GUNS OF THE FLEET AIR ARM

Anthony G Williams

Last amended August 2014

In the long history of the Fleet Air Arm many different aircraft have been adopted for service, but the guns carried by them have seen far fewer changes. At the time that the Fleet Air Arm was formed in 1924, fighter armament had become standardised around two rifle calibre machine guns (almost invariably .303" - 7.7 x 56R - Vickers) mounted in the fuselage so that they were readily accessible to the pilot in case any jams had to be cleared. This meant that they had to have their rate of fire synchronised with the engine revolutions so that they would fire through the propeller disc without shooting the blades off. Rear gunners were with equal uniformity given a single, flexibly mounted gun, usually the light .303" Lewis with ammunition contained in a 47 or 94 round drum, easier to handle than the long ammunition belts of the heavy Vickers.

The two guns were both of pre-Great War vintage and used different mechanisms. The Vickers was recoil-operated, relying on the rearward push of the firing cartridge to operate the mechanism in order to eject the spent cartridge case, pull forward the ammunition belt to put the next round in line with the chamber, push the round into the chamber and cock the mainspring ready to fire. The Lewis was gas-operated, tapping high-pressure gas from a point along the barrel as the gun fired in order to operate the ejection/reloading/cocking cycle.

Aircraft development for most of the interwar years was very slow and incremental and their armament changed even less. First World War pilots would instantly have recognised the twin-gun biplanes in service until the late 1930s. From the mid 1930s, however, the introduction of fast, sturdy monoplanes called for heavier firepower so the traditional armament had to be replaced.

The Second World War

There are two possible ways of increasing firepower; to increase the rate of fire (by adding more or faster-firing guns) or to increase the destructive power of each projectile by moving to a larger calibre weapon. The latter option has the disadvantage of reducing the number of guns that can be carried because the guns are much larger and heavier. They also tend to be slower-firing, so the chance of actually hitting the target is reduced.

In the mid-1930s when the new monoplane warplanes were being planned, aircraft were rarely if ever fitted with armour so the greater power of large-calibre guns did not provide an obvious advantage. Comparative testing in the UK between .303" and both Vickers and Browning .5" (12.7 mm) calibre weapons led to the conclusion that a larger number of faster-firing .3O3"s would be more destructive than the .5"s, so the smaller calibre was retained, although the .5" Vickers was adopted by the Royal Navy In a slightly different form as a light anti-aircraft gun. The traditional Vickers + Lewis combination was however replaced.

The larger number of guns required could not be mounted in the fuselage but had to be fitted in the wings, which meant they had to be much more reliable. Furthermore, neither Vickers nor Lewis could match the rate of fire of more modern designs. The standard belt-fed weapon became the recoil-operated American Browning adapted to .303" calibre (although during the War some saw service in the original .300" calibre of the American .30-06 cartridge), with the gas-operated drum-fed Vickers GO (or K) gun replacing the Lewis as a defensive weapon. There was one intermediate stage before the monoplane fighter era took over; in early 1939, the Gloster Sea Gladiator entered service as the FAA's new fighter, replacing the 1932-era Hawker Nimrod. Although still a biplane, it did at least feature four Browning guns.

The FAA's first attempts at monoplane fighters were cautious ones. They were convinced of the necessity to provide a second crew member and the aircraft they specified were inevitably larger and heavier than single seat fighters. Furthermore, the first to see service, the Blackburn Skua, was required to act as a dive-bomber as well, and was slower than the best of the biplanes. However, its four wing-mounted Brownings enabled it to be the first British naval aircraft of the War to shoot down an enemy aircraft; a Dornier Do 18, on 25th September 1939. The related Blackburn Roc, burdened by a four-gun turret, was even slower and only saw brief, land-based service. Fortunately for the FAA help was on the way, first in June 1940 in the form of the Fairey Fulmar; still with two seats but with a Merlin engine to give much better performance and equipped with eight Browning guns. This was rapidly followed by the Grumman Wildcat (initially known as the Martlet in British service) in September of the same year.

The Wildcat was an excellent fighter approximately equivalent to the RAF's Hurricane, but is particularly interesting in this context as it introduced the American .50" (12.7 x 99) Browning heavy machine gun into FAA service. This was one of the classic air-fighting guns which armed US aircraft of the period almost to the exclusion of anything else. The bullets were four times the weight of the .303's and the muzzle velocity was higher. The extra weight and slower rate of fire of the big gun were more than compensated for by its range and penetrative power, which was easily able to defeat the armour then being fitted to many combat aircraft as protection against rifle-calibre weapons. Initial deliveries of the Wildcat were fitted with four guns but six became standard equipment later on, as it was for the Grumman Hellcat which supplemented the Wildcat in FAA service from July 1943.

The RAF made very limited use of the heavy machine gun and went straight to the 20 mm cannon as a .303" replacement, choosing the high-velocity Hispano-Suiza HS 404 in 20 x 110 calibre. This gun first joined the FAA aboard the Sea Hurricane in 1941, the Mk IC and IIC versions carrying four of them. The next cannon-armed carrier fighters were the Seafire, operational in the autumn of 1942, and the two-seat Fairey Firefly which first went into action in 1944. The Firefly and later marks of Seafire had four cannon; earlier Seafires had a mixture of two cannon and four .303"s (or sometimes two .50"s).

The 20 mm Hispano was a formidable weapon in its day, combining high muzzle velocity with long range, excellent armour penetration and explosive shells. It was therefore carried forward (in the lighter, faster firing Mk 5 form) into the jet aircraft era as the RAF's and FAA's standard gun.

Postwar Developments

The 20 mm Hispano armed all FAA fighters until 1958 when the Supermarine Scimitar entered service. This was armed with four of the 30 mm Aden cannon which had first seen RAF service four years earlier in the Hawker Hunter. The gun had originated as a German design towards the end of the Second World War, using a radical new principle of operation in order to speed up the rate of fire. With conventional large cannon, the firing rate is limited by the time it takes for the breechblock to be driven rearwards to extract the fired case and then travel forward again to load the next round. The Aden and similar guns have a revolving cylinder behind the barrel containing several chambers. Ammunition is chambered, fired and ejected in gradual stages as the cylinder revolves, enabling the rate of fire to be doubled.

This "revolver" cannon should not be confused with the "rotary" cannon which is similar in operation but has a barrel attached to each cylinder, the whole contraption spinning round as it fires. This makes for a very bulky gun but barrel cooling is much less of a problem so the rate of fire can be considerably higher; up to 100 rounds per second. The six-barrel 20 mm M61 Vulcan rotary cannon has been the standard USAF aircraft gun since the late 1950s and is now in RN service as the business end of the Phalanx anti-missile system. The similar Goalkeeper fitted to the Invincible class carriers and the Type 22 Batch 3 frigates uses the GAU-8/A seven-barrel rotary cannon, firing far more powerful 30 mm ammunition, but this is now leaving RN service.

At only about 800 m/sec, the 30 x 113B Aden has a slightly lower muzzle velocity than the Hispano which might seem a retrograde step, but the British had been very impressed by the destructive effect of the low-velocity 30 mm MK 108 used by the Luftwaffe to knock down Allied bombers, and felt that the sacrifice of muzzle velocity in favour of shell size was worth making.

The FAA has so far failed to adopt three aircraft cannon selected for the RAF; the 20 mm Vulcan (used in the RAF's McDonnell Phantom), the 27 mm Mauser (in the Panavia Tornado) and the 25 mm Aden (intended for, but as it turned out not fitted to, the Harrier GR7/9). The first of these is a rotary cannon, the other two are revolvers. The Navy's Phantoms came without any cannon (as did their predecessor, the De Havilland Sea Vixen), although they could be fitted to accept a gun pod. While this restraint has meant that the FAA has avoided the RAF's problem of fielding three different aircraft cannon simultaneously, the Navy supply service has had little to cheer about, with two other 30 mm calibres (for the Goalkeeper and the Oerlikon KCB, which use similar but not interchangeable cartridges) and three different types of 20 mm ammunition; the original WW2-vintage 20 x 110RB Oerlikon (in service into the 1990s), the modern manually-aimed Oerlikon KAA in 20 x 128 which replaced it, and the Phalanx in 20 x 102!

After a pause of some ten years following the withdrawal of the Scimitar in 1970, the 30 mm Aden returned to FAA service in the Sea Harrier. Two guns were carried, in conformal gunpods fitted under the fuselage. This was a logical choice as the main justification for the Sea Harrier was to "see off" the huge Soviet Tu 142 Bear reconnaissance planes, although as it happens the only air combat they saw was against Argentinian fighter-bombers, helicopters and transports in 1982. Against small and agile targets the Aden's low velocity is a distinct handicap which explains the RAF's choice of a 25 mm version of the Aden, whose 25 x 137 ammunition fires a lighter shell but at a muzzle velocity of 1,100 m/sec and with a 50% higher rate of fire, for the Harrier GR7. Unfortunately this project turned out to be too ambitious and the gun was cancelled in 1999. In the early 2000s a decision was taken to discontinue the use of the Sea Harrier for cost-effectiveness reasons, and the last of them left FAA service in April 2006. The RAF's Harriers were taken out of service in 2010 and all sold to the USMC for spare parts.

The Future

The future of the FAA's aircraft is tied in with the acquisition of the RN's new aircraft carrier(s). They will be far larger and more capable than the old Invincible class carriers, and could carry a lot more aircraft. The American F-35B STOVL plane has been chosen after some wobbling over whether or not to reconfigure the carrier(s) for CTOL operations. In terms of gun armament, the F-35B would bring with it the possible use of GD's GAU-12/A 25 mm four-barrel rotary cannon in 25 x 137 calibre, as an optional extra in a conformal gunpod (only the CTOL F-35A has an internal installation). On current plans the F-35 force will be a joint operation between the RAF and the RN (as with Joint Force Harrier in the last few years of the plane's existence.

 

 FLEET AIR ARM GUNS AND CARTRIDGES

 

 Weight Kg

 Rounds Per Sec

 Bullet Wt Gm

 Velocity m/sec

 .303" Vickers

 11

 14

 11

 730

 .303" Lewis

 8

 15

 11

 730

 .303" Vickers K

 9

 17

 11

 730

 .303" Browning

 10

 19

 11

 730

 .50" Browning

 29

 13

 45

 850

 20 mm Hispano Mk l

 50

 11

 130

 880

 20 mm Hispano Mk 5

 42

 12

 130

 820

 30 mm Aden

 87

 20

 270

 800

NB: Special thanks are due to the staff of the Ministry of Defence Pattern Room, ROF Nottingham, for their assistance.

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