When the British tested the 0.5 in Browning vs their Vickers 0.5 in machine gun, they found very little to chose between the two, and neither to be particularly superior to the 0.303 in. I know there are people who still think the M2 was the be-all and end-all of WW2 aircraft guns, but both the USN and the USAAF were looking for a better gun throughout the war; they just couldn't get the 20 mm Hispano to work properly, because of poorly thought out design changes made by the US Army's ordnance division. Post-war, the USN abandoned the 0.5 in pretty quickly; they USAF got rid of it when they found it ineffective in Korea.
A bit simplistic.
During the British "trials" they were testing water cooled AA guns, not aircraft guns although the basic mechanisms were the same. One problem for the big guns was that they had very little difference in MV over the .303 so time of flight over useable distances were the same. They made a bigger hole, but other than that target effect was not that much better, The US ammo for the .50 was good for about 2500fps MV which meant that it's prime advantage over the Vickers was that it used a longer, heavier buller. But since it used a steel core again, not much difference in target effect. I don't think the test AA guns even got to 600rpm.
We have gone over this many times but it seems to be an enduring myth that the .50 remained little changed through it's service use.
Around 1940 the US did two things with the .50 cal.
1. They increased the velocity from 2500fps to about 2880-2910fps and since kinetic energy goes up with the square of the velocity this increased the hitting power of the bullets that hit something more than sheet metal. It also made air to air shooting a bit easier as the pilot didn't have to lead the target quite as much.
2. they increased the rate of fire from 600rpm (if you lucky and using short belts) to about 800rpm. A 33% increase in firepower for wing mounted guns (synchronized guns are another story)
Shortly thereafter they did something else, they adopted an Incendiary round close the British Dewilde round. However this got phased out in 1942-43 in favor of the M8 API round. in which a small quantity of incendiary material was placed in the nose of an otherwise unmodified AP round. However it did mean that just about every round in the belt was an AP round with that small amount of incendiary instead of using mixed belts.
The US was still not satisfied and addition to .60 cal programs (15mm) and .90 cal programs (23mm) they were working on high velocity .50s, high rate of fire .50s, and more destructive ammunition. The super high velocity .50s (and .60s) burned out barrels too fast. The high rate of fire .50 was finally produced using a T number in 1944 and was standardized as the M# at about 1200 rpm ( 50% increase in firepower over the M2 using the same ammo) and the more destructive ammo took a long and tortuous path. Dragondog's father may have been involved in early combat trials in WW II, it took until Korea to see widespread use. It was a super-high capacity incendiary round that was fired (due to lighter weight) several hundred fps faster than the standard ammo.
AS far as the USN goes, they had abandoned the .50 as far as planning/policy goes in late 1944. No new fighters (and few other aircraft) were
ordered after late 1944. But it took a year to two/three years for those new order planes to show up, and after the war was over the legacy planes (built during the war) took even longer to go away.
The AIr Force split, using 20mm guns for bomber interception (night fighter or all-weather fighters) and .50s for fighters. But please remember that an F-86 with six .50cal M3s had the fire power of nine WW II .50 cal guns and had the new M23 incendiary ammunition to boot. It may not have been good enough but the AIr Force was not trying to use WW II armament in 1950-53 thinking it was good enough.
It also took years to develop a new gun and ammunition, the 20mm guns trialed in the F-86s in Korea had been in development for a number of years.
One's view of the US .50 cal can be highly dependent upon which slice in time the view is taken.