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fapangel · 7 years

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WWII planes question- which is better, machine guns or cannon? Followup, does this change based on what country you are and therefore available guns? Do some kinds of planes like one more than the other?

Oh boy, a good fighterplane question! This exact question has been the subject of intenseand furious debate both during the war years, and especially amongmodern simulation nerds and angry Wehraboos.

Aerial gunnery has a great number of factors toconsider - destructive potential against the target, exteriorballistics, (which affect accuracy and range,) weight of fire, ammosupply/duration of fire, reliability and redundancy, and especiallythe weight efficiency of the armament - destructive potential perpound, which affects how much performance you sacrifice to lug theguns around. I'll get into the technical discussion of all thosefactors and how they manifested in different guns/aircraft during thewar, but there's two big caveats to cover first.

First - one must remember that to a significantdegree, questions of destructive power, accuracy, ballistics, et al.were a moot point, as most pilots didn't have the skill to reliablyland deflection shots (or snapshots, for that matter,) and weretrained to fill their windscreen with the enemy before firing. Fromdead astern at 50 yards, it really doesn't matter whatyou are shooting. There's outliers - such as Japanese Zeros with twin7.7mm pecking away at Wildcats - but those were the exception.

Second -for all the obsessive sperging various internet autists have lavishedon the topic, actual combat experience reminds us that weaponry mustbe matched to the target. On lend-lease.ru, one of the most valuableresources I've ever found on the internet, Soviet fighter pilot N. G.Golodnikov (who flew I-16s,Hurricanes, P-40s and P-39s during the war) saysbluntly that they quickly removed the .30 caliber guns from the wingsof their (early-model) lend-lease P-40s to save weight, as theyfound the twin Browning .50caliber machine guns in the cowling to be more than sufficient!Against the light, mostly unarmored Luftwaffe fighters they werecontending with - and the ranges they were shooting at - twin .50swere more than enough. (Theywould repeat this with their lend-lease P-39s.)

Now, for thetechnical discussion. Machineguns/autocannons have to make tradeoffs between several categories ofperformance - weight of shell fired, muzzle velocity, rate of fire,and gun weight. For instance, compare the famous 20mm Japanese Type99 Mark I - an incredibly weight-efficient weapon at only 51 pounds(the American aircraft version of the Browning, the AN/M2, weighedmoreat 61 pounds!!) It paid for this, however, witha terrible muzzle velocity of only 600m/s and sluggish rate-of-fire,only 520 rounds a minute - but the weight efficiency was crucialfor the power-challenged Zero, which kept it light enough thatmaneuverability more than compensated for the cannon's ballistics bysetting up point-blank shots. Compare that to the SovietShVAK 20mm cannon, which was similar to the Hispano or the MG/151-, a good rate of fire (800 rounds/minute, equal to the AN/M2,) anice, high muzzle velocity - but heavy, at a solid 40kg(88 pounds). The reasons for all this were fairly simple - to achievea higher muzzle velocity, the barrel and action had to contain moregas pressure from a more powerful cartridge (and possibly a longerbarrel), in turn requiring a stronger action/chamber, which meantthicker, which meantheavier. Likewise, anaction capable of cycling faster had to be stronger to survive theheavier battering it took, and the heavier the shells it was slamminginto the breech, the harder this was - so a higher ROF required moreweight as well. (This tended to synergize with better muzzlevelocity, as the action was already stronger for that reason aloneand the extra gas power available helped with loading - but you couldstill trade ROF for weight even then. The Japanese Type99 Mark 2 is a good demonstration of that; delivering a solid750m/s at only 34kg/75 pounds, but with an abysmal 480 rounds/minuteROF; worse than the original!)

Clearly, anyconceivable combination of tradeoffs could (and were) made, for alldifferent purposes. Both the Soviets and the Germans installed veryhigh-velocity, high-power, heavycannons on ground-attack aircraft (NS-37and BK-3,7respectively,) for tank-busting, as the only precision(non-cluster, non-napalm)direct-fire munition available for hitting hardened, mobile pointtargets was a cannon. (The famous GAU-8 of A-10 fame is the finallegacy of that dynamic - it didn't die till the PGM revolutionarrived!) And of course evenbigger cannons were used (57mm on the “Teste” Mosquito foranti-shipping work, the 75mms in B-25s, etc.) But these aren'tdebated over as they're clearly specialized weapons for specializedtasks that no other gun could accomplish.

Thus, the onlyapplication machine guns and cannons were really competing in wasfighter-bomber armament, and therefore that's where the debate did -and does - center. Since the monster guns mentioned above were simplytoo massive and heavy to be practical or useful on fighters, thislimits discussionto medium and heavy caliber machine guns versus20mm to 30mm cannons.

Again, we shouldremember that at close-range, bothends of this spectrum were perfectly lethal -asGolodnikov attests, rifle-caliber machine guns, with their impressivehigh rate of fire, could actually cut tails and wings right off enemyaircraft if you weren't stingy with ammo. Onthe far opposite end of the spectrum, he observed that the37mm M-4s horrid ballistics andROF didn'tmatter if you simply closed in close before firing. Ofcourse rifle-caliber machine guns had problems penetrating deep intobombers from the stern angle, big slow cannons were poor for takingsnapshots and both would've benefited greatly from the optionof effective long-range fire. Plus,these extremes of heavy cannon/light MG were usually mixed with heavyMG/light cannon to cover any weaknesses (Spitfires with .303s and20mm, Yak-9s with 12.7mms and 20mm cannon, etc.) Thoseexamples, in turn, show that the “balancing” armament was almostalways 12.7mm-13mm heavy machine guns, or 20mm high-velocity cannons- because both had high muzzle velocities, good rates of fire andwere flexibly potent against almost any target, ground or air.

In short, thewhole thing really boils down to 12.7mm HMGs versus 20mm cannons, andthat's where most of the debate has centered.Thisis especially because US forces explored moving to 20mm-primaryarmaments during the war, and because Wehraboos are eager to assertthe 20mm master race's ascendancyover .50 dumb yankee scum. Comparinghigh-velocity 20mms(Hispano/ShVAK/MG-151) versus 12.7-13mm, (MG-131, AN/M2,Berezin UB,) onefinds their external ballistics and ROF to be roughly similar; cannonvelocities ranging from 700-840m/s to machine-guns 750-890m/s, andROFsfrom 700-900 rounds/minute. Cannonshad far superior boom, of course, but also weighed as much as twomachine guns at least - so the argument, ultimately, comes to this:are cannons betteror worse than an equivalent weight of HMGs?

TheUsual Suspects passionately argue for the greater efficiency ofcannon with various metrics - often trying to quantify thedestructive power of the guns in some arbitrary fashion (total joulesof energy delivered over time and such) whichare further spun into firepower/weight coefficients like so.These analysiesare clean, neat, fully quantifiable, and have about as much bearingon reality as “muh stoppanpower” 9mm vs. .45 arguments do.They tell you nothing about howmunitions actually destroy a target. Aircraftare destroyed in one of two ways:

1.Score an effective hit against one of the three critical systems(engine, fuel tank, pilot.)2. Destroy theairframe's integrity.

Cannonsare obviously the most effective at #2, as their high-explosiveshells detonated on impact and did blast/frag damage to the immediatestructure. How many hits were required depended on the target and hitlocation, of course, and I couldn't find anything like authoritativeinformation, but my gut feeling is that, onaverage:

*four 20mm cannon strikes on a wing will probably do the job (more forbigger/tougher planes, less if they hit the wing root, which is alsowhere fuel tanks usually were,)

*two to the elevator(rendering the ship uncontrollable,)

*two to four to the empennage, toblow it off,

*one or two for a liquid-cooled in-line, two to four for a radial,

*oneinthe cockpit (the angles this is possible from depend on theaircraft's cockpit armoring,)

-and the fuselage... differs,sincethe fuselage aft of the pilot seat/radio was mostly empty space. Themore of it there is, the more hits required to make it crumple up.Plus,compared to the wing spars, the fuselage bears less structuralload in the air (which is why at least one B-17 camehome with a massive hole in its side just fine, but cracked in halfafter landing.) Asan example, Thunderbolt ace Robert S. Johnson's P-47 soaked up atleast24 20mm cannon shells from a Bf-109 sitting dead astern and hisship still came home (as he states in his autobiography, at leastthree detonated against the armor in the seat-back, meaning they flewthrough existing holes in the fuselage.) He also took fiveshells in the right wing, four in the left, two to the empennage(blowing away half his rudder,) andone shell that detonated in the cockpit, near his left hand. Soaccording to my cute list, Johnson should've been dead ten timesover, P-47 ruggedness or no. Thisalso demonstrates that dispersionmaters; hadthose five shells in his right wing all landed close together nearthe wing root...? (Alsonote that 20mm cannons couldanddid utilize AP or semi-AP ammo to go for the three critical systems,but this reduced their HE effect, and without the volume-of-firemachine guns enjoyed, getting hits in the right places was harder.For air-to-air they tended to stick with HE.)

Heavymachine guns, on the other hand, are king of kill option #1, as theywere often mounted in greater numbers, fired faster, and oftenbecause ofdispersion (from wing-mounted guns vibrating and/or convergenceissues,) they generally had far greater chances of landing a goldenBB somewhere important (like the pilot's skull.) This was enhanced bytheir armor-piercing ability - since they carried no fuzed explosive,they'd pop right through a fighter's aluminum skin and keep goingtill they met something substantial - like afuel tank, or thepilot. And they were typically AP-I (armor-piercing incendiary) toensure they'd do something nasty once they got there. Agood concentrated burst to a wing root or tail could indeed do enoughstructural damage to destroy the aircraft, but it was mucheasier to just go for the fuel tanks, engine, or pilot himself.

Thisall was exacerbated by the other tradeoffs cannons and machine gunsbrought - cannons are far more volumeefficient,which was vital for aircraft like the Bf-109 (you don't reallyrealize how small a Bf-109 was till you meet one in a museum.)Especially for the 109 and its thin wings, mountings were limited tothe propeller hub, the cowling, and the wing roots, almost mandatingcannon to achieve sufficient firepower. Incidentally, it meant thattheir whole armament had zero or near-zero convergence (same for theFW-190,) which allowed exploitation of the gun's full range (againstbombers) and with concentrating damage on target for good effects.(Evenplanes with a choice of mountings, like the Spit and Hurricane,mounted cannons in the wing roots for that reason.) Machine guns,naturally, were limited to two 12.7/13mms in the cowling at best, andthe rest in the wings, where greater vibration and the necessity ofconvergence dispersed their pattern a lot more. (Pre/Early war USaircraft had two .50s in the cowling and the .30 cals in the wings,before the six-fifties armament debuted.)

Thenconsider the targetsthey were shooting at - Americans were trying to nail fast, nimblelittle German fighters that had modest to no armoring, and Germanswere shooting at lumbering bombers and... well, fuckingP-47s.The Germans, twiceover, had literally no other choice for armament but cannons.

Whichbrings us back to the Wehraboos again and their favorite pasttime -bagging on the US Army Air Corps for not going all-20mm. Theyloveto give the Browning AN/M2 shit, because it was relatively heavy27.6kg(61lbs)and had a relatively low rate-of-fire (750-850 rounds a minute.) Thatdoesn't compare well to the best aircraft-mounted heavy machine gunof the war, the Russian BerezinUB, at 21.5kg (47.4 lbs,) with a ROF of800for prop-synchronized guns and up to 1050 for wing-mounts.It even had good muzzle velocity. (TheGerman MG131 weighed a mere 37 pounds and fired at a hefty 900 rounds aminute, but paid for it dearlywith a horridvelocity fo 700m/s - this was mostly because the gun had to be tinyto fit in the Bf-109s cowling.) Soyou get a lot of shitposting about how four Hispano-Suiza cannons(weight unloaded, 368 lbs or so) would've far better than six AN/M2Brownings (366pounds unloaded or thereabouts.)

Asusual, theWehraboos are always wrong.

Entirelyaside from the Hispano being a twitchy,jam-o-matic pile of shit who's need for armorer hand-fitting andbabying doomedUS efforts to reliably mass-produce it, QUAD-DAMAGE isn't thepanacea it's made out to be. One limitation is that cannons really,really don't fit in cowlings so well, which is why the FW-190 carriedtwo MG 151s in the wing roots, and two wing-mounted guns (which hadthe usual convergence problem, but without the rate/volume of fire tomitigate it and less volumeto store ammo.) They also have more vibration, and thus dispersion,further complicating one's efforts to land sufficient concentrationof firepower to blow important bits off a plane.

Still,four cannons is afantastic amount of firepower, and the volume of fire (actual numberof bullets in the air) wasenough to make higher angle deflection shots and snapshots a lot morelikely to land hits... unlessone of them jammed (especiallywith the damn Hispano.) The AN/M2 might've been heavy, but it alsoinherited the ground-based M2's rock-solid reliability. Thicker,heavier chamber, barrel, and so on isn't great for weight, but italso makes a gun a lot more tolerant of temperature extremes, poorammo lots, gremlins, negative space wedgies etc. Plus,you had redundancy. Ifone gun jammed, you were down 1/6th your firepower; if one cannonjammed, you were down by 1/4th.

Andthen there's dispersal problems. “Convergence” relates to theneed to aim wing-mounted guns inward sotheir rounds all land in the same place some fixed distance ahead.The closer to centerline the guns are (such as in the wing root) thelesser the angle, and the tighter the pattern will be further awayfrom the perfect “convergence range.” Closer or further away andthe rounds land in a wider pattern, which can significantlyaffect lethality. As theWikipedia page shows, convergence schemes that aimed to maximizeeffectiveness over the widest span of ranges abounded. Theycould get pretty complex. Pilots/units could also modify them toaccount for changing theater conditions, differenttactics or even personal preference.

Withfour cannons, however, the wing-root mounts have only slightconvergence issues, and the wing-mounted ones could converge at...one point. (Assuming ships like the P-51 could've installed cannonsin the wing-root at all; withthat thin laminar-flow wing, I'm not sure they would've fit...) Gunswere typically harmonized in pairs (one on each wing) so thesix-fifties gave you three possible convergences, and the cannons,two. The machine guns couldexploit their greater volume of fire to cover more sky withlead, which made a bigdifference when taking snap-shots or high-angle deflection shots.Cannons, with their lower volume of fire, really needed toconcentrate their firepower on one point to achieve proper structuraldestruction (you could mix AP/HE or all AP, but then you were reducing your target from the entire aircraft to the three criticalsystems...) More dispersed fire is superior for deflection shots(especially as rounds are coming in from higher angles that can rakethe whole planform, giving you the whole smorgesboard of criticalsystems to aim for - and there's no armor on the top of the canopy!)Conversely, cannons are better for delivering lots of firepower fast,when you don't have time for a leisurely tracking shot (especiallyrelevant with the sanic acceleration of most German fighters, beforeyou factor in boom'n'zoom fighting - play a simulator and you'll seewhat I mean-) but American fighters suited to boom’n’zoom were not volume-challenged and could set all their guns on asingle close-range convergence to achieve similar results (such asthe FUCKIT WE'RE GOING TO 8 GUNS Jug.)

Andthe final clincher was the AN/M2s muzzle velocity of 890m/s - prettymuch the best of the war (even the UB only clocked in at 814.) Theflatter trajectory made deflection shooting easier, but only incomparison to other HMGs, not M2-versus-Hispano; the Hispano made upfor its other failings here with an 870m/s velocity, leaving even theexcellentShVAK in the dust. Nordid this matter much for distance; a pair of cowling mounted .50s aren't going to out-snipewing-root mounted Hispanos. But it did matter for actual firepower,as it gave the excellent .50 calAP-I rounds that much more oomph to penetrate - sometimesthrough a decent amount of fuselage structure and reinforcements tohit a fuel tank or engine on the other side. Whenyou're spreading lead around to maximize the chances of the goldenBB, the performance of each BB matters - andthe US had the hardest-hitting HMG of the war to use.

There'salways porquenolosdos.png of course - had the Germans managed to makea sufficient number of heavy bombers that didn't conveniently setthemselves on fire,and had the Hispano not been a French fustercluck, four .50 calibersin the wings and two 20mms in the wing-roots would've been theabsolute best of both worlds (imagine a Spitfire with four .50sinstead of six .303s and the same cannons.)

Soin the context of the perennial WWII argument - which I'm sure you'veheard before - I find it a realstretch to say that cannons were superior to the six-fifties -especially considering that American pilots didn't have heavy bomberformations to fight. Andspeaking in general, thechoice really, really rides on what your intended targets are, andespecially on what you can actually fit into the aircraft inquestion. A HMG/cannoncombo is hard to beat. Two synchronized low-ROF, low-velocity 13mmscrammed into the cowling aren't gonna cut it alone, but if you've gota bigger plane to fit something like Brownings (as the Soviets did,with early P-40s and P-39s) or you just cram twice as many in thewings, it's great. The Germans had room for neither, so QUAD-CANNONwas really the closest they could get to that ideal loadout - andgiven their aircraft's sanic acceleration, suitability for energytactics and heavy bomber formations to contend with, it probably wasthe ideal armament.

So, in general -

- if you’re in a fast-accelerating fighter that likes to boom’n’zoom, your firing windows will be short, at close range, so deflection shooting is minimized and weight of fire matters the most, as you’ve little time to deliver enough dakka to kill the target. Advantage - cannons. Head-on attacks against a heavy bomber are identical.

- if you’re in a slower-accelerating fighter that can actually turn, you’ll often be taking “tracking” shots, with plenty of time to pour fire into someone; the challenge will be aiming and scoring. Advantage - machine guns.

- if you’re flying an interceptor, a cannon/machine gun combo is highly desirable, as you’ll want long-range (400, 500m!) firepower to engage bombers, and machine-guns to deal with fighters. A perfect example is the Spitfire.

Cannonsand machine-guns eventually merged when Soviet MiG-15s met AmericanF-86s over Korea and both found their armament wanting. Withthe speed of jets, F-86s couldn't put enough firepower into a MiGwith just machine guns, and the slow-firing cannons of MiGs justcouldn't put enough bullets into the air to land a hit. This isprecisely why the modern rotary-barreled 20mm aircraft cannon wasdeveloped - it's a merging of the machine-gun's ROF and the cannon'sfirepower in one package.

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