tucks luck,
when he got captured he did the same thing, put a 20mm round down the barrel of a flak and peeled it back. If the Germans hadn't thought this was so hilarious they would have probably killed him because he had just strafed and killed several of their buddies.
Are you sure there wasn't an enemy GV or another plane around?
Only time I've ever killed a GV with gun fire only (20mm or less) was a panzer. We had captured a field from CV and he was making the base flash afterwards. I was C-Hog.
Found him in some woods and did 2x top down all four 20mm guns blazing. I really didn't expect him to pop but he went BOOM with no-one else around either.
of course the PM accusations of cheating followed but you have to remember the force is strong in this one