Thanks Glider for the aircraft gun penetration tests, they are dated July 1942 so pretty much destroys the "myth" that the .303" had similar penetration to the 0.50".
The tests actually show the 0.50" has about the same penetration as the 20mm! Which may be how the earlier myth somehow got twisted with the Spitfire .303" -> 0.5" changeover.
Actually one of the faults with the Zero was it had low velocity 20mm (60-100 rounds) and 7.7mm (500-680 rounds) until mid '43. 7.7mm(.303") may work vs Me109's but were quite useless vs P-38 and Corsairs!
It doesn't destroy the myth at all, those penetration values are from M2 brownings and ammunition not M1's that were available in 1939-40 as used in the RAF tests, those results are also when the guns are fired directly against plate, that's totally different to the same bullet that has to pass through an aircraft structure before reaching the armour.
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