20mm cannon, best, worst, specs, comparison to LMG, HMG etc.

[Kryten]

That last bit may be a bit of hearsay as the Spit Vc with 4 cannons were used in Malta, some pilots preffered it for its ability to disable bombers, but most had two cannons removed to impove the rate of climb and alleviate the shortage of servicable cannons!

 

[renrich]

Don't know where the info came about the Air Ministry "discovering" that the 50 BMG would not penetrate any more armor than the 303 but that sounds a little iffy to me. The US fighters armed with 50s seemed to do fairly well against the armor in German AC.

 

[pinsog]

"The Air Ministry rejected the idea of using 2 x .5" with 2 x 20mm, at first, because they discovered that, from directly behind, a .5" bullet was no better at penetrating German armour than a .303","

I have learned more from this website in a month about WW2 airplanes than I did in my previous 30 years of studying WW2. Ballistics, on the other hand, make me shake my head. How could anyone that isn't drunk or smoking crack, possibly believe that a 180 grain .311 caliber bullet at 2500 feet per second could penetrate as well as a 650 to 800 grain bullet moving between 2800 and 3000 feet per second? Have you guys that are arguing for the .303 ever held one in your hand next to a .50 Browning?

The .303 wasn't even a good cartridge compared to the 30-06 or the 7.92, being at least 200 feet per second slower. Ballistically, it barely beats out the old 30-30. The simple fact is, the British made a terrible mistake choosing the .303 over the .5 Browning. It was a stopgap weapon and a poor one at that.

 

[renrich]

Don't know where the info came about the Air Ministry "discovering" that the 50 BMG would not penetrate any more armor than the 303 but that sounds a little iffy to me. The US fighters armed with 50s seemed to do fairly well against the armor in German AC. At 500 yards the AP bullet in the 50 BMG will penetrate .75 inches of face hardened armor. Double post, I apologise.

 

[Shortround6]

I doubt that the recoil of 4 20mm guns would tear the wing off. They do usually test these set ups on the ground before either flying them or tooling up for production. The also mocked up a wing with six 20mm cannon and flew the mock up. dummy guns but all six fairings. I don't know if they ballasted the wing or what internal modifications were made.

I am not sure I buy the the .50 not penetrating either but I would like to see the original source. One clue is the phrase "from the rear", bullets that first penetrate fuselage skinning and other obstructions a number of feet from the armor plate tend to yaw (tip). some projectiles yaw more than others and a .50cal, powerful as it is, doesn't penetrate well if the bullet has tipped sideways before it hits the armor. Tony Williams has some figures (perhaps not for the American .50 ?) in one of his books.

 

[Glider]

"The Air Ministry rejected the idea of using 2 x .5" with 2 x 20mm, at first, because they discovered that, from directly behind, a .5" bullet was no better at penetrating German armour than a .303","

I have learned more from this website in a month about WW2 airplanes than I did in my previous 30 years of studying WW2. Ballistics, on the other hand, make me shake my head. How could anyone that isn't drunk or smoking crack, possibly believe that a 180 grain .311 caliber bullet at 2500 feet per second could penetrate as well as a 650 to 800 grain bullet moving between 2800 and 3000 feet per second? Have you guys that are arguing for the .303 ever held one in your hand next to a .50 Browning?

I doubt that anyone is saying that the 303 penetrated armour as well as the 0.5 but it is true to say that when the British tested the 0.5 against the Me 109 from behind the 0.5 didn't penetrate the armour on the pilots seat. I suspect the two facts became confused. No one would disagree that the 0.5 would and did do a lot more damage to the target aircraft than a 303. Engines, fuel tanks and all the other vulnerable parts were far more vulnerable which is why from late 1944 the 4 x 303 were normally replaced by 2 x 0.5..

The simple fact is, the British made a terrible mistake choosing the .303 over the .5 Browning. It was a stopgap weapon and a poor one at that.

When the RAF decided to make the change to the 20mm the 0.5 M2 wasn't around, the M1 was part of the test and therefore the M2 wasn't an option. On that basis the stopgap choice of 8 x LMG whilst the 20mm was sorted out was a good one when you remember that in 1935 when the decision was made, no aircraft had armour or self sealing tanks. What the RAF were guilty of was not sorting out the 20mm earlier.

 

[Shortround6]

Glider is quite right. When the tests were done not only did the M1 have a firing rate of 600rpm at best compared to the late 1940 or 41 800-850rpm rate of fire for the M2 gun but at some point in the 1930s the ballistics were also changed. Several hundred FPS were added to the velocity of the .50 cal ammunition. The Original "M1" load, which is what the British would have tested used a 753 grain bullet at 2500fps. The M2 ball used a 711 grain bullet at 2935 fps (from a 45in barrel ?).

So the British had to decide between ( for weight) a single .50 firing (at best, they often fired slower) ten 753 grain projectiles a second or two .303 machine guns firing forty 174 grain projectiles. The .50 cal ammo weighs about 5 times as much as as .303 ammo so for the same weight as the 2672 rounds in the Hurricane they could have had around 540 rounds of .50cal to feed 4 guns. The more weight you add the slower the plane climbs.

 

[Shortround6]

The .303 wasn't even a good cartridge compared to the 30-06 or the 7.92, being at least 200 feet per second slower. Ballistically, it barely beats out the old 30-30. The simple fact is, the British made a terrible mistake choosing the .303 over the .5 Browning. It was a stopgap weapon and a poor one at that.

That is cold comfort to Allied airmen that were shot down by the Japanese army and navy pilots whose 7.7mgs were actually .303s

 

[pinsog]

That is cold comfort to Allied airmen that were shot down by the Japanese army and navy pilots whose 7.7mgs were actually .303s

I'm not saying an aircraft can't be downed by a .303. Thousands of elephant and cape buffalo have been killed with a .303. Doesn't make the .303 an elephant or cape buffalo gun. There is a reason the .375 H&H is the legal minimum for dangerous game in Africa.

I would guess that the majority of Japanese kills were with their 20 mm cannon. I would also say that we can all breath a big sigh of relief that the Zero wasn't equiped with 4 .50 Brownings. My guess is there would have been alot more Wildcats and Dauntlesses littering the ocean floor.

 

[Edgar Brooks]

Don't know where the info came about the Air Ministry "discovering" that the 50 BMG would not penetrate any more armor than the 303 but that sounds a little iffy to me. The US fighters armed with 50s seemed to do fairly well against the armor in German AC. At 500 yards the AP bullet in the 50 BMG will penetrate .75 inches of face hardened armor. Double post, I apologise.

Memo dated 12th January 1944, from the Director of Operational Requirements to the Chief of the Air Staff, headed ".5 Gun Secondary Armament in Spitfire" states 8th June 1942 "Letter from C-in-C. fighter Command asking that "Universal" wing of Spitfire V, VIII and IX be deleted and that armament be standardised on 2 x 20 + 4 x .5, or if this were not possible, 2 x 20 + 2 x .5. This change was required as performance was adversely affected with 4 x 20mm and the alternative 2 x 20mm + 4 x .303 was not considered adequate."
The response was "The above proposal was investigated and it was found that (i) 2 x 20mm + 2 x .5 would be at least 80 lbs heavier than the 2 x 20mm + 4 x .303. (ii) An assessment of the results of extensive firing trials indicated that the 4 x .303 secondary armament was at least as good as the 2 x .5 since the .5 was comparitively ineffective in attacks made from directly astern and at large deflection angles was not much superior to .303. The higher rate of fire of the .303 armament therefore gave a better chance of hitting at the greater deflection angles, i.e. strikes which would miss the armour."
"Air Ministry therefore proposed the matter should be reconsidered and as a result of a meeting at Fighter Command the C-in-C stated on 8th July that he now wished to retain the 4 x .303 secondary armament in Spitfires."
In December 1942, a new C-in-C started the whole thing up again, and it rumbled on until:- "12th November 1943 Air Ministry replied agreeing that action would be taken to equip Spitfire IX and XIV with .5 secondary armament as soon as possible, as the result of the trials was likely to be delayed. Further, the change was now considered justified in view of the greatly improved sighting accuracy, as demonstrated by trials just completed, to be expected with the Gyro Gun Sight. Thus, for deflection shooting the .303 would not have the advantage over the .5 which it now has with existing sighting method."
The file, and hundreds like it, is available at our National Records.
As always, hindsight is 20:20 and it should be remembered that, when Germany was first seen as the (only) potential enemy, it was not envisaged that France would fall, and fighters reach these shores, so only slower bombers were planned for, and it was felt that 8 x .303" would do the job. I have a memo, from Sholto-Douglas, in which he argues that pilot armour, for the Spitfire, would not be necessary, since, being the fastest thing in the air, only a dozy pilot would let anything get behind him; that changed, a bit sharpish, when France collapsed.
Edgar

 

[Shortround6]

The Ki 27 Nate used a pair of 7.7s, the Ki 43 used either a pair of 7.7s or one 7.7 and one 12.7 for the early part of the war, the Zero carried 60 rpg for it's 20mm guns for the first 6 months of the war if not longer. All IJA victories in the early part of the war were achieved without 20mm guns.

The .303 actually has an almost identical muzzle energy to a 7.62x51 NATO round, while this is a small step down from the 30-06 and the German 7.9mm it isn't quite down to a .30-30 either.



The weight of 4 50cal guns is about 30kg more than the weight of the guns ( two 7.7mm+ two 20mm) in the Zero and the weight of a useful amount of ammo would have added even more to the weight difference.

The M2 was a good gun but not great and it was not a good choice for planes with limited amounts of power. The gun itself was heavy for it's power and the ammo, while possessing good kinetic energy, was also heavy.

More than one early war US fighter may have suffered in combat due to being overloaded with guns and ammo for the power available.

 

[pinsog]

The Ki 27 Nate used a pair of 7.7s, the Ki 43 used either a pair of 7.7s or one 7.7 and one 12.7 for the early part of the war, the Zero carried 60 rpg for it's 20mm guns for the first 6 months of the war if not longer. All IJA victories in the early part of the war were achieved without 20mm guns.

The .303 actually has an almost identical muzzle energy to a 7.62x51 NATO round, while this is a small step down from the 30-06 and the German 7.9mm it isn't quite down to a .30-30 either.



The weight of 4 50cal guns is about 30kg more than the weight of the guns ( two 7.7mm+ two 20mm) in the Zero and the weight of a useful amount of ammo would have added even more to the weight difference.

The M2 was a good gun but not great and it was not a good choice for planes with limited amounts of power. The gun itself was heavy for it's power and the ammo, while possessing good kinetic energy, was also heavy.

More than one early war US fighter may have suffered in combat due to being overloaded with guns and ammo for the power available.

I wonder if the low firepower of the early Japanese fighters helped account for the reputation of Amercican fighters and bombers as being so tough. Maybe Amercan planes weren't so tough, maybe Japanese fighters, after running out of 20mm cannon shells, just had such pitiful firepower they couldn't finish them of.

Would 60 lbs really degrade the performance of a fighter plane that much? If someone handed me a Zero and said, go shoot down F4F's, choose your armament, I would give up 60 lbs to have 4 50's over 2 20's with 60 rounds each and a pair of 7.7's.

If I was flying an early P40, I would have removed the 30's,like the Russians did, and flown with 2 synchronized 50's, and if I was flying an F4F4 I would have the outer 2 50's removed.

 

[Shortround6]

Early Zeros had under 1000hp and the 66lb weight difference is just for the weight of the guns. the Zero carried 1000rounds of 7.7mm ammo which equals about 200 rounds of .50 cal ammo. The Japanese 20mm ammo used a cartridge case that was 27mm shorter than a .50 cal case and only 1.6mm fatter. While the projectile was much heavier the complete round didn't keep the same proportion. An empty .50 cal case weighs more than the bullet it fires. you are lucky if you would get much more double the number of the number of .50 cal rounds than the 20mm. Even throwing in the weight of the drums on the cannon is still going to mean adding even more weight than the 66lbs to get to even 200 rounds per gun for four .50s. You could be looking at 90-120lbs more weight with even a modest amount of ammo (200rpg). Wildcats with 4 guns carried 400rpg which would add 240lbs more than 200rpg.

Combat persistence is part of the problem. The Zero ran out of 20mm ammo in about 7-8 seconds but had about 33 seconds of ammo for it's 7.7mm. A four gun Wildcat had about 32-33 seconds of firing time, cutting the ammo to 200 rpg cuts the firing time to 15 seconds. Hurricanes and Spitfires had 15-17 seconds of firing time for their .303s.

If the Russians truly pulled the wing .30s from the early P-40s they must have been really desperate. The rate of fire for the synchronized .50s was really pathetic, some times down to 450rpm. Two of them would give you 900-1000rpm total, makes the 9600rpm of a .303 armed Spitfire look really good.
The US navy eventually agreed with you about the F4F-4. Later versions did drop back to 4 guns but they did it to increase the ammo carried from 1440 rounds for six guns to 1720 for four guns. Apparently they didn't like the shorter available firing time even with the extra fire power per second.

 

[pinsog]

Early Zeros had under 1000hp and the 66lb weight difference is just for the weight of the guns. the Zero carried 1000rounds of 7.7mm ammo which equals about 200 rounds of .50 cal ammo. The Japanese 20mm ammo used a cartridge case that was 27mm shorter than a .50 cal case and only 1.6mm fatter. While the projectile was much heavier the complete round didn't keep the same proportion. An empty .50 cal case weighs more than the bullet it fires. you are lucky if you would get much more double the number of the number of .50 cal rounds than the 20mm. Even throwing in the weight of the drums on the cannon is still going to mean adding even more weight than the 66lbs to get to even 200 rounds per gun for four .50s. You could be looking at 90-120lbs more weight with even a modest amount of ammo (200rpg). Wildcats with 4 guns carried 400rpg which would add 240lbs more than 200rpg.

Combat persistence is part of the problem. The Zero ran out of 20mm ammo in about 7-8 seconds but had about 33 seconds of ammo for it's 7.7mm. A four gun Wildcat had about 32-33 seconds of firing time, cutting the ammo to 200 rpg cuts the firing time to 15 seconds. Hurricanes and Spitfires had 15-17 seconds of firing time for their .303s.

If the Russians truly pulled the wing .30s from the early P-40s they must have been really desperate. The rate of fire for the synchronized .50s was really pathetic, some times down to 450rpm. Two of them would give you 900-1000rpm total, makes the 9600rpm of a .303 armed Spitfire look really good.
The US navy eventually agreed with you about the F4F-4. Later versions did drop back to 4 guns but they did it to increase the ammo carried from 1440 rounds for six guns to 1720 for four guns. Apparently they didn't like the shorter available firing time even with the extra fire power per second.

If the 2 popguns firing through the prop wont bring an enemy aircraft down, it doesn't matter how much ammo you have for them. F4F4's carried 240 rounds per gun. I believe some P40's carried about the same amount. I would rather have 200 to 250 rounds for a 50 than 1000 for the Red Ryders up on the engine cowling. Same if I was elephant or buffalo hunting, I would rather have 10 rounds of 375 H&H than 100 of 223. Sherman tanks with a 75mm carried twice the number of rounds as a Tiger with an 88mm, which would you rather have?

According to Robert Johnson, a Fockwolfe sat behind him and hosed his P47 with LMG fire until he ran out of ammo. According to Saburo Sakai, he did the same thing to a Wildcat until he finally switched back on his cannon and finished it off.

I would argue that in either case, particularly the Wildcat, 50's would have finished the job without cannon, especially at point blank range like both of these examples were.

 

[Ratsel]

what was the average time of a dogfight? 1 on 1? I think the answer is there, thats were the superiority of the 50cal or
13mm rounds come into play. now vs a bomber like a landcaster or B-17's 8 x .303's firing for a minute is utterly useless,
50cal/13mm is quite a bit better. 20/30mm shells will do the job.

20/30mm shells vs a fighter, well it dosn't matter how many are fired and it dosn't matter at what rate, 1 or 2 hits will
get the message across.

as far as removing the outer 50cals, some P-40's did that in N.Africa. result: increase climb rate, slight increase in acceleration.
the practice was abandoned though, as the extra 50's were needed against the Bf109F/G's.

conclusion: 2x13mm w/ cannon fuse mount was great for head on and rear attacks. 8 x 303 or 6 x 50cals great for deflection shooting.

 

[pinsog]

what was the average time of a dogfight? 1 on 1? I think the answer is there, thats were the superiority of the 50cal or
13mm rounds come into play. now vs a bomber like a landcaster or B-17's 8 x .303's firing for a minute is utterly useless,
50cal/13mm is quite a bit better. 20/30mm shells will do the job.

20/30mm shells vs a fighter, well it dosn't matter how many are fired and it dosn't matter at what rate, 1 or 2 hits will
get the message across.

as far as removing the outer 50cals, some P-40's did that in N.Africa. result: increase climb rate, slight increase in acceleration.
the practice was abandoned though, as the extra 50's were needed against the Bf109F/G's.

conclusion: 2x13mm w/ cannon fuse mount was great for head on and rear attacks. 8 x 303 or 6 x 50cals great for deflection shooting.

AFAIK the only wing guns removed by the Russians were always 30 caliber Bronwings, not 50's. I didn't know they removed guns in North Africa also.

 

[post76]

I think the 50 cal was as effective as it was versatile.
With a platform of 6 guns, you have an effective air weapon as well as a disruptive ground weapon.
This means little retooling or rigging for mission objectives and an aircraft could be ready quickly and still pose an air threat if loaded for a ground attack mission, or vice verse. This would be where 303s fall short. They didn't have the same punch and they lacked the range where a heavier projectile might be beneficial. I just imagine a strafing run where the target is out in front of the plane more so the pilot doesn't have to stick his nose in the dirt to pull off a shot. You've seen the gun cams where they are waxing trains from a much further distance out than they might engage an aircraft. This is where the issue of velocity and trigger time would be more beneficial, something that only the 50 had both of.

as far as removing the outer 50cals, some P-40's did that in N.Africa. result: increase climb rate, slight increase in acceleration.
the practice was abandoned though, as the extra 50's were needed against the Bf109F/G's

Hitting an air target is much less of an issue and i do think a 303 could be effective particularly in the case of water cooled engines that carry their radiator vents on their wings. One hole there or elsewhere in the plumbing would ground the airplane. Carrying one less pair of guns for weight might depend on pilot needs and aircraft versatility.

 

[Shortround6]

If the 2 popguns firing through the prop wont bring an enemy aircraft down, it doesn't matter how much ammo you have for them.

2 popguns did often bring down early war aircraft. Not often enough it is true, but if two "popguns" could do the job a fair amount of the time trying to claim that 8 such popguns were a huge mistake doesn't make much sense.
The Bf 109 fought in Poland and France with , at times, four "popguns" and the rest of the time with two 20mm guns very close to the Japanese Zero and two "popguns" firing through the prop. They kept the two "popguns" for another 2 years. Mostly because they didn't have anything better. Same situation as the Zero, 20mms are empty in about 8 seconds and any additional fighting is done with the "popguns".

F4F4's carried 240 rounds per gun. I believe some P40's carried about the same amount. I would rather have 200 to 250 rounds for a 50 than 1000 for the Red Ryders up on the engine cowling.

Something of a mixed message here. the 240 rounds per gun for the F4F-4 weighed 532lbs. that is just the ammo. you are comparing the ammo for a single gun to the ammo for two cowl guns. Are you saying that single .50cal MG would have done any better in the situations you describe than the paired "popguns" with their much higher rate of fire and much higher hit rate?

Same if I was elephant or buffalo hunting, I would rather have 10 rounds of 375 H&H than 100 of 223. Sherman tanks with a 75mm carried twice the number of rounds as a Tiger with an 88mm, which would you rather have?

Bad analogy's, you don't "spray" an elephant (or even spray the area it is in which is what some air combat came down to) with gunfire and hope to hit a vital part. your tank analogy might be better if compared a Sherman with a 75 to a Sherman with a 17pdr. and even then it depends on the target.

According to Robert Johnson, a Fockwolfe sat behind him and hosed his P47 with LMG fire until he ran out of ammo. According to Saburo Sakai, he did the same thing to a Wildcat until he finally switched back on his cannon and finished it off.

I would argue that in either case, particularly the Wildcat, 50's would have finished the job without cannon, especially at point blank range like both of these examples were.

Ah yes, two famous incidents that do not tell us how many times Japanese or German pilots used the same tactic and succeed in shooting down the target. Now going back to the Wildcat, there is little doubt that with either four or six .50cal guns they would have made short work of the target but let's examine the Johnson case a little more closely. Why was the German pilot using his "popguns"? to save 20mm ammo or had he used up all his 20mm ammo already? Fw 190s carried 200rpg for their wing root 20mm guns that fired a bit slower than .50 cal mgs. Any fighter armed with .50s with 200-240rpg would have been out of ammo and in no position to attack Johnson with any gunfire.
Now by 1943 the "popguns" had certainly seen their day and carting around a pair of them with the ridiculous amount of 1000rpg ( around 35-40 seconds more firing time than the 20mm guns) seems a bit of a waste, but claiming that a pair of guns that guns that couldn't bring down a P-47 in 1943 means that eight such guns were a really bad armament in 1940 against such aircraft as as a Do 17 which was only about 20% heavier than a P-47 empty and a much bigger target doesn't follow.

 

[Shortround6]

what was the average time of a dogfight? 1 on 1? I think the answer is there, thats were the superiority of the 50cal or
13mm rounds come into play. now vs a bomber like a landcaster or B-17's 8 x .303's firing for a minute is utterly useless,
50cal/13mm is quite a bit better. 20/30mm shells will do the job.

I think you are confusing the time line. By the time the B-17 was in combat (or the Lancaster) the eight .303 armament was history.
There is no question that a battery of six .50 cal guns with a good supply of ammo is better than a battery of eight .303mgs with a good supply of ammo.
The argument is being made that the British made a mistake in using the .303 in 1935-1940 instead of adopting the .50cal.
What is being ignored is both the actual capability of the .50 cal gun and ammo in the 1935-1940 time period as compared to later and the weight associated with a .50 cal battery.
While the decision to use the .303 didn't exactly coincide with the Spitfire and Hurricane it would be well to remember that the late 30s versions of these planes had Merlins that were good for 880hp for take-off and had fixed pitch propellers. The Gladiator had an even lower powered engine with a fixed pitch propeller. shoving too much weight in the form of guns and ammo into the planes would have resulted in lower performance. To be an effective fighter the fighter not only has to have an effective armament, it has to get that armament into a firing position. As power and performance increased due to later model engines, better fuel and the adoption of constant speed propellers the British did increase the armament of their fighters.

20/30mm shells vs a fighter, well it dosn't matter how many are fired and it dosn't matter at what rate, 1 or 2 hits will
get the message across.

Actually the number of rounds fired and the rate of fire are important. With many pilots only hitting with 2-5% of rounds fired, a 2-3 second burst may result in no hits if the number of rounds fired in that burst are too low.

 

[Njaco]

what was the average time of a dogfight? 1 on 1? I think the answer is there, thats were the superiority of the 50cal or
13mm rounds come into play. now vs a bomber like a landcaster or B-17's 8 x .303's firing for a minute is utterly useless,
50cal/13mm is quite a bit better. 20/30mm shells will do the job.

20/30mm shells vs a fighter, well it dosn't matter how many are fired and it dosn't matter at what rate, 1 or 2 hits will
get the message across.

as far as removing the outer 50cals, some P-40's did that in N.Africa. result: increase climb rate, slight increase in acceleration.
the practice was abandoned though, as the extra 50's were needed against the Bf109F/G's.

conclusion: 2x13mm w/ cannon fuse mount was great for head on and rear attacks. 8 x 303 or 6 x 50cals great for deflection shooting.

IIRC the majority of dogfights lasted a mere few seconds or less than a minute. Others lasting 5-10 minutes were rarer but on occasion happened. But even then, the rule was short bursts of 3-5 seconds.