A short overview of British aircraft weapons of WWII.
There are some excellent on-line references that go into great detail about individual weapons (see a list of links at the foot of the page). However, I felt there was a need for a simpler, short overview that put the weapons into some historical context. Here is the result.
At the end of the First World War, the Royal Air Force was using an air-cooled variant of the water-cooled belt-fed Vickers machine gun and the Lewis drum fed machine gun. The Vickers gun was used in installations where the gun had to fire through the spinning propeller, its closed-bolt system being compatible with the synchronisation system that entailed. The rate of fire of the aircraft Vickers gun was increased to double that of the infantry gun. The Vickers gun came in two versions, the most common one firing the standard British .303 calibre rounds and another built by Colt in the USA that fired larger French 11mm rounds that were used for "balloon bursting" (shooting down enemy observation balloons). The Lewis gun fired from an open bolt and was unable to be used with synchronisation gear so it was used on movable mounts by observers and dedicated gunners or was fixed to fire outside the arc of the propeller, one method was the "Foster mount" where the gun was mounted on top of the wing but could be slid back on a rail until it was vertical.
This combination of the Vickers and the Lewis saw the RAF through the 1920s and into the mid-1930s. In the early 1930s the need for increased fire-power to bring down larger aircraft, built primarily of metal, was recognised and trials led to the adoption of a variant of the American M1919 Browning machine gun as the fixed machine gun for the next generation of British fighters. This was slightly lighter than the Vickers and had a much greater rate-of-fire and it was recognised that batteries of at least 6, (then increased to 8) ¹ of these guns would be required (some marks of Hurricane could mount up to 12 guns). Built in the UK by BSA (Birmingham Small Arms), it was redesigned for British use, changing to .303 calibre and being altered to fire from an open bolt to avoid "cook-offs". Using an open bolt would normally preclude a guns use with synchronisation gear, but the British Browning could use a mechanism that closed the bolt when the trigger was pulled but only fired the round when the synchronisation gear engaged. In the event this was only put to use in late production Gladiator biplane fighters (early ones used the older Vickers gun) and a handful of Brewster Buffalos converted to use them. All other British fighters that used the gun had them fitted in the wings to fire outside the propeller arc. The Browning was also used in the new enclosed turrets for the next generation of British bombers, the Wellington and Whitley (along with the Defiant turret fighter), with up to four guns in some turrets.
The old Vickers gun was still in use in older designs such as the Fairey Swordfish into the Second World War. The Australian Wirraway two-seat army support aircraft was still using them at the end of the war in the Pacific.
At the same time, the Lewis gun was replaced by a new drum-fed design from Vickers called the Vickers "K" gun, also known as the "VGO" (Vickers Gas Operated). This had a higher rate of fire than the old Lewis gun, was more reliable, and if stoppages did occur they were much easier to clear. The Vickers K fired from an open bolt and so could not be used with synchronisation gear. It was used primarily as a moveable machine gun for observers and gunners although the turret of the Avro Anson and early Blenheim bombers also mounted them. At the outbreak of World War Two, the RAF was switching from the Lewis to the Vickers K and it would be very unlikely for the earlier Lewis to be used in combat. However, the FAA (Fleet Air Arm) retained the Lewis gun through until the end of 1940. ²
Early experience in World War Two showed that the need to change the magazine on the Vickers K when under sustained attack was a big drawback, so wherever space permitted a belt-fed Browning was substituted, indeed where possible a pair of Brownings were used instead. The Hampden and the Lysander were two aircraft so upgraded. The turret of the Blenheim was also redesigned to accommodate two Brownings instead of the single "K" gun. Many of the Vickers "K" guns were then passed onto Commando units, the Parachute Regiment and the SAS.
All these British machine-guns used a combination of normal "ball", tracer and armour-piercing rounds (the latter only of use against thin armour). The lethality of the British guns was greatly increased by the introduction of "de Wilde" incendiary rounds. These had been developed in secret at Woolwich Arsenal and were only just ready in time for the Battle of Britain. Named "de Wilde" ammunition by the British this was a ruse to make the Germans think it was based on the work of a Mr de Wilde in Switzerland. In fact, it had been found that "proper" de Wilde bullets could only be made by hand, whereas the British design could be mass-produced by machine. The British "de Wilde" bullets were the invention of C. Aubrey Dixon, a Captain in the Bedfordshire and Hertfordshire Regiment (he retired with the rank of Brigadier). The de Wilde ammunition produced a small explosion that would ignite anything flammable being particularly effective against fuel tanks. The speckles of light it produced when hitting its target confirmed that a pilot's aim was true.
Spitfire and Hurricanes in the Battle of Britain carried 350 rounds for each Browning gun which gave only 18 seconds of firing time. The Fleet Air Arm's Skua and Fulmar carried 600 rounds per gun which gave 31 seconds firing time. The Defiant's turret guns also used belts of 600 rounds. British pilots and gunners were taught to count the seconds of each burst of machine-gun fire to keep track of how much ammunition they had left, unlike German pilots who had ammunition counter gauges in the cockpit. It should be noted that none of these machine guns were designed to fire very long bursts of fire, they would overheat and risk damage. Bursts of 2-3 seconds were the norm with bursts longer than 6 seconds only in exceptional circumstances. During the early night blitz on the UK, the gunner in the turret of a Blenheim night fighter was surprised when the pitch black of the night sky was suddenly illuminated by the cockpit lights of a Heinkel 111 bomber that appeared out of clouds directly above him. He poured the entire contents of the 96 round drum magazine of his Vickers K gun into the cockpit of the bomber, causing it to crash to earth, but overheating his gun in the process. Night victories were rare at this stage of the war so the crew were feted when they landed. The celebration was cut short when one of the station armourers appeared brandishing the damaged barrel of the Vickers K gun and demanding the arrest of the gunner for damaging government property! ³
At the start of WW2 Britain was not alone in fielding rifle calibre guns in its aircraft, most of the major powers used them although the Germans, French and Dutch were supplementing them with the new cannon designs. What was not appreciated at the time was that the ShKAS rifle-calibre guns adopted by the Soviet Union were able to fire at twice the rate of the other nations guns while it still weighed about the same. Thus one ShKAS gun would have been worth two Browning .303 machine guns. However, its design was complicated and that complication could lead to jamming if not maintained properly or if there were inconsistences in the ammunition used (the ammunition for the gun was specially made to a high standard).
Many of the US aircraft supplied to Britain came with the .50 calibre Browning. This large calibre gun had a longer range and the projectile did much more damage. However, it did weigh considerably more and had a lower rate of fire (a post-war development of the .50 did have a higher rate of fire). Some late-war Spitfires used a pair of .50 Brownings alongside 20 mm cannon. The .50 calibre Browning served the Americans well, providing their needs for both fighters and bombers throughout WW2 and even into the Korean war (the gun is still in widespread use today). These days there is a lot of online criticism of the British for not adopting the .50 Browning in place of the .303 before the war started. There are a few factors to take into consideration. Firstly at the time that the .303 Browning was adopted (the early to mid 1930s) the prime consideration in British thinking was volume of fire, penetration and range were seen as secondary. The Air Ministry had a preoccupation with the brevity of modern aerial combat and the need to score as many hits in the minimum time was seen as paramount. It was only later that armour and self-sealing tanks became commonplace on military aircraft. Secondly, if they had considered a move to .50 calibre the British Vickers company already produced their own .50 calibre gun which they were trying to sell to foreign air forces. This gun had already been adopted by both the British Navy and Army. The Navy used it on quad mountings for anti-aircraft defence and the Army used it in light tanks and the Matilda I infantry tank. The Army and Navy had only adopted it after comparative tests with an early example of the American Browning .50 gun, in which the Vickers was deemed to be the better design. So if the RAF was going to adopt a .50 machine gun it is likely to have been the Vickers. This would have been a disaster, the Vickers .50 round was much less powerful than the American round resulting in a shorter range and less penetration. In service with the Navy and Army, it proved a troublesome, unreliable weapon and was soon abandoned by both services. If the RAF had gone to war with Hurricanes and Spitfires fitted with Vickers .50 guns the Battle of Britain might have had a different result. The Vickers .50 round did give Britain one great service however, the Italians adopted it for their SAFAT machine guns where its lack of range and penetration no doubt saved many an Allied life! On top of that, the Japanese, in turn, adopted the same size of round for their Ho-103 machine gun which in all other respects was a copy of the Browning .50 gun. Thus again the reduced penetration and range made it inferior to the US original. As it was when the British first received aircraft from the USA fitted with the Browning .50 there were problems with its ammunition. The cartridges on armour piercing rounds would split causing guns to jam, tracer rounds were in short supply and when used would damage the gun barrels. Finally, there was no .50 equivalent to the de Wilde round. Such was the frustration that some of the Buffalos were converted to use the rifle-calibre .303 Brownings. All the issues with the .50 calibre were resolved but by then it was too late for the RAF Brewster Buffalo pilots in Malaya and Singapore. ⁴
As it was, even before the Second World War had started the British had realised that the .303 calibre was going to be inadequate and adopted the Hispano 20 mm cannon and a quartet of these weapons became the standard weapons fit for all late-war British fighter designs. The slower rate of fire of the cannon was offset by their much-increased range and lethality. The explosive shells they fired would do extensive damage. Towards the end of the war, it was found that solid armour-piercing rounds were producing better results against the types of targets then being engaged. When first used the Hispano cannon used drums containing 60 rounds giving only 7 seconds firing time. The early drum mechanism often jammed in service but was later improved and a belt feed mechanism was developed which was very much more reliable and was usually used with belts of 120 rounds giving 14 seconds firing time.
In all the pre-war planning by the RAF for fighter armament one concept that kept repeating was that of "no-allowance sighting". You can read about this on my web page at the link below. As part of this concept, the RAF experimented with the large 37mm cannon produced by the Coventry Ordnance Works (the so-called "COW" gun). It was used to fire upwards on a variety of experimental types and briefly used in service mounted in the bow position on the big Blackburn Perth flying boats.
The COW gun was developed by Vickers into the 40mm "S" gun. First seen as a weapon for the defence of bombers (a turret for it was designed and tested on a Wellington bomber), it was later adopted as an anti-tank weapon, two being mounted under the wings of Hurricanes. It was used to great effect in the North African desert and the Far East. The ultimate heavy gun used by the RAF in WW2 was the 57mm "Molins Gun", an automatic version of the 6 pounder anti-tank gun used on the "Tsetse " version of the Mosquito.
It should be noted that in the 1920s and early 30s, with a background of international disarmament talks at the League of Nations, there was every chance that whole ranges of weapons would be banned. The use of explosive and incendiary rounds was a particular topic for debate. Indeed some interpretations of the St Petersburg Declaration of 1868 saw it as banning all such rounds under the size of 37 mm (which is often cited as the reason for 37 and 40 mm being so prevalent for smaller calibre field pieces). So the use of explosive cannon shells of 20mm and explosive incendiary ammunition in rifle calibres could be viewed as illegal under international law. All parties seem to have just ignored the international legal ramifications of the new developments in armament once the rearmament of Germany was revealed by Hitler in 1935. ⁵
In the first months of the war incursions by British Battle, Wellington and Hampden bombers met with heavy losses. It was realised that the defensive power of the .303 machine guns was inadequate against German Bf109 and Bf110 fighters. So the British switched to night bombing where the short range of the .303 was less of a disadvantage. Increasingly the gunners were seen to be there to give warning of German fighters to the pilot and the main defensive measure was the violent "corkscrew" the aircraft would enter into to escape, any German fighters shot down or damaged by the gunners were a welcome bonus. At this stage, the British did plan to return to the offensive in daytime, even before the war had started they had called on British industry to design turrets with four 20mm cannon in them, and the bomber designs to carry them (Air Ministry specification 1/39). This led to the Bristol 159 four-engined bomber armed with two of the four-cannon turrets (one dorsal, the other ventral). The Bristol 159 was one of the casualties of Lord Beaverbrook's decree that all production should be concentrated on existing types during the emergency period of 1940. American .50 calibre guns were in short supply to Britain and it was only in late 1944 that British built bombers started to use them. The Avro Lincoln bomber just coming into service at the end of the war was armed with a mixture of .50 Brownings (in tail and nose turrets) and 20mm cannon (in a dorsal turret).
Table of RAF aircraft weapon specifications
* The Lewis gun's rate of fire would "accelerate" the longer the trigger was pressed. The Lewis gun was prone to more stoppages than the Vickers K gun, and those stoppages were harder to clear. This made the Vickers K much more popular with air gunners. The Vickers K was in widespread use by the RAF early in the war and it would have been rare for a Lewis to have been used in combat by the RAF. Meanwhile, the Fleet Air Arm still retained the Lewis gun early in the war, but Vickers K guns were "obtained" whenever possible.
Non- aircraft machine guns for comparison
** Depending on model. Early model Besa guns could be switched between high or low rates of fire. The higher rate of file mode was deleted from later models.
Weight of fire of 3-second burst from various marks of Spitfire and contemporary German fighters.
¹ Air Marshal Sir Ralph Sorley is usually credited with the adoption of 8 machine-guns in the Spitfire and Hurricane, however, the full story is a lot more complicated. I recommend reading chapter 5 "the Quest for Fighter Firepower" in Colin Sinnott's book "The RAF and Aircraft Design 1923-39" ISBN 0-7146-5158-3
² To confirm the FAA continued to use the Lewis through the first year of WW2 read Matthew Willis' book "Blackburn Skua and Roc" (ISBN 978-83-89450-44-9).
³ This incident happened on 31st October 1940, the gunner was W.S. "Sticks" Gregory (later Wing Commander) flying with Plt Off Rhodes. - Details can be found in Chapter 10 "Battle Day of an RAF Pilot" of "The Battle of Britain - The greatest battle in the history of air warfare" by various authors, this particular chapter by Richard Townshend Bickers. ISBN 1 85613 025 8.
⁴ For details of the problems with the .50 calibre gun on the Brewster Buffalo see the early chapters of "Buffaloes over Singapore" by Brian Cull, Paul Sortenhaug and Mark Haselden ISBN - 1-904010 32 6
⁵ The St Petersburgh Declaration does not actually specify 37mm as the minimum calibre for exploding shells but to confirm that at the time this was seen as the minimum calibre by the Air Ministry see chapter 5 (page 117) "the Quest for Fighter Firepower" in Colin Sinnott's book "The RAF and Aircraft Design 1923-39" ISBN 0-7146-5158-3
The books and website of Anthony G Williams at the link below.
The Website of Anthony G Williams, foremost writer on the subject of aircraft gun armament.
Vickers machine gun on the "Plane Encyclopaedia" website.
Vickers K machine gun on Wikipedia
Browning .303 on Aviation History website.
Browning .303 on Military Factory website
Browning .50 on Aviation History website
Browning .50 on Wikipedia
Hispano 20 mm Cannon on Wikipedia
Hispano 20mm Cannon on "The Spitfire Site".
COW gun on Wikipedia
COW gun on Perth Flying boat (YouTube)
Tsetse Mosquito with 57mm Molins gun (YouTube)