Why did US fighters not use cannon in WW2?
 

Leafing through some old William Green books I noticed that throughout the total production run of the main US WW2 fighters - P-47,Thunderbolt, P-51 Mustang, Wildcat, Hellcat all were only armed with machine guns . The Lightning carried machine guns and one cannon.

Only the later versions of the Corsair carried 4 cannon.

When you consider than in Europe cannon started to be fitted from 1939 across the fighter types of all nations this seems strange. Especially as the US was relatively quick to adopt self-sealing tanks, cockpit armour etc etc

Any idea why?

Not an answer, but expanding the question.

The US carried on with this preference even with jet a/c such as the F-86. The F-86's 6 x 50 calibre machine compared to the Hawker Hunter 4 x 30mm Aden.

Having zero practical knowledge on this subject conjecture would suggest that a machine gun round has a higher muzzle velocity than a cannon shell so there is less deflection and more accuracy.

Is think it is down to effect, for the same weight penalty you can carry more guns/ammunition than the same in cannon along with the extra airframe bracing. Greater weight of fire, quantity has a quality all if it's own, cannon would put less rounds in the air but granted, probably only takes one to hit to have the effect.
Uneducated rationale

They simply didn't to, they had the 50 cal already.

Multiple fifty's were more than adequate for the task at the time (and lets face it, the things are still being used today!) so with a huge production and logistics train already in place to support the 50 cal, there was no real need to change, and as mentioned, this continued well past WW2 and into the 1950's and the jet age.

yes - the idea that they had a pretty big industry set up anyway for 0.303 & 0.5 inch did strike me - far easier to ramp up production rather than go for something heavier

But carrying it into the jet age was rather bizarre............... especially when a Mig-15 carried 3 cannon................

From some reading I've done around the issue, they had the licence to make the same 20mm Hispano cannons as the UK but for some reason had massive quality control issues producing them. I believe the USN was at one point planning on standardising on the 20mm but due to the production issues went with the .50 cal instead.
I think the P-61 (?) Black Widow was armed with 20mm though as they'd just about sorted things out by that stage.

As others have noted, they decided (don't have the source material to hand) that for simplicity the standard armament for a fighter was 6x 0.5 inch guns. Don't forget the P-40 - more numerous than any of the naval fighters! This policy was varied slightly to suit specific airframes: the unusual layout of the P-38 and P-39 allowed space for a cannon as well as the machine guns, and the size of the P-47 allowed it to have two extra. Conversely, most of the F4Fs had to make do with 4.

The reasoning (IIRC) was exactly as Sloppy Link and Geeram have stated, and I suspect that the Americans felt vindicated when they saw the difficulties which other nations had at times with cannon-equipped fighters (the early Hispano-equipped Spitfires but also the German MK108). And even a typical mid-war Spitfire was pretty impotent against everything apart from a Zero once the 20mm ammunition had run out and all it had was 4x .303 guns.

P-61 - 4 cannon plus 4 x 0.5 mg in the turret

Wikipedia on the Corsair

F4U-1C:

The prototype F4U-1C, appeared in August 1943 and was based on an F4U-1. A total of 200 of this variant were built from July to November 1944; all were based on the F4U-1D and were built in parallel with that variant.[82] Intended for ground-attack as well as fighter missions, the F4U-1C was similar to the F4U-1D but its six machine guns were replaced by four 20 millimeter (0.79 in) AN/M2 cannons with 231 rounds of ammunition per gun.[127] The F4U-1C was introduced to combat during 1945, most notably in the Okinawa campaign.

Aviators preferred the standard armament of six .50 in (12.7 mm) machine guns since they were already more than powerful enough to destroy most Japanese aircraft, and had more ammunition and a higher rate of fire.[128] The weight of the Hispano cannon and their ammunition affected the flight performance, especially its agility, but the aircraft was found to be especially potent in the ground attack role."


All later Corsairs retained the cannon tho' I think

USAAF Beaufighter night fighters used 20mm and machine guns. P-39 used 37mm and 20mm cannons. On a Thunderbolt for example :Eight .50's with a total of 2400-3200 rounds had many advantages, and a few disadvantages, over four 20mm with 500-600 rounds. The .50 was a standardised weapon for logistics and had a much higher muzzle velocity than the then 20mm...remember E=mV2

The Mig 15 was designed from the outset to sweep the skies of the Soviet Union of any US A-bomb carrying aircraft. The suitability of the three cannon armament arrangement in this role was proven when the Migs forced the USAF to cease daytime operations over Korea with the B29.

The prime reason for the US retaining the .50 calibre well past its sell by date was cost. They retained huge numbers of this weapon and its associated ammunition in inventory after the end of the second world war. In the post war period they had no desire to fund starting production of a new weapon when they already held vast stocks of weapons already paid for

In terms of weight of fire, four 20mm cannon outguns six 50 calibre by a significant margin.

For the same reason the UK choose to arm the Hunter and Javelin with four 30mm Aden guns. This new generation was designed to protect the UK from A-bomb carrying Soviet jets. As it was considered that any defending aircraft may only have a fleeting opportunity for one burst of fire, a high volume of fire was of prime importance.

I think the starting points were different for the US and UK. Here in the UK we had many aircraft fitted with light machine guns effectively firing a rifle bullet. The performance of this small calibre was marginal and we took a fresh look and migrated towards cannons.

The US started with a .50 cal HMG round which provided them with a superior capability from the outset. The US recognised the advantages of the cannon but the capability increase from the .50 cal was not as urgently needed compared to those who pitched-up to a war firing a bullet around a 1/4 of the weight and half the effective range of the .50 cal.

(NB some US aircraft had .30 cal weapons fitted and $ price comes into everything)

http://upload.wikimedia.org/wikipedi...comparison.jpg

I believe this, along with the previous couple of posts, are most relevant.

It wasn’t a matter of what you were shooting from - it was what you were shooting at. In the Pacific War the vast majority of targets were unarmoured and the extant machine guns were more than adequate for the task and it would not have been logistically efficient to build another logistics chain and aircraft models for the European theatre.

TBM - almost Beaufighters carried 4 cannon and 6x 0.303 -

Same reasoning as given by JTO - you might only hit the enemy with a few rounds and they had better do damage - W European military aircraft were generally better protected than either US or Japanese aircraft - certainly at the start of war .

Several US fighters imported into Europe early on showed substantial performance loss due tot eh need to fit armour, armoured glass and self sealing tanks....... the original P-51 being a case in point.........................

It seemed to me that a big factor was the prioritization of muzzle velocity (flat trajectory) and rate of fire, both of which would help compensate for less-than-excellent marksmanship. That carried over to the 1950s and the present day, with the quad-M-39 setup on the F-100 and F-8 and, of course, the M61.

Project Vulcan itself (I recall from history briefings) resulted from post-WW2 retrospective studies of gun lethality, which concluded that the average pilot was best served by something combining the characteristics of a rifle and a shotgun.

of course you can overdo the weight of shot - the 57mm, 6 pounder Mollins gun........


https://www.pprune.org/members/32618...957-us57mm.jpg


https://www.pprune.org/members/32618...956-tsetse.jpg


https://www.pprune.org/members/32618...5-molinsa2.jpg

I would guess that the flatter trajectory and higher round count would allow for chancing shots in a dogfight and also allow fighters to sit outside the range of a bombers turrent.
Otherwise for cannon fighters the strategy sounded like "shoot when the enemy fills my windscreen"

Sometimes I consider the .50 BMG America's secret weapon of WWII.

............... but one cannon hit did a hell of a lot more damage than a load of machine gun bullets

Well, that sweeping statement needs a small 'it depends' addendum. Putting little incendiary .303 rounds into the unprotected fuel tanks of contemporary German light bombers didn't really leave a whole lot left. The argument only changed when the need to shoot further and harder became relevant.

So it really boils down to two major points:
1. Logistics/commonality. It was easy to mass produce large numbers of .50 BMG quickly, and provide lots of ammo for those guns which were common to lots of other vehicles, to include a dozen or more on every B-17 and B-24 bomber, plus the numerous guns on B-20, B-25, B-26, etc etc., And it was easy to get all those guns and all that ammo from the USA to the far end of the Atlantic and the Pacific. This was essentially the same reason that US tanks used gasoline engines rather than diesel. Despite the far greater fire danger posed by gasoline, it simplified logistics by having a single type of fuel for everything from light jeeps to heavy tanks.
2. Good enough. The targets being shot at were light enough that a .50 BMG did the job. Neither the Germans nor the Japanese had large bombers that needed to be shot down. The exceptions of cannons on US aircraft was for air to ground use, not air to air. And even then, large numbers of .50 BMG rounds on a truck or other ground vehicle (even armored ones) were effective at disabling them even if they did not destroy them. And often disabling was all that was needed. So the crew flying them usually preferred the machine gun equipped aircraft over the cannon equipped aircraft.

There is a third lesser point. Both the AN/M2 (the lightweight aviation version of the ubiquitous "Ma Deuce") and its ammo was relatively small and compared to cannon, very light. Both are very important considerations in a fighter.

I recall reading that Capt Eric "Winkle" Brown shot down 2 FW 200 Condor whilst flying the Grumman Martlett (Wildcat) from HMS Audacity. Thus the .50 cal ammunition was effective against a large 4 engined aircraft. Although the Sea Hurricane with only 8 .303cal mg did manage to down 3 FW 200 Condors, there were a number of occasions when they were unable to inflict sufficient damage to shoot them down.

The FW 200 was a minimally altered airliner that had serious issues with both fuselage and wing strength - the literature is full of pictures of them falling apart (literally) on landing or taxying. So not the worlds toughest target

IIRC two of the toughest were flying boats - the Sunderland and the Kawanisi 8K "Emily" could take an outstanding amount of punishment apparently

Ken. WW2, most German and British tanks also used gasoline.
As far as fighter use of Cannon goes, the Germans had quite a lead in the availability and the installation of cannon type weapons. They seem to have appreciated the advantages of the larger calibre well before the RAF and had the 20mm weapons in the wings of the Bf109E before WW2, having learned the lessons in Spain. The RAF seemed to agonise about cannon and the Spitfire had to wait till Nov '40 for the Mk1b to get 20mm cannon in the wings working satisfactorily, while the Luftwaffe outgunned them through the BoB. By the time of the Spit Vb, the Germans had sorted the engine mounted cannon in the 109F and, shortly after, the Fw190 with at least two wing root mounted 20mm, sometimes four and, nothing below 12.7mm. The Germans preferred the bigger weapons and soon moved on to the 30mm MK108. This weapon suffered from early poor quality but did become reliable enough and, with the thincase "minen" type blast shells they were a very effective weapon, only an average of 4 hits req'd to down a heavy bomber. For the Americans, it would seem that the .5 BMG was sufficient to deal with many situations, particularly against fighters and small bombers. Certainly, there was an element of strength in numbers, with 6x .5BMG and generally huge numbers of P51's hunting the Jagdwaffe in the later stages of the War. However, the pure effective weight of fire from the 4xMK108 in a 262 was impressive. Interestingly, the late developed MG213 20mm and MK213 30mm revolver cannon were the basis of many postwar weapons, including the ADEN 30mm and latterly, the Mauser 27mm in the Tornado. Of course, the Americans developed some excellent revolving barrel weapons. However, the instantaneous rate of fire achieved by the revolver type Mauser can be an advantage in a .5sec snap burst and, the installed weight of the Mauser is low.

OAP

Six or Eight .50 Caliber machine-guns focused at a point some several hundred yards in front of the aircraft put a lot of Rounds in a very small area.

There is no comparison between a .50 caliber round and a .30 caliber for effect.

The Spit with four 20MM cannon must have been a distinct improvement over the .303 equipped versions.

Hi. I recall that Eric Brown describes having to engage the Fw200 head on because of it's defensive firepower. His .5 firepower was effective against the vulnerable cockpit of the bomber.

OAP

Not quite correct I am afraid. 19 Squadron had Spitfire Mk1bs with cannon in May/June 1940, but suffered from problems with their operation. I remember reading somewhere years ago that a pilot fired on a ME 109 ( and I am sure this was over Dunkirk) and immediately the windscreen of his Spit turned red!
There was also a Hurricane at North Weald called the 'old cow' (IIRC) with 20mm cannon ,that was somewhat slower @300mph than the Browning armed ones. It has been suggested by numerous BoB pilots, that had cannon fighters been more available, the outcome would have been even more decisive.

Thanks rolling. I have corrected my post. The Spit Mk1bs issued to 19 Sqn were withdrawn because of constant weapon failures. The cannon installation in the 1b only became acceptable by Nov '40, after the BoB.

OAP

I think this is the wrong way round:

Rather than the USAAF to cannon, the RAF should have gone for 0.50 cal.

We all know the 0.303 is too light and 20mm too slow and low rate of fire for A2A.
Carrying both didn't make up for the deficiencies of the other!

Pleasure OAP, thank you.

The thinking at the time was one of accuracy. The powers that be weren't that confident of a pilots ability to hit a target and a spread of .303s offered the best chance of hitting a target.
Cannon were thought about pre WW2, but ditched for .303. The 8 gun fighter was a potent weapon when thought of in the mid 30s. The idea was even toyed with 10 or 12 guns. I believe tests were carried out pre war on .5s, but the results were disappointing.

Interestingly, reading about the Hispano-Suiza 20mm cannon development, it seems that the weapon development in the USA was never satisfactory during wartime and so, the continued use of the .5 Browning was something of a force-majeure.

OAP

I think it was recognized that they needed something bigger than the 0.50 cal HMG. The USN held a "Joint Fighter Convention" in 1944 and the moderator told the attending aircraft companies that they should provide room in their designs for the oncoming T17E3 .60 cal Aircraft MG. It was pretty impressive gun, had a very high muzzle velocity but never made it to use. Some large production contracts were canceled.
This link will provide you with more than you probably want to know about the T17E3 .60 Cal MG. Just scroll down on the posts.
https://www.milsurps.com/showthread.php?t=32509

The simple fact of life of cannon v machinegun is quite simply that the increased size of the cannon round allows the incorporation of high explosive charges, complex fuses and incendiary components. Late in the WWII the RAF carried out extensive evaluations of the effectiveness of many different calibres and types of ammunition. The result was that the 20mm Hispano semi-amour piercing incendiary round was by far the most effective round in aerial combat. It was also very good when used for strafing and was quite capable of penetrating the upper armour of the Panzer Mk4.

Subsequently in the Korean war it was established that .5" Browning ammo. used by the F86 Sabre was being deflected by the airflow over the wings and fuselage of the Mig 15 once speeds got up to round Mach .85. It was established that this was largely due to the light weight of the .5 cal. bullets.

As others have noted, the US fighters didn't often face heavy bombers in either the European or Pacific Theaters, and the 50 cal. did the job quite well on the smaller aircraft (especially the Japanese which lacked defensive armor and such).
My dad fought in the South Pacific - Guadalcanal, New Guinea, Philippines. By the time he got to Guadalcanal in late 1942 the P-39 was pretty much outclassed for air-to-air, but was very effective for air to ground with that big cannon in the nose (TBM - I thought all the P-39s had 37mm cannon but perhaps some used the 20mm). According to my dad, when the P-39 fired that 37mm cannon during a strafing run, he could see the aircraft 'stutter' from the recoil. If his observation was correct, I can't help but think that firing that big cannon during a dog fight would make aircraft control very tricky. Further, for air-to-air, mixing (relatively) slow cannon with higher speed 50 cal. would make leading the target for both nearly impossible.
The picture that Just This Once posted helps, but it is hard to appreciate how big a 50 cal round really is if you haven't seen one. I have a bottle opener made out of an inert 50 cal round and it is HUGE!

One analysis.

WORLD WAR 2 FIGHTER GUN EFFECTIVENESS

P-39D-1 and P-400 [ex-RAF ordered Airacobras] used the 20mm cannon...

Megan

Thanks for that link - it really covers all the bases!!

Many of the fighters had the different weapons boresighted to give a concentration of the different trajectory weapons at a certain range. This gave a reasonable sighting solution. Also, many of the cannon were giving high rates of fire that allowed their mixed use (most notably by German piston fighters) simply as a combined weapon. The use of very large slow firing guns was different, and they would usually be used to take individual aimed shots.
Cheers

OAP

Well it could be, in the right hands: there are also accounts of Ki-43s downing American heavies (from memory, early model B-17s ie without turrets) with the absurd armament (for 1941-2) of 2x 7.7mm guns! The quality of the IJAAF pilots early in the war made up for the apparent deficiencies - not just armament - of their mounts. Even the later Oscars had 1x 7.7mm gun and 1x 12.7mm...


The Hurricane IIB entered squadron service with 12x .303s I think? Although the IIC and IID appear to have been more successful, albeit mainly air to ground.