At the time of the decision on the next armament for fighters, current fighters where still canvas, and the Lewis and Vickers were all that RAF personal knew.
A single hit from a 20mm cannon was assumed to take a canvas aircraft of the period.
The high speed of newer fighters only gave fractions of a second to fire and achieve hits.
Hence why 8 Brownings or 4 cannon was chosen (but never actually tested against aircraft targets!!)
The real world showed one must hit the pilot, fuel tank, or destroy the engine or main spar to down an aircraft.
Some have suggested a FF 20mm cannon, but it failed against modern / armoured aircraft. 530 Rpm is to slow, 60rds is not enough.
It could not penetrate the armour protecting the pilot and tank from behind, or shatter an engine or main spar.
20mm Hispano requires a new wing for spitfires ("C" wing), which means a break in production when Britain needs fighters, any fighter.
The Hurricane IID is a slug with 4 cannon. Brown is quite uncharitable about this model.
There is limited HE for it, until production starts, especially for fuses. 50% as plain ball until late 41, little better than any other MG ball ammo.
Most pilots are green, and can't hit the side of a barn. They need lots of fire to hit.
There is a huge stock pile of .303 ball, but not AP or Incendiary rounds.
The M2 Browning was ready by 1933 and in production. The problem of the '20s model M21 were solved.
Aircraft were been designed with M2 in the mid -'30s. One of the earliest is the P-36 Hawk, that first flew in 1935.
As said above .50 AP will penetrate 10mm at 1200m, and 19mm at 500m. It easy penetrated aircraft armour way beyond air combat range during BoB.
It would shatter an engine block, etc.
One could have put the M2 into a standard "A" wing, and removed the divider between to two inner .303' ammunition bays for the larger .50" rounds.
M2 weighs 29kg, 9kg heavier than 2 .303' brownings. 150rds .50"' ~ 17kg vs ~17kg of 2 x 350rd .303" ( 18kg difference)
(BTW Hispano is 60kg with 60rd drum, 15.5kg for 20mm 60 rds, or 38.5 kg per side or 77kg heavier!!)
IF the M2 had been dropped in, the production of .303" AP and incendiary would not be required, and production focused on the more effective .50" versions.
As these are pressed filled, fuseless, they are cheaper and easier to mass produce.
Although adopted for service British authorities were unhappy with the American Incendiary M1, considering it not to be bore safe due to the incendiary composition being pinched between the steel sleeve and the envelope. The development of a British incendiary was considered a high priority. The
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Lastly there was a shortage of .303" Brownings at the start of the war, as production could not keep up with aircraft.
Fighter were fitted with only 4 early on. Buying US M2 could have solved this problem.
Later production .303" Browning could have being fitted with heavy barrels and used by Royal Armoured Corps.
No need for BESA and 7.92mm. BESA production should have gone to M2 and .50".