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.