We have a lot of threads that mention this. The US may have had a different mindset, but they were also trying out a bunch of different things in experiments/development programs.
The US air force did NOT think the M2 was adequate let alone optimal or they would not have funded multiple programs that lead to the M3 .50 cal machine gun.
This gun was used to a very limited extent in WW II, 6-8,000 produced but how many got overseas I don't know. However it was the standard gun in the jets that fought in Korea.
While pretty much identical in appearance to the M2 the M3 fired at 1200rpm (about 50% faster) so six guns in a jet were equal to about nine guns in an old piston plane.
The US air force was also NOT using WW II ammo in the guns. They were using the M23 incendiary round which carried a much higher amount of incendiary material than WW II incendiary ammunition and due to the bullets being lighter they had significantly higher velocity which reduced the amount of lead needed on deflection targets. Please not that this round was service trialed during WW II with not great results but the Air force persevered with not only several redesings but changing manufacturer twice in the search for better reliability/safety. Not quite what one would expect from a an air force that throught the WW II guns/ammo were "optimal".
During WW II the US ordnance dept also developed and number of guns and cartridges of larger caliber. This included 23mm guns (.90 cal) and .60 cal guns (15mm) in addition to high velocity .50cal guns. One of these programs involved necking the 20mm Hispano case down to .50cal. They got a lot velocity but you also need a gun the size and weight of 20mm Hispano to fire it. Very short time of flight to target but barrel life was poor.
The 20mm cartridge used in the Vulcan gun and the M-39 revolver cannon (used in the F-100 and others) is one of the .60 caliber rounds necked up to 20mm.
Think slightly over grown Russian 14.5mm anti-tank rifle round.
The US air force had an obsession with high velocity/short time of flight in order to reduce the problems with defection shooting.