AIRCRAFf ARMAMENT

The Second World War aircraft gun controversyWhich was themost effective form of combat-aircra ft armament duringWorld War Two?Abatteryof

small-calibremachine-gunscreating ahail of bul lets? or fewer but heavier cannon,perhaps capableof deliveringadecisive blow with each shell? ANTHONY G. WILLIAMS weighs upthe pros and cons

hub. An initial experiment w ith a prototypeHeinkel He I 12 in the Spanish Civil War showedthat this installation was unsatisfactory for aer-ial combat, though effective in ground attack.

Germany then went to the opposite extremeand selected the small, low-veloci ty Swiss Oer-likon FF 20mm cannon for development. Amodified version, the MG-FF, was put into pro-duction by Ikaria Werke Berlin, and enteredLuftwaffe service . Initial attempts to fi t this asan engine gun ran into reliability problems, sothe Messerschmitt Bf 109E-3 of 1939 carriedtwo MG-FFs in the w ings and a pair of 792mmMG I 7 guns synchronised to fire through thepropeller disc. However, many Bf 109s were stillarmed with four RCMGs at the start of the war.

The Battle of France caught the French withfew fighters capable of dealing with the Bf 109s.Most French fighters carried a mixture of 20mmcannon and RCMGs; the best was the Dewoi-tine D.520 w ith its four wing-mounted 75mmMAC34 M39 MGs and the powerful, fast-firingHispano-Suiza HS.404 moteur canon. The USA-built Curtiss Hawk H-75A, with four or six 75mmFN-Browning guns, wa s too lightly armed.

One lesson of early fighting was that the RAFfighters' battery of RCMGs was less effectivethan expected. Al though the eight guns be-tween them fired no fewer than 160 rounds persecond (r.p.s.) . they were initially adjusted toconcentrate their fire at the long range of 400yd,which led to the bullet strikes being spreadacross the target at shorter ranges. As self-sealing fuel tanks and armour became muchmore common during 1940, it proved necessaryto concentrate fire at much closer ranges.

The problem was that the small bullets fi redby the RCMGs could not carry enough incendi-ary or explosive to guarantee success, and alsohad insufficient penetration to defeat armourreliably. A series of ground tests carried out bythe British, firing at a redundant Bristol

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EARLY FIGHTINGIn 1939 the RAF's new Spitfire and Hurricanefighters were remarkable for their heavy batteryof eight wing-mounted 0303in (7'7mm) RCMGs;tw ice as powerful as that of any other majorfighter. In con trast, Germany's Luftwaffe hadbeen watching the French use of engine-mountedcannon. In 1932 this resulted in a requirementfor a fighter wi th two RCMGs or a single 20mmcannon. The weapon considered was the large,very powerful but slow -fi ring Rheinmetall -Borsig MG C/30L, firing through the propeller

HE SECOND WORLD WAR stimulated animportant advance in aircraft gun arma-ment. The fighters of most combatantnations began the war w ith a few rifle-

calibre machine-guns (RCMGs) of 7'5- 8mm cal-ibre, but, by the end of the conflict, cannon of20mm or greater calibre were standard. Theone exception was the USA, which reliedoverwhelmingly on the 127mm (0'50in) M2Browning heavy machine-gun (HMG) . Studentsof armament history continue to argue: why didthis happen, and were the Americans right?

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ABOVEThe Oewoitine0.520, with its four 75mmwing-mounted machineguns and a fast-firingHispano-Suiza moteur-canon, was the best fighterin the French inventoryatthe outbreak of war.

OPPOSITE,TOP Armourersreplenishthe magazines ofthe two nose-mounted050in and four wing-mounted 030in machine-guns of a Bell Airacobra of601 Sqn, RAF. The barrel ofthe 20mm cannon, firingthroughthe propeller hub,

IS conspicuous.

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ABOVE The MesserschmittBf 109E-4 's modified MG-FFM gun could fire Minen-geschoss ammunition.

BELOW Even the 12 O303inBrowning machine-guns ofthe Hurricane II oftenlacked sufficient pun ch todo decisive damage.

and Hurricanes would have had far more effecton the Luftwaffe's bombers had they been armedwith four MG-FFM cannon, which would haveweighed little more than their battery of RCMGs.

THE SEARCH FOR BIGGER GUNSThe RAF had realised years before the war thatthe 0303in MG might become inadequate as aprimary aircraft gun, given the steady increasein speed, strength and toughness ofaircraft. Theyconsidered, but rejected, O'SOin (12'7mm) gunsas giving insufficient advantage over the 0303in .Instead, they sought a good 20mm cannon,which they thought would be far more effectivedue to its explosive ammunition, and found onein the new French Hispano-Suiza HSA04. Thiswas selected in the late I 930s, and specifica-tions were written for a new fighter to carry it(the Westland Whirlwind), but problems in set-ting up UK factories meant that only a few gunswere available for the Battle of Britain .

To save time the Hispano was fitted to existingsingle-engined fighters for testing, and a squad-ron of Spitfire IBs saw service during the Battle.However, the gun was designed to be mounted

Blenheim from 200yd to the rear wi th variousBritish and German guns and ammunition,revealed that the 0303in incendiary B Mk VIbullets would set light to a Blenheim wing tankwith only one hit in five; during the Battle onlyone or tw o of the eight guns were normallyloaded wi th this ammunition. Other 0303inand 792mm incendiaries were only half aseffective. Similarly, while both German andBritish steel-cored armour-piercing (AP) roundscould penetrate up to 12mm of armour plate iffired directly at it from 200yd, most of the bul-lets were deflected or tumbled by first passingthrough the fuselage skin or structure. In con-sequence, only a quarter to a third of the bulletsreached the Blenheim 's 4mm-thick armourplate at all , and very few penetrated it.

The Luftwaffe relied more on their 20mm MG-FF cannon, which had some interesting advan-tages and disadvantages. The guns were morethan twice as heavy as the 792mm MG 17 andfired only half as fast, about 9 r.p.s. The muzzlevelocity was also low er, giving a di fferent tra-jectory and time-to-target. Moreover, the 60-round capacity of its drum magazine limited thew eapon to only about 7sec of firing. However,the big shells weighed about 12 times as muchas the RCMG bullets, and their high-explosivefilling caused blast damage, as well as throwinghigh-velocity shell fragments around.

In May 1940 the Bf I 09E-4 entered service withthe modified MG-FFM gun, adapted to fire thenew Minengeschoss (mine shell) ammunitionwhich had very thin walls and a doubled capac-ity for high-explosive (HE) and/ or incendiary.As a bonus the shells were lighter, permitting ahigher muzzle velocity. The theory was that solidbullets or AP cannon shells relied on hitting therelatively small vital areas to have an effect,whereas an HE shell could cause serious dam-age regardless of where it struck . There is nodoubt that this theory was correct: the Spitfires

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AIRCRAFT ARMAMENT

"Throughout the Hispano cannon's life, fine-tuning of insta IIABOVE AWestlandWhirlwind has its fournose-mounted 20mmHispano cannon serviced.The drum-shaped objectsare the belt feeds.ABOVE RIGHT,TOP Reloadingthe nose MG 1S1/20 of aJGS4 Bf 109F. Ease of accesswas a great asset.ABOVE RIGHT, BOTTOM Fourwingmounted 20mmHispano cannon made theHurricane IIC a formidableopponent.

BELOW The Hispano cannoninstallation in a Spitfirewing. The change fromdrum magazine to belt feedgreatly increasedammunition capacity.

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on a massive engine block, and did not like beinginstalled in a far more flexible wing. Moreover,the initia l installation had the guns on their sidesto bury the big drum magazines in the w ings,and the Hispano did not like this at all. Theresult was such appall ing unreliability, the gunsfrequently jamming after only one shot, that thesquadron requested its old aeroplanes back.

Throughout the Hispano's li fe, fine-tuning ofinstallations was required to make it reliable inw ing mountings. Guns mounted in the muchmore rigid fuselages of such aircraft as the Whirl-wind, Beaufighter and Mosquito caused far fewerproblems. Once working properly, the big can-non was a formidable weapon. compared w iththe MG-FF it was slightly faster-firing at 10 r.p.s.,and its much bigger cartridge cases generated aconsiderably higher muzzle veloci ty, w hichimproved both hit probability and penetration .While the Allies never introduced the Minen-geschoss technology, so their shells only car-ried half the HE/ Incendiary (HEI) load, the heavyHispano shells ' better penetration meant theycould inflict considerable damage. The Hispanosoon changed from a eo-round drum magazineto a belt feed giving much greater ammunitioncapacity (which the MG-FF never had, other thanin an obscure nightfighter installation). The pen-alty was that the Hispano was almost twice aslong and double the weight of the German gun;unwelcome features for Wing-mounted weapons.

RAF fighters carried a mixture of HE or HEIand plain steel "ball" (practice) rounds (whichhad good penetration) until the emergence ofthe semi-armour piercing incendiary (SAPI) in1942, after which the standard loading was50/ 50 HEI and SAPI. The German cannon werenot exclusively loaded w ith Minengeschoss, butused them mixed with older-type HEI-T shells(retained because, unlike the M-Geschoss, theycould carry a tracer) and later some API roundsas well, in varying proportions.

The Germans were not satisfied with the MG-

FFM, which had been adopted as an interimmeasure pending a purpose-designed cannon.This duly emerged as the Mauser MG 151 , w hichgradually took over from 1941 . Ini tially, the Mau-ser had been designed to use a high-velocity15mm cartridge, but it saw relatively little serv-ice in this form. Wartime experience led to thecartridge case being modified to accept the20mm shells from the MG-FFM , surrenderingmuzzle velocity and penetration in the interestsof far greater destructive effect. The 15mm ver-sion was available with HE shells, but they wereconsidered too small. The resulting MG 151 / 20was intermediate in size, weight and muzzleveloci ty between the MG-FFM and the Hispano,but was faster-fi ring at 12 r.p .s. It was a superbdesign w hich the Americans tried to copy, pro-ducing some 300 guns in 060in (15mm) calibre,designated T I 7, but they never adopted it.

Later in the war, various new German gunsemerged w ith cal ibres of up to 50mm. The mostsigni ficant ones were the Rheinmetall -BorsigMG 131 , MK 103 and MK 108. The MG 131 wasa 13m m HMG intended to replace the RCMGs inboth fixed and movable mountings, and there-fore kept as small and light as possible, makingit the least powerful HMG to see service. Theother guns were both in 30mm calibre but oth-erwise very di fferent. The MK 103 was a huge,high-velocity slow- firing gun, while the MK 108fired much smaller, low-velocity cartridges at avery creditable 10 r.p .s., and was only hal f thesize and weight. Their use was really promptedby the difficulty the Luftwaffe was experiencingin shooting down USAAF B- 17 bombers. Their30mm M-Geschoss HEI shells were highlyeffective, containing four times as much HE asthe 20mm version, and three or four hi ts withthese could bring down a bomber, comparedw ith 20 hits with 20mm ammunition.

In 1941 the USSR was drawn into the war. Aswell as the very-fast-firing (30 r.p.s.) 762mmShKAS, it had in service an excellent HMG, the

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llations was required to make it reliable in wing mountings"

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127mm Universal Berezin, and a 20mm cannon,the ShVAK. The ShVAK had actually started lifeas a 127mm gun. It weighed the same as theMG 151 / 20 and was slightly faster-firing, butthe ammunition was less effective as the shellshad only a small HE capacity. Towards the endof the war the Berezin was also altered to firethe same 20mm ammunition as the ShVAK; at2Skg the resulting B-20 was the lightest gun ofthis calibre to see service. The Soviet prefer-ence was to install all guns in the fuselage oftheir little fighters rather than in the wings, toconcentrate firepower and make the aircraftmore agile. Because this limited the number ofguns, a relatively small number of aircraft wasequipped w ith larger guns as engine cannon,including the powerful 23mm VYa-23 and 37mmNS-37. Because of the weight, recoil and rela-tively low rate of fire of these weapons, thesewere deemed suitable for expert pilots only.

ENTER THE AMERICANS - &THE JAPANESEThe end of 1941 also saw America and Japanenter the wa r. Their aircraft weapons were verydifferent. America relied almost entirely on theBrowning machine-gun, not just in OSOin(12'7mm) calibre, but, for the first couple of years,in 030in (762mm) as well. The Japanese armyand navy air forces followed their own paths, thearmy steadily up-gunning from 77mm through127mm machine-guns and later 20mm can-non, with 30mm and even larger guns seeinglimited service by the end of the war. The navystarted with a mixture of 77mm MGs and Type99 20mm cannon and continued to rely on thiscannon, although it did make some use ofHMGs and was also introducing 30mm guns bythe end of the wa r. As in Germany, the destruc-tion of American heavy bombers was a strongincentive for the development of weapons of30mm or larger calibre.

The Americans did not intend to make such acommitment to the Browning MGs. Both before

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and during the war considerable efforts weremade to secure alterna tive aircraft guns. Tria lsof foreign equipment resulted in the selectionof the 20mm Hispano-Suiza HS.404, large num-bers being made. The Browning-designed 37mmM4 cannon was also introduced, although usedalmost exclusively by the Bell P-39 and P-63.However, the standard fighter armament be-came a battery of six O'SOin Browning M2 HMGs.

This has led to the often-expressed view thatthe O'SOin M2 was the best all-round fightergun of the war. After all, the USAAF and USNavy fighters unquestionably came to dominatethe skies in which they fought. If there had beena better gun, America would have used it.However, the truth is not quite as simple asthat. There are two issues here; how good wasthe O'SOin M2 compared w ith other HMGs, andhow effective was it compared w ith cannon?

The most obvious comparator was the Sovi etUB, which fired ammunition of virtually identi -cal power. The UB weighed 2Skg, compared w iththe M2's 29kg, but the Soviet gun fired at 175r.p .s., compared with around 13 for the M2. Interms of power-to-weight ratio the Berez in wastherefore clearly superior. The Browning faredbetter against most other HMGs, as they all firedless-powerful ammunition, so the M2 enjoyedadvantages in range and penetration. However,it had a slightly lower rate of fire than the Ger-man and Japanese guns (both around I S r.p .s.),and was also bigger and heavier. The mostpowerful of all of the HMGs was the I Smm MG151, but this was heavier and slower- firing thanthe M2. Overall, therefore, the O'SOin M2 wasnot the best of the HMGs but was about aver-age, w ith reasonable performance for its weight.

How did HMGs compare w ith 20mm cannon?The first problem is that the cannon variedhugely in size, weight and performance. The MG-FFM, Type 99- 1 and B-20 were all lighter thanthe M2, but the first two were significantly worsein terms of muzzle velocity and rate of fire, al-

ABOVE Ade HavillandMosquito Mk VI fighter-bomber provides a graphicillustration of its firepowerduring a night-time firingof the four 20mm cannonand four O303in machine-guns installed in its nose.This was the most widelyused of all the Mosquitofighter variants.

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AIRCRAFT ARMAMENT

RIGHT AFocke-Wulf Fw190A-S/Ull shows off itsunderslung 30mmRheinmetall-Borsig MK 103slow-firing gun.FAR RIGHT The Soviet Union'sYakovlev Yak-9 was armedwith a 20mm ShVAK enginecannon and a single127mm UBS heavymachine-gun.

ABOVE The French Hispano-Suiza HS.404 cannon, witha 60-round drum magazine.

though the B-20 matched the M2's rate of fireand was not far behind in velocity. The Japa-nese Ho-5, Type 99-2 , ShVAK and MG 151 / 20were all somewhat heavier. Muzzle velocitiesand rates of fire were closer to the M2's but gen-erally sti ll not as good. The Hispano was signif-ica ntly heavi er and slower- fi ring until theBritish Mk V emerged near the war's end, mat-ching the MG 151120 in weight and rate of fire.

The foregoing compares only the guns' effi -ciency; it takes no account of ammunition, thearea in which the HMG loses most ground. The20mm cannon shells were not only two to threetimes heavier than HMG bullets, but their HElcontents greatly increased their effectiveness.Although HE ammunition was available formost HMGs, their small bullets severely limitedthe quantity of chemicals carried, so the Ameri -cans decided not to use them. Initially, the M2used a mix of incendiary and AP bullets, w ithsome tracers, but in 1944 the MS API began totake over. Rather curiously, this was based onthe Soviet B.32 API used in the Berezin.

It is di ffi cult to assess the effectiveness of dif-ferent ammunition types, but various tests sug-gest that a typical World War TWo-era HE orincendiary shell, w ith chemical contents form-ing about 10 per cent of total shell weight, wasabout twice as destructive as a plain steel shellof similar size and weight. This makes it clearthat 20mm cannon were considerably more de-structive for a given total weight of armamentthan any HMG could hope to be. For example,

the 050in MS API contained less than one gramof incendiary, whereas the 20mm Hispano SAPIachieved similar armour penetration but car-ried more than ten times as much incendiary.

This advantage was recognised by the US Navy.After comparing the 050in M2 and the 20mmHispano they estimated that the cannon wasthree times as effective. In other words, the typ-ical RAF armament of four 20mm cannon wastwice as destructive as the USAAF's six 050inHMGs, for very little more weight. Proponentsof the Browning HMG point to its excellent bal-listics, which enhanced its range and hit proba -biit y. But the Hispano's muzzle velocity wasvery similar, and although the blunt-nosed shellswere less aerodynamic the difference overtypical air-combat ranges was not significant.

Data on the principal aircraft guns is shownin the table below. Of the RCMGs, only the0303in Browning has been included, most ofthe others being very similar in effectiveness.

The cannon's advantages are clearly shownin the decisions made as a result of combatexperience by air forces w ith a choice of goodHMGs and cannon. We have already seen howGermany preferred the 20mm version of the MGl SI despite its poorer ballistics. In the 127mmBerezin the Soviets had arguably the best HMGof the war, but they still preferred the heavier,slower-firing 20mm ShVAK. Japan had severalgood HMGs available; the army's Ho- I 03, andthe navy's 13mm Type 3, a 050in Browningchambered for slightly larger-cal ibre arnrnuni-

BElOW Three sectioned 20 x80mm cartridges for theGerman MG-FFM, showingthe HE-T (left) and thehigh-capacity M-Geschossshell (centre). On the rightis an API.

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Principal aircraft guns compared .Gun Calibre Bullet Muzzle Rate of Weight of Destruct. Gun Gun NB 1. The "Destructive Effect"

Weight g Vel m/sec Fire r.p.s. Fire Kg/sec Effect Weight Efficiency column produces an approx-imate indication of the fire-

0303in 7'7x56 11 750 20 022 20 10 20 power rating of the gun,MG 131 13x64 34 750 15 051 48 17 28 calculated by multiplying fourHo 103 12-7x81 33 770 15 050 45 23 20 factors: projectile weight,127 Breda 12'7x81 33 770 12 040 36 29 12 muzzle velocity, rate of fire,0'50in M2 12'7x99 43 890 13 056 60 29 21 and a multiplier for the127 Berezin 127xl08 48 840 17 082 97 25 39 average percentagecontentMG 151 15x96 57 960 12 068 94 42 22 of incendiary or HE materialType 99-1 20x72 128 600 8 102 108 24 45 (5% content =xt -s,MG-FFM 20x80 92 700 9 083 126 28 45 10% =x2, 20%=x3).MG 151120 20x82 92 800 12 11 0 192 42 46 2. The "Gun Efficiency"Ho 5 20x94 79 730 14 110 154 37 42 column is calculated byShVAK 20x99 95 790 13 123 143 42 34 dividing the "DestructiveType 99-2 20xl01 128 750 8 102 120 35 34 Effect" by the "Gun Weight".Hispano II 20xll0 130 860 10 13 200 50 40 3. Guns which wereHispano V 20xl10 130 830 125 1-62 250 42 60 synchronised to fire throughVYa-23 23x152 200 900 9 180 234 68 34 the propeller disc would haveMK 108 30x90 330 505 10 33 580 60 97 their rates of fire reduced byMK 103 30x184 330 860 7 23 693 141 49 between 10% and 40% from37mm M4 37x145 608 610 25 152 160 96 1-7 those shown

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AIRCRAFT ARMAMENT

tion, but they made increasing use of cannon.So why did the Americans not make more use

of cannon, speci fically the 20mm Hispano theyalready had in mass production? There weretwo main reasons. One was certainly that theM2 was adequate for its purpose. In WesternEurope the main adversaries were fighters,which were much easier to damage and shootdown than bombers. In the Pacific Theatre theJapanese aircraft were ini tially poorly protectedand easy to shoot down. Later Japanese aircraftwere better protected, but again these wereusually fighters. If the Americans had faced theneed to stop raids by heavy, well-protectedbombers, it is likely that the HMG's shortcom-ings would have been starkly revealed.

There was another reason, however, whichexplains why the US Navy, despite rating thecannon very highly and facing the need to dealwith attacking bombers and kamikazes, fitted itto few aircraft. That was serious production prob-lems with the American Hispano, which gave i ta reputation for unreliability. Despite produc-tion running well into six figures, the AmericanHispano failed to achieve an acceptable relia-bil ity standard for the duration of the war.

To return to the original question, were theAmericans right to rely so heavily on the 050inM2 when all other combatant nations had aclear preference for cannon of at least 20mmcalibre? The answer has to be yes. It was ade-quate for its purpose, and was the only satis-factory aircraft gun in production in the USA. It

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was very reliable (except where the installa-tions created problems), was made in hugequantities, and the simplification of supply bycomparison with the diversity of weapons usedby the Axis powers gave a major logistical advan-tage. However, the Americans could get awaywith using a weapon so deficient in destructivepower not only because of the nature of theiropposition, but also because the size and enginepower of their fighters enabled them to carry abattery of at least six guns, thus making up inquantity what they lacked in destructive quality.

The advantages of the 20mm Hispano M2 werenot entirely ignored. It was carried by LockheedP-38s, together w ith four 050in Brownings. Itwas also installed in nightfighters, which need-ed maximum firepower to convert a short firingopportunity into a kill. Four were installed inNorthrop's P-61 Black Widow, and two could bemounted in a Grumman F6F-5N. The cannon'sextra fi repower was also appreciated forground strafing.

After the war the US Navy quickly changedover to the 20mm cannon in its improved.faster-firing and more reliable M3 form, butthe USAF stayed with the 050 M3 until thefighting in Korea demonstrated once and forall that the HMG had had i ts day. From themid- 1950s the USAF at last replaced the oldBrowning with 20mm cannon, initially the M39revolver and then the M61 rotary - just asmost of the rest of the world was moving Dup to 30mm! ~

LEFT Acomparative photoof the ammunition for theprincipal WW2 fighterguns. From left to right:British 0303in (NxS6mm- representative of allRCMG cartridges); Ameri-can O'SOin M2 (l 207x99);French/British/American20mm Hispano (20x11 0);Soviet 1207mm UB(1207xl08) and 20mmShVAK (20x99); German13mm MG 131 (13x64),lSmm MG 151 (1Sx96),20mm MG-FFM (20x80),20mm MG 151/20 (20x82),30mm MK 108 (30x90) and30mm MK 103 (30x184);Italian/Japanese 127mmBreda-SAFATIH0-103(l207x81); Japanese 20mmType 99-1 (20x72),20mmType 99-2 (20xl0l) and20mm Ho-S (20x94).FAR LEFT The six O'SOinBrowning M2 heavymachine-guns for the NorthAmerican P-Sl Mustang.

Anthony Will iams is theco-author, with EmmanuelGustin, of Flying Guns:World War 2 - theDevelopment of AircraftGuns, Ammunition andInstallations 1933-1945,published by Airlife in 2003and distributed by TheCrowood Press. His webaddress is http://www.quar-ry.nildram.co.uk

BELOW Comparativedrawings of the principalSecond World War fighterguns, to approximately thesame scale.

"So why did the Americans not make more use of cannon, specifically the20mm Hispano already in production? There were two main reasons"

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