Ralph Barker in his torpdeo bomber book mentions how a 1944 convoy off the Dutch coast was first attacked by rockets to disable the larger escorts present (presumably with the best AA suite) and then the aircraft closed into to use cannon/MG on the remaining vessels. Entire convoy claimed as sunk.
On another note regarding the lethality of the 20mm also recall a Uboat commander claiming the 20mm on his uboat did not appear to bother the Wildcat which was strafing his boat. Attacked by aircraft from a CVE I am guessing.
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