USAAF Beaufighter night fighters used 20mm and machine guns. P-39 used 37mm and 20mm cannons. On a Thunderbolt for example
ight .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