Armament of competing fighters. It would seem .50 calibre was considered and discarded in preference to 20mm.
As to why the guns were modified to use .303, the following is given as a reason:
"The combination of severe economic problems and vast quantities of standard .303" weapons and ammunition in store after the First World War militated against the adoption of any new calibre. The 1930s rearmament programme therefore saw such weapons as the American Browning aircraft machine gun and the Czech ZB30 (better known in its British incarnation as the Bren Gun) expensively redesigned to fire the rimmed cartridge....."