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.