Just found this:
about fitting .50 cal Brownings to Spitfires.
An interesting paragraph:
It should be noted that compared with other aircraft weapons of the day, the performance of the Browning was rather undistinguished, especially in comparison with aircraft cannon widely used by other combating nations. The American gun was also very heavy. On the other hand, the USAAF had found it extremely reliable and simply “good enough” in air-to-air combat. This way the Browning became standard armament on American fighters – the P-51 Mustang, P-47 Thunderbolt or the F6F Hellcat, and more, with the same arrangement retained even for F-86 Sabre in post-war years.