Ju-88 P-5 “Flying 88”
On the topic of stupid big guns on planes, let’s look at the Ju-88 P-5 which was some genius’ brainchild to arm a Ju-88 with a 8.8-cm gun. Only one prototype was built before the project was sensibly scrapped.
Conceivably it would’ve looked something like the top picture but with a big-ass 8.8-cm gun sticking out of instead of 7.5-cm.
The bottom picture is a schematic of the semi-auto mechanism of the 8.8-cm pod gun.