Caliber of guns on American Spitfires?

I know the US used Spitfires starting with the Mk V, many with the 4 x Browning/2 x Hispano "b" wing, and the later "C" wing fitted with the same armament. I assume that the US would have swapped the British .303 Brownings for their own .30 cal M1919 guns, as used on other American aircraft, but I can't find anything about it at all. I assume the two guns are basically a bolt-in swap (easily exchanged anyway), and it would make sense since I doubt the US would want to obtain .303 ammo just for a single style of fighter in temporary service. Against that is that A.) they had to have a special logistics train for the Spitfire anyway, not being a standard type, B.) the British extensively tweaked the Browning, raising the ROF and converting it to open bolt, so they aren't exactly direct equivalents, C.) having a non-standard type never stopped the British from using the M2 Browning in "E wing" Spitfires (or the 7.92mm BESA in their tanks...and wasn't there some 15mm BESA they used in Bren carriers or something?).
Anyway, US Spitfires, .303 British or .30-06 Springfield? Cannot find any place online that even mentions it, yet it seems like an obvious question to me.

The three Eagle squadrons formed in UK were part of the RAF until the USA entered the war, they transferred with their planes in September 1942 until those were replaced with USA aircraft.

The U.S. Navy also used Spitfires, VCS-7 used a mixture of second-hand Spitfires Mk.V...Spitfires of the US Navy — Articles | 1944 | history | Spitfire Mk. V

I would say that most of the US Spitfires were used at the ETO and MTO where the RAF was involved operationally. So the supplies with the .303 ammo weren't any trouble. As a result there was no reason for replacing the British made armament with the US stuff.

pbehn said:

The three Eagle squadrons formed in UK were part of the RAF until the USA entered the war, they transferred with their planes in September 1942 until those were replaced with USA aircraft.

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Maybe true, but they were not the only units. The 31st and 52nd Fighter Groups were sent to England without their P-39's, and were given Spitfires as an interim until their P-39's could be shipped in, or in the end, a better type of fighter. I'm pretty sure there was also at least one USAAF fighter group flying Spitfires over North Africa and/or the Med, whether it was one of these same groups or a 3rd, I don't know. SO you both say they just gave them British-spec Spitfires and they used them as they came? No new radios or anything? I thought US and UK radio systems were often totally incompatible.
spitfiresite.com/2010/04/uncle-sams-spitfires.html
EDIT: Nevermind, it says right in the article here that it was the same two groups that went to N. Africa. But they were not ex-Eagle squadrons.

Johnny .45 said:

But they were not ex-Eagle squadrons.

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It doesn't matter I woud say. Just they used Spitfires. But there was no need of replacing the British armament with the US MGs. Both the ETO and MTO including the N.Africa area were the frontlines where the RAF was involved .. so a part of the supplies for the RAF could be delivered to the US squadrons as well. Also IMHO there was no the need of changing of the radio sets because the commading systems at the operational areas based on the British stuff mostly.

From what I read it may or may not be possible to replace the actual guns and mountings in the field, replacing the ammunition boxes feeds etc is a bigger job. I believe the standard fitment for a 0.5" mg was in the position of the cannon when with 2x0.303 and the cannon moved slightly outward. In any case it is only replacing one type of weapon with the USA standard, it would still have the 20mm cannon.

I agree. There could have been the differences between the feeding systems of the guns. This would result in a need of introducing changes to the wing structure what couldn't be done in the field rather.

Wurger said:

I agree. There could have been the differences between the feeding systems of the guns. This would result in a need of introducing changes to the wing structure what couldn't be done in the field rather.

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The two MGs on the standard armament each have a box at either side, these boxes are too small for a decent number of 0.5" ammunition. Although nominally the same wing at the start, changing one to the other probably involved more work than fitting a new set of wings. As I have read the USA use of Spitfires was just a stop gap measure waiting for newer types to arrive.

He's not talking about 50cal but 30-06 which is virtually the same size as the .303.

Crimea_River said:

He's not talking about 50cal but 30-06 which is virtually the same size as the .303.

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It's a slightly longer round a 63 mm case as opposed to the .303s 57 mm case. There's probably some allowance in the mechanism of the ammo feed for variations in length but an extra 6 mm might just be enough to cause a jam.

I would say that it wasn't thought worth the effort to change as the Spitfire was only a stopgap.

As far as I know, the armament remained standard British equipment. American Spitfire pilots also used standard RAF flight equipment - parachutes, helmets (with British radio connection ) etc etc.

The .303 case diameter was 0.401 v. 0.441 in our .30-06. I suspect that the respective ammo links were therefore not compatible. Concur that the abundance of RAF/ETC ammo stocks would serve the AAF Spitty units rawther well...

Yep, the 30 06 links don't work with 303's. Someone sent our group a bunch of links for some 303 ammo we are making up for our Hurricane restoration and the links looked very similar but were too loose. Turned out they were for the 30 06 ammo.

You could probably make the change but why bother.

You need 4 guns per aircraft plus spares. Close to 100 guns per squadron.
You either need new, US guns or a major rebuild of the British guns, New barrels, bolts, extractors, recoil springs, perhaps ejectors. I have no idea if any items in the feed system need changing or not (stop pawl?). British guns fired open bolt, US guns fired closed bolt but that might not need to be changed.

Now what does this really get you? The only US planes using .30 guns in quantity in NA are the P-39s (perhaps A-20s?) . I don't know what mix of ammo the US was using in the .30 cal guns on the P-39.
US Army ground guns, rifles and machine guns, used a much lower ratio of AP and incendiary ammunition than aircraft guns. Perhaps 10%? By late 1942 the British may be supplying aircraft .303 in pretty much 50/50 AP and incendiary with a few tracers thrown in. The US ammo is a bit more powerful but if you don't have an almost equal mix of bullets you may be throwing some of your advantage out the window.

The C type 'universal' wing on the Spitfire had mounting provisions for four 20 mm Hispano cannons and 4 .303 Browning guns, though two cannons and 4 MGs were most common due to the weight penalty of 4 cannons. The 20 mm mounts could be used or adapted to .50 M2 browning and US lend-lease facilitated the use of 2 .50 and 2 20 mm guns.

4 .50 cal guns could potentially be fitted as well, and would make the most sense from sustained US logistics perspective (also reduce weight somewhat and be adequate for fighter vs fighter combat, less so for bomber interception), but the situations relevant to USAAF Spitfire use don't seem to merit that sort of swap.

.30-06 would definitely not work in .303 Browning guns and even if the receiver tolerances allowed it (they may not) a change in bolt (or bolt face and extractor) and barrel would be required for the different cartridge dimensions and case-head. The same ammunition boxes might be compatible, but if tolerances were too tight, the longer .30-06 might foul them, too. (7.62x51 NATO and 7.62x54mmR both tend to work in receivers designed for .303 British with barrel and bolt modifications, or just barrel and extractor modifications for the latter case as the flat bolt face used for rimmed cartridges already works fine with the larger, Russian case-head, but the larger rim usually requires extractor changes ... rimless cases could also work with the flat bolt face, but extractor changes are required there for sure) Some examples of .303 actions were even converted to 8x57 mm Mauser, but that too is closer to the overall length than .30-06 and only very slightly longer than 7.62 NATO.

So of .30-06 chambered AN/M2 aircraft guns were available, you'd tend to go with outright replacement/swapped guns, but given few fighters in the AAF were even using .30s (the P-40B/C, P-39, and very early P-51s being pretty much it), that wouldn't be super-appealing for logistics either. (the conversion to 4 .50s might be more worth the effort required, more for performance than logistics concerns and perhaps moreso if British 20 mm ammo was in short supply, given the supposedly questionable quality of some US batches, albeit it was worst when matched with US Hispano guns; in the P-38 it was less an issue as it had a re-cocking mechanism accessible in the cockpit, but the Spitfires, Mustangs, and USN aircraft did not ... I'm not sure if the few 20 mm armmed P-39 variants had that ability or not) The matched ballistics, rate of fire, lower weight, and ammo capacity/firing time of 4x .50s might have been a genuinely appealing field modification to make.

Now, if ammunition quality and 20 mm reliability was not in question, it'd be easier to just remove the .303 guns entirely to save weight and use just the pair of 20 mm cannons at max ammo load. (assuming they were the belt-fed Hispanos at least; if drum-fed then that's one more factor for switching to .50s)