The British were working on the .303 Browning around 1930?
They knew they could not use the Vickers gun in remote locations (where the pilot could not beat on it with gloved fist) and trying to use Lewis guns, even in multiple, was not going to be a long term solution either what with the 97 shot drum.
They had entered into negotiations for the Hispano gun in 1936 and were building the factory in 1938 so any scheme that advances the US .50 cal has to fall well before 1937-38.
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This and it's classmates had been built to a 1930 specification with four guns desired.
It just took a while for adjustments to be made and factories built
They still had plenty of time to take another look at the .50 Browning once the M2 variant was introduced in 1933 (the same basic receiver was used across different M2 models from the M2HB heavy-barreled air-cooled infantry weapon to the water-cooled version and the AN/M2 aircraft version). That could've ether been with plans to rechamber it in the .50 Vickers, adopt .50 BMG, or potentially consider another caliber, like 13.2x99 Hotchkiss. (though I don't believe the British had adopted the Hotchkiss 13.2 mm AA gun in any capacity, so the interest there would be less appealing logistically, even if the higher explosive/incendiary payload might be appealing ... though both the .50 vickers and .50 BMG have much more potential there to exploit than any rifle caliber gun, and using the Hotckiss ammunition as a basis for internal R&D shouldn't have been too difficult)
The M2 variant addressed many of the shortcomings of the earlier iterations of the M1921 and would've been the point to start looking at it seriously. Prior to that, the .50 Vickers itself was a better option in all practical contexts. (prior to the M2, aside from reliability issues, the .50 Browning could also only feed from one side, making installations in aircraft more problematic and possibly contributing to the USAAC's use of a .50 cal nose gun only on one side, albeit the M2 variant was introduced around the same time any .50 cal installations on fighters seems to have occurred ... though at least the YP-24's use of a .50 cal gun predates it)
I'm not sure of the rate of fire of the initial AN/M2, but it should've been at least 750 RPM, given that was already achieved at the highest recoil buffer calibrations with the older 1924 version of the M1921 tested in British trials in the 1920s. (both the water cooled Vickers and 1924 Browning were capable of 750 RPM there, and the only noteworthy thing mentioned was that the Browning became less reliable at low rates of fire than the Vickers)
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In hindsight, had the British adopted the M2 Browning in SOME form in the late 1930s, they also could've taken advantage of the modifications the FN variant made in 1939 and potentially add those modifications to their own production lines.
It's also worth noting that the British manufacture of .50 BMG ammunition for export in the 1930s was a bit more powerful than the American stuff, though like the American ammo, the "Ball" ammo was actually mild steel core with a thin lead envelope between the jacket and core (to provide sufficient plasticity when taking the rifling). Kynoch and Woolwich both made .50 BMG in Britain, with the former using 800 grain bullets and the latter 804 grains with a velocity of 2,580 ft/s. That's less muzzle energy than the 710 grain M2 ball, but significantly more than the 750 grain 2500 ft/s .50 ball ammo in use for most of the 1930s, however it was flat based, so despite the higher sectional density, the ballistic coefficient might not be as good as a lighter boat-tailed bullet. (though flat based bullets reduce barrel wear and also are more tolerant of worn our out-of-spec bore sizes as the base can expand under pressure, at least for the case of open-base FMJ bullets with sufficient amounts of lead plug material at the base to allow for expansion: note a base cavity is not needed for this except possibly for very low pressure ammunition)
That bullet weight of 804 grains = 52.1 grams, and just slightly more than the standard 802 grain 13.2 mm hotchkiss bullet and at nearly the same velocity (786.4 vs 790 m/s) so it should compare quite favorable in ballistically in that regard. (energy at range, trajectory, armor penetration, etc ... assuming similar core material)
See:
The .50 Browning gun and ammunition were being developed in the United States at the end of the First World War, but the war ended before the weapon entered service. Britain was aware of this development and first considered use of the Browning in late 1918 to counter German ground attack aircraft
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By 1927 Vickers also held the license in the UK for manufacturing the .50 Browning, as Armstrong Whitworth had held it and Vickers acquired Armstrong in 1927. Albeit obtaining a license for the improved M2 model may have required additional negotiation. (Vickers could've also worked on improving the existing M1921 design independently of Colt and possibly using some of the work being put into the .303 browning development as well given the similarities to the M1919 and M1921 such developments could be in .50 BMG or .50 Vickers in a possible lighter faster firing form akin to what the IJA did later on)
In addition to all that, the .50 Vickers could still have been adopted in its existing (or a slightly improved) aircraft form at 53 pounds and 700 RPM (probably 750 RPM possible) and used exclusively in synchronized mountings with the receivers/cocking levers accessible enough for the pilot to clear malfunctions (as minor as they might be) as installations on the P-40/B/C/Tomahawk and P-39 allowed (and various aircraft using the .303 Vickers, and the Hawk 75 export models if fitted with .303 or .50 Vickers as they had provisions for).
Given the relative performance of the Vickers to the M1919 (and derviatives) in synchronized mountings, it's reasonable to assume the .50 cal variant fared similarly and thus had its rate of fire reduced significantly less than the Browning. So the .50 Vickers would at least be faster firing than the M2 browning when synchronized. (perhaps 500~583 RPM if the Japanese Type 97's performance is representative of a typical .303 Vickers with 900 RPM reduced to 600~700 RPM when synchronized)
The Gladiator should've been simple enough to re-arm with .50 Vickers guns for the fuselage guns, but more modifications would've been needed to fit them on the Hurricane or Spitfire. In both of those cases, the upper nose area (like on the Tomahawk) seems like the best fit as there's little room in the fuselage sides. The instrument panel would need to be re-arranged a bit and portals in the front bulkhead/firewall cut to allow for access to the rear of the receivers (or at least actuator rod extensions connected to the charging lever to allow recocking and smacking of the cocking lever).
The Spitfire would need to have its main fuel tank reduced (the upper portion cut down to make room for the guns) and the Hurricane might be best with its reserve tank removed entirely (and avoid the issues with fire ... which might be even worse with hot gun barrels near by). Both could have their wing armament partially reduced both to save weight and make room for additional fuel tankage in the wings. (removing just the inboard two guns on the spitfire would add some space for fuel cells where the gun was, though the ammo box space wouldn't gain much given it's shared with the next gun over, but more importantly, you clear the barrel of the inboard gun from the wing leading edge, so you have the entire section of the leading edge from the root to the next gun over to put fuel tankage in; you might also add a small tank behind the pilot, possibly to help balance CoG with the added weight of the guns, though they're also pretty close to the CoG, so the LE tanks might be simplest and least vulnerable). So you'd end up with 2x .50s in the nose and 6x .303 guns in the wings.
The Hurricane could remove its two inboard guns and make room in both the leading edge of the wing and internal space where the guns and ammo boxes were for additional fuel. Ending up with 2x .50s in the nose and 4x .303 guns in the wings, rather like the Tomahawk. Alternatively, you could delete the outboard pair of guns along with inboard ammo boxes, increase the ammo load of the remaining two guns (stored outboard of the guns) and still have the space of the removed inboard ammo boxes to use for fuel, along with possibly additional tankage directly ahead of the spar there and in the wing leading edge. (that ammo box space alone is rather substantial, including some open space not occupied by the ammo boxes themselves, and far more than enough to make up for the missing 28 imp gal reserve tank) The guns could also probably be spaced far enough apart to avoid fouling the coolant header tank too much or requiring too much modification there.
They could even have considered installing .50 Vickers guns on their Tomahawks once they got them (and any of the Hawk 75s they got) given the weight savings, better rate of fire, potentially slightly larger ammo capacity, and closer matching ballistics to the .303 guns in the wings.
Given the mixed use of .303 and .50 guns, having closer ballistics would definitely be noteworthy, and the .50 Vickers Ball, AP, and SAP are a reasonably close match for the .303 AP Mk.VIIW (2,500 ft/s 174 grains). The SAP .50 would be closest at 2,470 ft/s with the others 2,540 (AP) and 2,580 (Ball), all 580 grains. (so sectional density would be higher and trajectory would differ a bit at longer range, with the .50 retaining energy better; though this just means the .50 guns could be used at slightly longer ranges and the .303s harmonized closer in for their more effective range limits). The .50 SAP tracer was 542 grains 2,380 ft/s.
OTOH the .303 Incendiary and Tracer bullets were ballistically poorer with the modified "De Wilde" type incendiaries were 152 grains at 2450 ft/s for the Incendiary B Mark VI, and 168 grains at 2370 ft/s for the Incendiary B Mark VII. TheTracer Mark VIIG (used up to 1939) was 158 to 168 grains and 2380 ft/s, but the later Tracer G Mark II (standard WWII .303 tracer) was 154 grains and only 2,300 ft/s. I believe it contained significantly more tracer composition and was even longer than the earlier mark (so less room for propellant, plus higher pressures for a given amount of propellant, as the bullet takes up space in the catridge case). The longer bullet length increases ballistic coefficient somewhat, but the lower sectional density combined with the lower velocity should have made for significant deviation in trajectory from AP or ball ammo. Though if you stuck with exclusively incendiary and tracer ammo,it would work out better. The .50 SAP tracer would be the best match there, though still flatter shooting and longer ranged. I'm not sure what the velocity was for the .50 version of the De Wilde style incendiary. (that also appeared relatively late, or seems to have been introduced part way into the war, though probably could've been introduced much earlier had there been demand, ie if being used on fighter aircraft)
Of the existing .50 Vickers ammo trialed/introduced prior to 1940 (in this case trialed in 1935 and introduced with the Navy in 1940), the SAP tracer would also have been the only .50 Vickers round in British use with any sort of incendiary effect. Other than that, you'd have to rely on sparks or heat generated by the AP or SAP rounds to ignite fuel. And this would be the main interest for .50 guns over purely using .303, it could defeat the self-sealing fuel tanks used at the time by the Germans and greatly improve the effective target area. Armor plate penetration would be significant, but the ability to punch holes too big to seal would have been a much bigger consideration. (once the advent of self-sealing fuel tanks was recognized, at least) And even if there was some delay in getting better incendiary and/or explosive projectiles available in .50 Vickers, if used in combination with .303 incendiaries, the effect should have been quite good.
Some of these considerations are especially important if the .50 Vickers gun had been adopted late in the lead-up to the war as a stop-gap measure while the Hispano cannon was being sorted out. Vickers was already tooled-up to produce the .50 Vickers and it was in large scale use as AA batteries with the Navy (and limited use in tank and armored vehicle mounts with the Army), so introducing the aircraft version wouldn't require the same sort of investment or delays involved with trying to license and adopt yet another type of gun while trying to get the Hispano worked out.
They were also using the Oerlikon type S AA gun, but attempts to use the type S as a stop-gap for the Hispano didn't work out very well in underwing pods on the Spitfire or Hurricane. (though perhaps might have worked OK in a nose mount on a twin-engine fighter, and at least you have the potential for 100 round drums there vs the 60 round Hispano; it also weighed more than the Hispano at ~68 kg, much more than the lightened FFS, and roughly 2.83x as much as the .50 Vickers aircraft gun, or 1.42x as much as a pair of them ... and even with the weight of the synchronization gear included, the pair of .50 vickers guns would likely be close or or less than just one Oerlikon type S and not far off from a single Hispano either)
This would've been easier if the air ministry had been thinking ahead and requested that the Hurricane and Spitfire at least have provisions for synchronized guns as well. And had they realized the need for a stop-gap soon enough, the RAF should've had proper incendiary ammo introduced for the .50 as well. Low explosive or incendiary rounds would be further significant as good "spotter" rounds to make hits on targets obvious (the De Wilde type incendiary bullets were great for this). There's a good argument that this sort of ammunition was more important to have than tracers as an aiming aid, and especially useful for this case as the .50s would out-range the .303s yet have close enough trajectory at shorter ranges to allow for ranging-in with the .50s and then opening up with all guns. (provided the two batteries of guns were on separate trigger mechanisms, somewhat like the 109Es did with the 8mm nose guns having significantly longer range and flatter trajectory than the MGFFs in the wings)
Albeit this latter point also would've still partially applied to a simple installation of .303 guns in the nose with larger ammo capacity than the wing guns. (lack of convergence zone limitation would allow firing at a wider array of ranges and use of the nose guns as an aiming aid without wasting wing gun ammo) This was useful on some American fighters with synchronized guns and separate firing triggers on wing vs nose guns. I don't think the P-36 or P-40/Tomahawk allowed this, but I think the F2A did. I think the P-39 only had the 37mm (or 20 mm) cannon on a separate trigger and I'm not sure about the early Mustang variants with synchronized nose guns. I think the P-47 allowed the inner and outer pairs of .50s to be selected separately, though that's not quite the same thing. I'm not sure about the P-43.
