That's assuming that the round would stay inside the barrel and not punch right through or detonate before it ever got to the breach. Do you know how unbelievably lucky a gunner would have to get to line up a PERFECT down the barrel shot? Just saying.
Well there was one very lucky guy that I read about --- Robert Stanford Tuck. While reading 'Fly for your Life', the book about his exploits, he describes the time when he was shot down over France. He had been on a sortie with his wingman when his aircraft's engine took a round which stopped the engine dead, and as they were at low level he only had a split second to find a field to land in and the one he happened to pick was slap-dab in the middle of an AA battery. The problem was, even though his aircraft was spewing smoke and everything else, as he was gliding in to land the AA crews kept shooting point blank at him which pissed him off, so he aimed at one of the gun placements and gave a quick squirt back at them.
Moments later he did a wheels-up landing almost right in front of the AA gun which was all shot up, and didn't make the other AA crews very happy so they started to beat him up after he got out of the Spit. But then to his amazement the Germans started to slap him on his back and shake his hand. He couldn't figure out what was going on because he thought they were going to shoot him for killing all their mates. Turns out one of his 20mm shells had gone right down one of the barrels of the 20mm quad AA gun which when exploding peeled the barrel back like a banana and this was what killed all the gunners operating it.
The Germans were quite impressed with his marksmanship ---- but it could also been the fact they noticed his aircraft had 29 'kill' marking on the side of his Spit and the Germans realizing they had captured one of the best --