetrating inch thick armor plate, at 600 yd. Obviously, no airplane can be sufficiently armored against such shells. The 23-mm is a special design that employs a shell with considerably greater effective explosive force than that of the 20-mm while the weight of the 23-mm cannon and complete round of ammunition remains the same as for the 20-mm cannon.

A very novel and highly effective installation of Madsen aircraft cannon was made before the war by Fokker of Holland, in his twin-engined, two-seater attack plane Fokker G 1. Two 23-mm cannon plus two 7.9-mm machine guns were fixed into the nose of the plane in front of the pilot's cockpit and within his reach. The 23-mm cannon were supplied with belts carrying 100 rounds of shells each, and all the guns were loaded and operated by means of a combined pneumatic and hydraulic system. All the pilot had to do was to make his selection of what guns were to be fired on a selector switch board, and depress a lever when he wished to open fire. The ammunition belts for all the guns were of the flexible, disintegrating type made up of loose links, which fell into a container, as did the ejected cartridge cases, during firing.

A study of some of the salient characteristics of the weapons in the table lead to extremely interesting factual conclusions. The most important considerations of these aircraft guns in their order of listing, include weight of the gun, rate of fire, muzzle velocity, type of ammunition feed, type of ammunition, and weight of the ammunition to be carried.

Inasmuch as it is desirable to carry a number of guns on an airplane, the weight factor is quite a problem. Machine guns of either .30 or .50 caliber do not weigh so much, nor does their ammunition. A number of these could easily be carried by modern high-powered planes. But the machine gun has a tremendous disadvantage in using ammunition that is practically ineffective. The machine gun, however, has a high tiring rate and a high muzzle velocity, which, however, does not assure flatness of bullet trajectory, as small arms cartridges start to curve very early in their flight.

The 20- and 23-mm cannon compare very favorably with machine guns as to weight, although they do weigh somewhat more. Their firing rate is almost that of a .50-caliber machine gun, however. The type of ammunition feed is disintegrating belt, which means that a 23 -m cannon can fire automatically as long as there is ammunition in the belt to feed it and installations are now available for 100 and more rounds of shells for these arms. They have a high muzzle velocity assuring a very flat curve or trajectory over distances such as would be common in aerial combat, up to 600 yd, although their actual effective range is much longer, as the table shows. The intermediate-caliber shells are also very effective, and when fired from either fixed or flexible mountings, can be used against enemy ground objects, including gas storage tanks, and other targets which must be set afire or destroyed. This cannot be done with machine guns alone.

The increased explosive force of the 23-mm shell is obtained by making the actual shell longer and the shell case smaller. The shell consequently contains more explosive matter, while the case holds less propellant. Due to this, the muzzle velocity is decreased considerably, giving the 23-mm shell a shorter range. But as aerial combat takes place at comparatively short ranges, the only reasons why high muzzle velocity is desirable is to give the shell a flat trajectory, and also to reduce the time of flight of the shell. The latter is important, as every fraction of a second counts while aiming and firing, keeping in mind the many factors that combine to cause misses of both machine gun and cannon fire.

Practically all cannon armament on present day aircraft are mounted in either fixed inboard or outboard positions, with the exception of installations in large bombers and heavy flying boats. There are special types of small aircraft that do mount free guns in turrets, but these are all of machine gun caliber of .50 or less. The British Boulton Paul Defiant is an outstanding example of a super-armed fighter airplane, with numerous guns in the wings, fuselage, and a multiple machine gun turret aft of the pilot. The favorite position for cannon is in the nose of the plane, just in front of the pilot, though they are also mounted in the wings or slung underneath them. Planes such as the British Spitfire and Hurricane now carry four machine guns in each wing. These might conceivably be replaced by two cannon each. The total recoil force is felt in the plane but insufficiently to affect it either structurally or to change its flight characteristics to any great extent, under conditions where the guns are in fixed positions firing forward, inasmuch as the recoil is in the opposite direction in which the plane is flying.

When these cannon, and particularly the heavy caliber 37-mm cannon, are fired from free or turret positions, the effect of the recoil is much more noticeable and would actually hamper the maneuverability just at the moment when freedom from outside forces is most desired, ie, when enemy planes are swooping down and around the defending craft.

The type of ammunition feed is an important factor although for fixed gun positions, an automatic belt made up of disintegrating links is more or less standard for both machine guns and cannon because it allows the feeding of a greater quantity of ammunition and because the belts can be fed from advantageous positions in the wings or fuselage and the boxes holding the cartridges or shells can be placed where most convenient. Nevertheless drums fixed to the guns have the advantage of giving a more positive feed than the belt type, which may lag or jam under violent combat maneuvers. Most drums will hold around 60 rounds of ammunition and up to 100 rounds, while belts can be endless up to any amount of ammunition that can be carried.

Among the types of ammunition are armor piercing, explosive, tracer and incendiary. The latter three are of importance in aircraft guns, with explosive taking the lead, the tracers being necessary for aiming control and incendiary to fire the enemy craft or ground objects. Referring again to the table (see item 15), it will be noted that the weight for 100 rounds of ammunition rises rapidly as the guns increase in size.

Designing airplanes of the fighter type to carry a maximum load of armament is far more difficult than heavily arming huge bombers or flying boats. The latter are structurally so sturdy that they can easily carry the weight of a number of guns in various turrets, and also withstand the combined recoil forces. The British and German fighters, however, are more heavily armed than our American planes. The British particularly have an advantage in owning high powered engines of 2,000 hp and more, so that the plane and wing structure is designed to cope with speeds of 400 miles plus per hour.

The sketches show recent inboard and outboard wing mountings of cannon. There must be sufficient space allowed in the wing sections to accommodate the guns, their mountings, and the ammunition boxes. An inboard mounting is generally preferable, although it is more difficult to design, because it gives the fighter a smoother, and faster appearance, in keeping with the general streamlining principle. In some of the belligerent craft, the gun barrels do not even protrude from the leading edge of the wings, and the gun ports are covered with fabric until such time as they are fired in combat, when the coverings are blown away.

Reference to the table will show that the recoil force of the small-caliber cannon amounts to about 400 lb per gun. This is a considerable force which must be reckoned with in wing design, especially if more than one cannon is to be carried. Moreover, provision must be made for carrying the ammunition boxes with a sufficient supply of shells. With machine gun cartridge boxes now holding up to 1,000 rounds for each gun, it is evident that for cannon-equipped planes the shell supply must be proportionately large in order to obtain the necessary firepower and to be able to continue this power over long enough periods of time, so as not to be helpless against enemy fire, or have to return to their bases for reammunitioning.

Structurally the problem of wing mounting of cannon boils down to the principle of centering them as much as possible near the plane's own center of gravity, and that is why cannon are usually fixed in the nose or fuselage of twin engined planes. If set in the wings, they are placed near the fuselage and in any case grouped tightly together, with due consideration for head room for both gun and shell feed. A prediction for the future is that cannon mountings in the wings will be made an integral part of the wing structure itself and so serve a dual purpose, and not merely be adaptations of fixing existing guns on planes designed to fly without these guns.

The 37-mm gun characterized in the table for turret mounting only, has a surprisingly low weight for the gun alone. With the mounting, the complete installation would weigh 250 lb. 100 rounds of ammunition would weigh 164 lb. The rate of fire is theoretical, as fire is not as continuous as that of the machine guns or 20-23-mm cannon, which are belt-fed. The 37-mm cannon is fully automatic as far as the five round clip is concerned, but after each clip is exhausted, a new one has to be inserted manually by the gunner. To fire 100 rounds, he must insert new clips 20 times. To offset this, the 37-mm shell is extremely effective and every one that would register against an airplane would almost certainly bring it down. The 37-mm cannon we are here considering is for mounting on large bombers primarily and not on small fast pursuit planes.

Summarizing the three classes of aircraft arms in use, it is apparent that all have their sphere of action. Machine guns, it is evident, are becoming increasingly ineffective. The 37-mm cannon projected for turret mounting is limited to certain types of planes, and for fixed mounting of this caliber cannon, using a longer gun with a higher muzzle velocity, would result in prohibitive weight of gun and ammunition.* The 20- and 23-mm automatic cannon appear to be the golden medium, and indeed, they will play a large part in all future aerial warfare.
*Unless used as the only fixed gun on the plane, or at most two on twin-engined and larger craft.

Comparative Table of Characteristics
of Aircraft Machine Guns and Aircraft Cannon
Aircraft Machine Gun,
caliber .30 (7.6 mm)
(American Browning)
Aircraft Machine Gun,
caliber .50 (12.7 mm)
(American Browning)

Aircraft Cannon
(20 mm)

Aircraft Cannon
(23 mm)
Aircraft Cannon
(37 mm) (Turret)
American Armament Corp
1. Weight of gun in pounds24.152118.8 118.8 135
2. Overall length of gun in inches 40.25478.478.435
3. Length of barrel in inches2436474729.14
4. Rate of automatic fire, rounds per minute1000-1250400-650350 drum
500 belt
5. Muzzle velocity, feet per second2.6602,5502,9202,3951,250
6. Type of operationrecoil, full automaticrecoil, full automaticrecoil, full automaticrecoil, full automaticrecoil, full automatic
7. Type of cartridge or shell feedingdisintegrating beltdisintegrating beltdisintegrating beltdisintegrating beltfixed clip 5 rounds
8. Length of recoil, in inches4.43.93.910
9. Type of ammunitionArmor piercing tracerArmor piercing tracerExplosive shell,
Tracer incendiary
Explosive shell,
Tracer incendiary
Explosive shell,
Tracer incendiary
10. Weight of complete round of ammunition395 grains1,825 grains10.5 oz11.9 oz1.4 lb
11. Weight of bullet or shell only743 grains4.64 oz6 oz1.1 lb
12. Force of recoil, single shot firing, in pounds4403301,000
13. Force of recoil, automatic firing, in pounds4843961,000
14. Twist of groove in barrel (rifling) in calibers363625
15. Weight of ammunition with belt or clip in pounds/100 rounds6.63076.682.5164
16. Maximum effective practical range, in yards2003001,2001,000300
17. Maximum effective theoretical range, in yards6009002,4002,000600
This article was originally published in the April, 1941, issue of Aviation magazine, vol 40, no 4, pp 36-37, 136, 138, 140.
The original article includes 2 photos and 5 drawings.
Photos not credited.