The Development of RAF Guns and Ammunition

from World War 1 to the Present Day

 

Anthony G Williams

 

This is the text of a presentation given to the Royal Air Force Historical Society in October 2008

with a selection of the illustrations used. The presentation is based on material in the trilogy co-authored with Emmanuel Gustin: Flying Guns: Development of Aircraft Guns, Ammunition and Installations

Details of the books are on THIS page.

 

Last amended (some new photos): January 2017

 



World War 1

 

The initial use of aircraft in the Great War was for reconnaissance and artillery spotting. However, it was soon realised that if such flights were useful, it made sense to try to deny them to the enemy. Airmen therefore began to take guns aloft to take pot-shots at the opposition, but these were aircrew rather than aircraft guns; a variety of pistols, revolvers, shotguns, rifles and carbines were carried. Special ammunition was even developed for some of them: incendiary bullets for firing at observation balloons, and even a shotgun cartridge firing a type of chain shot for slicing through aircraft bracing wires.


It was soon realised that a machine gun was the ideal weapon for shooting at other aircraft, but at that time the standard British Army MG was the .303 inch Vickers Gun; a heavy, water-cooled device weighing some 40 lbs which the primitive early-war planes struggled to lift off the ground. Fortunately, BSA had acquired a licence to manufacture the American Lewis Gun, which was far more suitable. It was much lighter at around 26 lbs, and its ammunition was held in a pan magazine clipped to the gun, instead of the rather cumbersome fabric belt of the Vickers.  The Lewis was rather bulky because its barrel was surrounded by light-alloy fins which were themselves covered by an external sleeve, but such elaborate cooling arrangements were not required when the gun was mounted on an aircraft, so they were soon stripped away leaving it with a bare barrel. This, in conjunction with removing the stock, reduced the weight to about 17 lbs.

The Mk II version, used later by the RFC, had a smaller sleeve fitted to protect the mechanism. Two other changes made during the war were to increase the capacity of the magazine from 47 to 97 rounds and to speed up the rate of fire from around 550 to between 700 and 750 rounds per minute (rpm). The final Mk III version reverted to a stripped barrel, shown here with the double-height 97-round drum.

The light weight and magazine feed meant that the Lewis was particularly suited to flexible mountings designed to allow a gunner to move the gun around to point it in different directions. Mounting the gun for the pilot to use proved rather more problematical, especially as it was discovered that the optimum arrangement for a fighter aircraft was to have the engine and propeller in front of the cockpit. Early attempts to mount the gun to fire at an angle past the propeller were less than successful. It soon became clear that the most effective shooting could be done if the gun was fixed to fire straight ahead.



Lewis Mk III (left) and a stripped Mk I. Note the double-height 97-round magazine on the Mk III



Some brave souls fixed the gun to fire through the propeller disk, hoping that a couple of bullet holes through the propeller could be tolerated, but there was always the risk of chopping off a propeller blade. The first satisfactory solution for biplane fighters was to mount the gun on the top wing, so that the bullets would miss the propeller. With early mountings of this type the gun was fixed in place, which meant that the pilot had to stand up to change the magazine, leading to some exciting incidents. Eventually, the Foster mounting was developed: this allowed the pilot to pull the gun down towards the cockpit for magazine changes, and incidentally permitted the gun to be fired upwards.




Foster mounting for a Lewis Mk II on an SE5a

 

The top-wing mounting was not an ideal solution, however. The need to change magazines and to clear the frequent stoppages mainly caused by poor quality ammunition made this inconvenient. The French came out with a makeshift alternative solution in 1915 by mounting a gun in front of the pilot, where it could easily be reached, and fitting the propeller blades with deflectors to prevent bullets from penetrating them.

 

The Germans adopted a more sophisticated solution (actually first proposed before the war) of timing the shots from the machine gun so that they would pass between the propeller blades. These synchronisation systems involved converting the gun so that it could fire single shots each time the firing line was clear, thereby slowing the rate of fire to a degree which varied according to the relationship between the gun's reloading speed and the constantly changing propeller revs. This required precise timing of each shot and therein lay a major problem, for the Lewis Gun was not capable of this. It could only fire from an "open bolt" (with the bolt held back and the chamber empty) so each time the firing signal was sent, there was a pause while the bolt started to move forwards, collected a cartridge from the magazine, loaded it into the chamber, locked the breech and then fired. Despite many attempts to modify the Lewis, this all took far too long for precise timing.

At this point the Vickers Gun came back into the picture. It fired from a closed bolt; when ready to fire there was a round in the chamber, the action was locked and all that was needed to fire the gun was for the firing pin to be released. This was ideally suited to synchronisation. Furthermore, in a fixed mounting the belt feed was less of a problem and saved the pilot from having to change magazines. And by then, the planes were powerful enough to cope with the extra weight. So the Vickers became the standard fixed gun of the RFC and RNAS, although the Lewis was still preferred for certain purposes, which I will come on to. The Vickers was modified, first by emptying its water jacket and punching holes in it to let cooling air through (the jacket could not be removed as it was needed to support the front of the moving barrel) reducing the weight to around 28 lbs, and subsequently by providing it with a slimmer and neater jacket, although this Mk 3 version wasn't adopted until after the War.

Its free rate of fire was also increased from 550 to 850 rpm by fitting a Hazleton muzzle adaptor, although the actual RoF of a synchronised gun would have been much less than this.  Synchronisation was always problematic and inclined to slip out of phase, but the Constantinescu-Colley, or C-C, hydrosonic system performed relatively well.

 Vickers Mk I (right) and the post war Mk III.


 

Fabric ammunition belts (used in Army Vickers guns until the 1950s) had the disadvantage that the empty section tended to flap around in the wind, plus the belt could get wet and then freeze. Steel disintegrating-link ammunition belts were perfected in the UK by Prideaux in mid-WW1 and became standard for aircraft guns thereafter.

 

The methods of sighting the guns also developed during the Great War. At first, only a couple of bits of metal were used to line them up, but the British developed the Aldis optical sight, mainly for fixed guns, and everyone worked on complex sights for flexibly-mounted guns, designed to compensate for the ballistic problems of shooting to one side. Even so, holding fire until as close as 50 yards or less was recommended.

 

I now want to turn to ammunition developments, since aerial fighting in the Great War prompted considerable development efforts, in two directions. One was to improve the variable quality of the ammunition (a problem affecting all combatants). A certain percentage of stoppages was acceptable in a ground gun, since the gunners could usually quickly clear the jam, but was a different matter in an aircraft, especially if the gun was mounted out of reach. In an attempt to resolve this, the British introduced in 1917 "Green Label" (or "Green Cross") .303" ammunition specifically for synchronised guns. This was taken from standard production lines, but carefully selected from batches which complied with tighter manufacturing tolerances and gave reliable ignition. This proved successful and was followed up in 1918 by establishing special production lines to make high quality ammunition for this purpose. This was known as "Red Label" (also as "Special for RAF, Red Label", "Special for RAF" and finally "Special").


The second line of development was the production of a variety of specialist bullets, initially prompted by the need to destroy hydrogen-filled spotting balloons and airships which were little affected by having small holes drilled through them.

Several attempts were made to devise bullets filled with various explosive and/or incendiary chemicals. Initial work was in larger-calibre guns simply because the bullets were bigger, but this was soon replaced by .303" ammunition. Some types of incendiary, such as the Buckingham which contained a phosphorous/aluminium mixture, were ignited on firing and burned slowly throughout their flight leaving a smoke trail, while others ignited on impact. The Pomeroy or PSA explosive bullet contained nitro-glycerine and was purely explosive, but the Brock, which contained potassium chlorate, and the RTS (Richard Threlfall and Sons) with both nitro-glycerine and phosphorous, had both explosive and incendiary effects, so were known as HEI bullets. Note in this photo that the first three rounds were loaded with Cordite propellant (so-called because it was extruded into cords), the last with nitro powder. Use of these bullets was initially somewhat hazardous as the early versions had a reputation for premature detonations, and elaborate handling precautions were required. These bullets were at first reserved for home defence, partly because they were needed to combat the German airships attacking British cities, partly because of concerns that they were technically illegal (explosive/incendiary bullets were banned as inhumane by international agreement). However, they were used on both sides, and after the war it was recognised that they were acceptable as they were intended to be used against aircraft rather than people.

.303 rounds sectioned: Brock incendiary, PSA Mk I HE, PSA Mk II HE, RTS Mk II HEI (Peter Labbett)

 


The home-defence fighters retained the top-wing Lewis guns rather than the synchronised Vickers, for several reasons. First, the gun was lighter which was an advantage given the high rate of climb needed to reach airship altitudes; secondly, it could be tilted to fire upwards; thirdly, its location meant that the pilot was shielded from the muzzle flash by the wing, so didn't lose his night vision; and, last but far from least, it was unsafe to fire the early explosive/incendiary ammunition from a Vickers because the bullet left in the hot chamber after firing a burst could "cook off" from the heat. In this instance, the Lewis Gun's open-bolt firing was an advantage.

 


The Interwar Period

 

By the end of the Great War the Vickers and Lewis guns in .303" calibre were the established RAF armament and remained so until the late 1930s. They were also widely sold abroad, including to Japan, which was still using them at the start of World War 2.

 

However, many experiments had also been made during the War with large-calibre shell-firing guns, later known as "cannon". Some of these were manually-loaded, including the recoilless Davis guns. Others were big machine guns, notably the 1 Pdr (37 mm) "COW" gun (Coventry Ordnance Works). None saw significant use in the War. A few dozen of the 37 mm COW guns were completed and these featured in various interwar projects including planes specially designed to mount them, but they didn't generate much enthusiasm. The COW enjoyed a swansong as an airfield defence gun in World War 2.




2 Pdr (40mm) recoilless Davis Gun on Felixstowe flying boat (Lewis gun for sighting)




1 Pdr (37mm) COW gun on Blackburn Perth flying boat


 

There was little money to buy armaments after the end of the Great War, but that didn't stop lots of theorising and experimentation, particularly in investigating the potential of larger-calibre guns. Three different classes of aircraft gun began to emerge in various nations: improved rifle calibre machine guns, heavy machine guns, and automatic cannon. Rifle calibre guns were those which use the same ammunition as the standard military rifle, firing bullets of around .30-.32 inch in diameter (7.5-8 mm calibre). Heavy machine guns fired much bigger cartridges with bullets of around .50-.60 inch diameter (12.7-15 mm) which were three to six times as powerful as rifle-calibre ammunition. Cannon fired projectiles of 0.8 inch (20 mm) or greater diameter, which was generally considered to be the smallest worthwhile size to use high-explosive ammunition, although some smaller HE shells were used by Germany, Italy and Japan during World War 2.

 

Vickers was in the process of developing a scaled-up version of their .303" MG, chambered for a new .5" (12.7 mm) cartridge. This was produced in three versions for army, naval and aircraft use and was tested by the RAF in the mid-1920s against the new .50" Browning heavy machine gun, which was bigger and more powerful. The conclusion was that neither offered sufficient advantages to replace .303" MGs, since the slightly bigger hole they could punch wasn't adequate compensation for their greater size and weight and their lower rates of fire. The Swiss Oerlikon 20 mm cannon, developed from the German Becker of the Great War, was also tested in the late 1920s and early 30s and proved more promising since its explosive shells could do a lot more damage than just punching bigger holes, but it was big, heavy and slow-firing.

 

As a result of all of this, the RAF decided in the mid-1930s to stick with the .303" calibre for the time being, while noting that a 20 mm gun would be the preferred replacement if armour protection were applied to warplanes. After competitive tests, two new machine guns were selected; the US Browning and the Vickers Gas Operated (known also as the VGO or Class K), a modification of the Vickers-Berthier light MG. The Browning was considerably modified over the American original. It was not just converted from .30 to .303 inch calibre but also modified to fire from an open rather than a closed bolt because the cordite-loaded .303 rounds tended to explode if left in a hot chamber. The Browning was belt-fed and initially intended for fixed fighter installations (although later adapted for use in turrets). In contrast the VGO used a pan magazine of 100 rounds and was for flexible mounting. It bore a close resemblance to the Lewis, although internally it was quite different. Rates of fire were around 1,200 rpm for the Browning, 950 rpm for the VGO. It was with these weapons that the RAF fought the Battle of Britain.


The rifle-calibre guns used by different air forces were quite similar in performance, weighing around 20-25 lbs and mostly firing at 1,000-1,200 rpm.  There was more variation in the characteristics of heavy machine guns, with weights ranging from 40 to 90 lbs and rates of fire generally between 700 and 900 rpm. There was an even greater variation in size and power among the 20 mm weapons (let alone the few even larger-calibre cannon), with weights from 50 to 120 lbs, rates of fire from 400 to 800 rpm, and considerable variation in muzzle velocities, which affected their hit probability.



303" Browning machine gun



303" Vickers Gas Operated machine gun





The three standard wartime RAF rounds: .303", .5" and 20 mm Hispano (left), compared with representative German ammunition: 7.92 mm, 13 mm MG 131, 15 mm MG 151,    20 mm (MG-FF) and 20mm MG 151



The photo above indicates how the ammunition differs in size and power, with three famous cartridges used by the RAF during World War 2 the .303" rifle-calibre round, the American .50" Browning HMG, and the 20 mm Hispano cannon compared with Luftwaffe ammunition in the same classes. The considerable power of the Hispano is obvious.



 

World War 2

 

In 1934 the Air Ministry had decided to accept the advice of the Operational Requirements Branch that in view of the increasing speeds of both fighter and bomber aircraft, gun firing opportunities would be brief, so a six or preferably eight-gun battery should be installed in fighters. This of course led to the specification which eventually resulted in the adoption of the Hurricane and Spitfire. Fitting so many guns around the engine wasn't feasible, so they were all mounted in the wings and the complications of synchronisation were dispensed with. The removal of the guns from near the hot engine caused a gun freezing problem at high altitudes, which was addressed, not always successfully, with special lubricants and heating systems.


Sectioned .303" rounds, from the left: tracer, armour-piercing and B Mk VI incendiary (Dixon/De Wilde)



Work was also done on improved .303" ammunition. The steel-cored armour-piercing and Buckingham incendiary/tracer (designated B.Mk IV) rounds were based on old designs, but a new incendiary, the B.Mk VI, was developed by Major Dixon, loosely based on the Belgian De Wilde design. In this picture you can see the steel core for the AP bullet and the construction of the famous B Mk VI incendiary.

 

In firing tests, the B. Mk VI had a 20% success rate in setting fuel tanks alight, twice that of the Buckingham or the equivalent German 7.92 mm round, and also had the happy side-benefit that the flash of ignition on impact told the pilot that he was on target. Incidentally, the Americans adopted the Dixon design in a simplified form for their .30 and .50 calibre incendiary ammunition, and the British subsequently copied the simplified design as the B.Mk VII. Unlike the practice in other air forces, which used mixed ammunition belts, the RAF preferred to load each .303 fighter gun with only one type of ammunition. The Dixon ammunition was first issued in June 1940 and was at first in short supply, the initial fighter loading being one gun firing Dixon incendiary, two with Buckingham incendiary/tracers, two with armour-piercing and three with plain vanilla "ball" rounds with lead cores. By 1942 the standard loading for fixed guns was half with AP and half with incendiaries.

 

As a result of early battle experience, aircraft armour and self-sealing fuel tanks were rapidly applied, and the .303 guns lost effectiveness accordingly. In the Battle of Britain, the performance of .303 ammunition was initially adequate but it was found that the German bombers often survived large numbers of hits. The reason became clear in further tests which involved firing .303 and German 7.92 mm armour-piercing ammunition against the fuselage of a Blenheim light bomber from behind not the toughest of structures, and with only a 4 mm armour plate protecting the gunner. This AP ammunition could normally penetrate up to 10-12 mm of armour plate, but it was found that the aircraft structure it had to plough through before reaching the armour deflected, absorbed or disrupted the flight of the great majority of the bullets, and of those which reached the armour, very few had enough energy left to penetrate it. Some improvement was achieved by reducing the gun harmonisation range from 400 to 250 yards in order to concentrate the firepower of the RAF's fighters, but it was clear that a more powerful gun was needed. This eventually arrived, just too late for the Battle, in the form of the 20 mm Hispano.




20 mm Hispano with drum feed on engine mounting


 

The Hispano (technically the Hispano-Suiza HS 404) was designed and developed at the French arm of the European Hispano-Suiza company in the mid 1930s. A firing demonstration of a prototype to British officers in Paris in 1935 banished all thought of the Oerlikon; the Hispano was similar in size and weight, slightly more powerful and fired nearly twice as fast. Unfortunately, the processes of obtaining approval to buy the gun, setting up a subsidiary Hispano factory at Grantham (the British Manufacturing And Research Company, or BMARCO), redrawing the gun to imperial rather than metric units, testing and debugging the prototypes, then fitting them into aircraft and debugging the installations, all took too long for the cannon to achieve anything in the Battle of Britain.

 

A key problem was that the Hispano was designed for engine mounting, which meant that it would be bolted to a rigid crankcase. An aircraft wing is nowhere near as rigid, and this caused problems with all wing mountings, which had to be fine-tuned to achieve reliable gun functioning. In the initial Spitfire installation, which did see brief use in the Battle, matters were made worse by mounting the guns on their side in order to bury as much as possible of the bulky drum magazine within the wing thickness. The Hispano took a marked dislike to its unfamiliar environment and jammed as often as it fired. Much modification was needed to both the gun and the mountings before acceptable reliability was achieved. Even so, the stoppage rate by 1944 was still three times that of the US .50 Browning. A major improvement was the replacement in 1941 of the original 60-round drum by a belt feed.


Belt-fed 20 mm Hispano on a Hurricane Mk IIC



Work was also needed to the ammunition, as it was found that the fuze of the standard explosive shells was too sensitive, causing them to burst on the aircraft skin rather than within the structure where they would do most damage, and plain steel practice shells often proved more effective. By 1941 both a delayed-action fuze and an explosive with added incendiary filling had been developed, but the practice rounds remained in use alongside the HEIs until they were replaced by a new semi-armour piercing round (SAPI) which was essentially an HE shell filled with an incendiary compound and capped with a hard steel tip instead of a fuze. From 1942 on, the standard Hispano loading became 50% HEI, 50% SAPI.





Types of RAF 20 mm Hispano ammunition, from left: Practice/Ball; HEI; SAPI; AP


Compared with other Second World War 20 mm aircraft cannon, the Hispano was a powerful and effective gun, but only averagely fast-firing and unusually long and heavy. Its weaknesses were addressed in the late-war Mk V, shortened, lightened and speeded-up from 600 to 750 rpm. Below you can compare the size of the .303 Browning with the .50 Browning, the short-barrelled Mk V Hispano and the standard Mk II.




 



Wartime RAF guns. From the top: the .303" Browning; the .50" Browning; the short-barrelled Hispano Mk V and the standard Mk II




The Hispano Mk V could lay claim to being the best aircraft gun of the war, but this mainly saw action in the Hawker Tempest. What became the standard RAF armament of four Hispanos was also probably the best all-round fighter armament of the war, weighing more or less the same as the standard American armament of six .50" Brownings but being about twice as destructive.

 

Sadly the same claims could not be made of the RAF's bomber defensive armament. The initial advantage of the power-operated multi-gun turrets disappeared as the .303" gradually lost effectiveness. Various attempts to introduce more powerful guns virtually all failed; the long and heavy Hispano, which needed substantial support, was far from ideal for the purpose and did not enter service in turrets until very late. The .50" Browning was eventually fitted to some turrets by the end of the war, as well as being used in some fixed mountings, most notably late-model Spitfires which carried two .50" Brownings alongside two Hispanos, apparently because gun heating arrangements were inadequate to keep four Hispanos functioning in sub-zero temperatures.

 

These were not the only guns used by British aircraft during the Second World War. Two others deserve mention; the Vickers 40 mm Class S and the Molins 6 pounder. The Vickers was designed around the same ammunition as the naval 2 pounder pom-pom, but the gun was based on a much-developed 1 Pdr COW gun. It was originally intended for aerial combat and fitted in a dorsal turret to a much-modified Wellington bomber, but this idea was abandoned. Later, a need arose for a gun capable of penetrating tank armour which could be fitted to ground attack planes. The S gun was duly dusted off and issued with armour-piercing ammunition.



Vickers 40 mm S Gun


Hurricane armed with a pair of uncowled 40 mm Vickers S Guns



It saw service in the Hurricane IID (with one slung under each wing) and was an alternate armament for the Hurricane Mk.IV, which otherwise carried rocket projectiles, conversion between the gun and rocket armaments being quite rapid. The S gun performed very well in North Africa, South-East Asia and in 1943/44 over northern France, flying from bases in England. Compared with the rocket projectiles more usually associated with "tank-busting" the S Gun was far more accurate, scoring in practice shoots around 25% hits compared with 5% for the RPs (and according to Operational Research, the peculiar flight characteristics of the RPs made them very difficult to aim, which meant that in action, pilot stress caused the hit rate against tanks to decline to 0.5%). Unfortunately, the S gun wasn't powerful enough to penetrate the latest tanks, and the Hurricane Mk.IV was withdrawn from the European theatre only three months before D-day.

 

 The RAF continued to show interest in airborne anti-tank guns, leading to the development of the DH Mosquito FB XVIII (better known as the Tsetse). This carried an army 6 pounder anti-tank gun fitted with an autoloader developed by the Molins company. This combination worked very well, scoring a 33% hit-rate against tank-sized targets, and the 57 mm ammunition was far more effective than the 40 mm, but the RAF changed its mind and handed the planes over to Coastal Command for anti-U-boat work since it was the only gun which could reliably penetrate a pressure hull.


6 pdr Molins gun (above) and Mosquito FB XVIII (below)



In 1946 a Tempest fitted with a pair of Vickers 47 mm Class P anti-tank guns was tested, but after that official RAF interest in powerful ground-attack guns disappeared for good.

 

Gunsights also developed, the pre-war reflector sights being supplemented by gyro sights in the last couple of years of the War in order to make deflection shooting easier, as without them average pilots were unlikely to score hits unless they were directly behind their targets.

The comparative sizes of 20 mm, 40 mm, 47 mm and 57 mm cannon ammunition conveys some impression of their relative power



 

The Modern Era


At the end of the Second World War, there was, as usual, very little money for new armament developments and the Hispano remained in service until the mid-1950s, not just in fighters but also in the Shackleton MR plane. However the Allies did have a new gun to play with; the Mauser MG 213C.


The German firm had designed a new type of gun to meet a Luftwaffe requirement for a very fast-firing, high-velocity 20 mm cannon. This addressed the main restriction on rate of fire ammunition handling by breaking it down into several stages. Instead of one chamber formed as a part of the rear of the barrel, five chambers were used within a cylinder whose axis of rotation was parallel with the barrel, so that as the cylinder rotated, each chamber was brought into line with the barrel in turn, and its cartridge fired. At the same time, the other chambers were engaged with loading a fresh cartridge or ejecting a spent case. This allowed rates of fire of well over 1,000 rpm to be achieved. As this layout bore some resemblance to the traditional revolver type of handgun, it became known as the revolver cannon.  During the development of the MG 213C a low-velocity 30 mm version was also produced, considered more suitable for bomber destruction. This became the focus of interest in both the UK and France, who continued the development of the gun. It took several years before the resulting Aden and DEFA guns were ready for service, but they were eventually introduced using slightly different versions of the 30 mm ammunition.


 

30 mm Aden gun


Further joint development saw the ammunition altered to fire a lighter shell at a higher muzzle velocity, and this became the NATO 30mm round still used by the Aden Mk 4 and DEFA 550 series guns, and by the M230 Chain Gun used on the Apache AH-64 attack helicopter in British Army service. However, the Aden, DEFA and M230 all use slightly different versions of the ammunition which are not completely interchangeable.


The 30 mm Aden Mk 4 was the standard RAF and FAA gun from the late 1950s until the 1980s, and remains in service with the Hawk trainer (the last combat aircraft to carry it being the Sea Harrier and the Jaguar). It was exceptionally hard-hitting for its day, firing shells weighing twice that of the Hispano's at an only slightly lower muzzle velocity, but at a much higher rate of about 1,300 rpm. The difference in destructive effect compared with the Hispano was even greater than these figures indicate, because the Allies also benefited from another German development; the Minengeschoss or mine shells.



Luftwaffe 20mm MG-FF ammunition: HE-T, Minengeschoss and API


 
The Minengeschoss were high-capacity shells with very thin walls which approximately doubled the HEI content of the shells, as you can see in the above picture of sectioned Luftwaffe 20 mm ammunition: compare the space for explosive in the HE-T and the Minengeschoss, even allowing for the tracer in the former. When used in the Aden, this resulted in the 30 mm shells having four times the blast effect of the Hispano's. Aden ammunition also used another German development, tungsten-cored AP projectiles.

A 1955 paper compared the performance of the RAF's standard armament of four Adens with the USAF's alternatives of four M39 20 mm revolver cannon or one 20 mm M61 six-barrel rotary "Gatling" gun.  The rates of fire quoted were 6,000 rpm for the US systems and 5,200 rpm for the Aden fit. The 20 mm guns also had a higher muzzle velocity. But in the weight of high explosive fired per second, the Aden fit was six times higher. Still, the RAF was not entirely satisfied, feeling that a higher muzzle velocity would increase the hit probability. It was also noted in 1957 that 20% of Aden shells would ricochet off the target and another 55-60% would detonate on the surface, seriously reducing their lethality. The RAF later greatly admired the 30 mm Oerlikon KCA revolver cannon fitted to the SAAB Viggen fighter, which fired heavier shells at a much higher velocity than the Aden and matched its rate of fire, at the cost of a relatively modest increase in size and weight.

Despite the dismissal of the 20 mm M61 Vulcan rotary gun, it did see British service in a gunpod which could be carried by the RAF's FGR-2 ground attack version of the McDonnell Douglas F-4 Phantom II aircraft.




Above: SUU23 gunpod under Phantom II




27 mm Mauser BK 27 revolver cannon



The next gun to enter RAF service was the 27 mm Mauser BK 27 revolver cannon, fitted to the Panavia Tornado aircraft. This is similar to the Aden and weighs very little more, but uses 27 mm ammunition which fires shells of the same weight at a muzzle velocity which is 30% higher and at a rate of fire which is 25% faster, at about 1,750 rpm. The same gun, modified for a linkless feed system, is also fitted to the Eurofighter Typhoon.

 

In the 1990s there was an abortive attempt to produce a new version of the Aden gun, chambered for the NATO 25 mm cartridge and known unsurprisingly as the Aden 25. It was initially intended to arm the RAF's Harriers from the GR5 onwards but was defeated by various technical problems,  the final and insurmountable one being the sharp curve required of the path of the ammunition belt between the magazine and the gun, which caused unreliable feeding. It was abandoned at the end of the last decade after about 100 guns had been built, and the Harriers have remained gunless ever since, which reportedly proved a disadvantage in Afghanistan.

 

The proposed adoption of the Lockheed Martin F-35B Lightning II STOVL strike fighter could see another gun and ammunition entering the inventory: the General Dynamics GAU-22/A four-barrel rotary gun in the NATO 25 mm calibre, which has been selected as an optional fit in a gunpod for this aircraft. This offers an even higher muzzle velocity than the BK 27 (albeit firing lighter shells) and will fire at a maximum of about 3,300 rpm.

 

The picture below shows the  ammunition used in  RAF guns, plus the 25 mm NATO round which may be introduced into service with the F-35B. Note the two types of Aden ammunition: the early Low Velocity (which was almost identical to the Mauser original) which was soon replaced by the High Velocity, which sacrificed some shell weight to make space for more propellant to raise the muzzle velocity.



 From left: .303"; .50" BMG; 20 mm Hispano; 30 mm Aden Low Velocity; 30 mm Aden High Velocity; 20 mm M61; 27 mm BK 27; 25 mm NATO


 


Some final observations: how useful are guns?

 

In the days before guided missiles, guns were what fighter aircraft were all about: the sole purpose of the aircraft was to get some guns into a position where they could harm the enemy. The introduction of guided air-to-air missiles led to the rapid abandonment of guns in the 1960s, which was promptly regretted when experience in Vietnam revealed that, for various reasons, the impressive missile hit rates achieved in trials were not replicated in combat. Since then, missile performance, in both the air-to-air and air-to-ground roles, has greatly improved. Even so, new fighter designs still come with guns or at least, a gun. This is despite the problems which their vibration and noxious gas emissions cause to the aircraft, as well as the cost in purchasing, feeding and maintaining the guns plus training those who use and care for them. Indeed, the Ministry of Defence did its best to cancel the acquisition of the guns for the RAF's Typhoon, but these were eventually fitted and cleared for use, at least in the ground attack role.

 

Why are guns remaining so popular? I think that several reasons can be identified. The current motivation is for aircraft acting in close support of troops in Iraq and Afghanistan to deliver very precise fire which is limited in effect, so that enemy forces very close to our troops can be engaged. This is likely to be a continuing need as the present generation of guided bombs and missiles, while precise, have a considerably greater radius of destruction. Staying with the surface attack role, a gun also has the ability to fire warning shots or inflict limited damage to a ship, for instance in a display of determination. In the air-to-air role, a gun may also fire warning shots (when using tracer ammunition), may be used to destroy low-value targets such as Unmanned Aerial Vehicles, and provides a last-ditch backup should the missiles run out.

 

Ultimately the gun may be replaced by a combination of small guided surface-attack missiles, such as the laser-guided 70 mm rockets which are now in service, plus lasers in the air-to-air role, but that day seems likely to be many years away.


 

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Other articles on this website dealing with aspects of this subject in more detail are:

 

The Cannon Pioneers: the Early Development and Use of Aircraft Cannon

 

37mm and 40mm Guns in British Service

 

The .5" Vickers Guns

 

The Battle of Britain: Armament of the Competing Fighters

 

WW2 Fighter Armament Effectiveness

 

Tankbusters: Airborne Anti-Tank Guns in World War 2

 

The Vickers 40mm S Gun with Littlejohn Adaptor

 

The 6 pdr 7 cwt and the Molins Guns

 

The RAF's 47mm Class P Gun project

 

Modern Fighter Gun Effectiveness

 

Will the Fighter Gun Survive?

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