Those will have been the tests that led to the 8-gun interceptor concept - a Sqn Ldr whose name I forget calculated that in a modern engagement, no-one could be expected to track the target for more than seconds 2, and hence armament had to destroy HM Enemies within that time. He concluded that this required 8x 1000 rounds per minute machine guns. Brownings had just become available and fitted the bill.
They were originally either .3 or .5, but the British version was bored out to .303. No idea why, but the proposal that it was for commonality sounds sensible.
Since when are our decisions sensible?