Any more context on this image? My understanding is that AA guns are shockingly inefficient, and I'd expect a tank gun to be almost useless unless it's a specific tactical scenario.
I imagine it was most used against close air support, not high-altitude bombing. Seeing what to bomb in the dense woods would require flying pretty low and slow, which makes you a great target for a big semi-auto gun like this.
Another good motivator is that a bigass rifle is really flexible (sort of). You can shoot at vehicles, bunkers, snipers, harass tanks, and apparently also planes.
A twin-mounted full-auto variant was kept in service till the late 70s, mostly for anti-helicopter purposes, so it's probably a pretty decent piece of kit.
It probably was almost useless, but in WW2, such ad-hoc anti-aircraft measures were still attempted against dive bombers and the like.
Oooh, lovely detail about this beast: to counter the murderous recoil, the massive spring inside is so big the gun comes with an arbalest-like cocking mechanism instead of a normal handle.
Also, 20mm is .78 inch for those of you who hate metric, which explains the above.
