@Olivier5,
In our navy, we used 20 mm "maschine-gun-like" gun (20/70 Mk 10) on landing crafts (LCM) as anti-airplane gun.
I've shot with it, not terribly successfully, since you had to move it with your feet and didn't have a proper training (wasn't my job either, but "they thought, since I was the second in command [during my time as a reserve officer] ...)
When I was as a conscrit on a coastal minesweeper, I'd been for some months on the the second team for our anti-aircraft gun, a 40 mm (40/70 Bofors 315).
During an air target practice shooting, our team nearly got the Noratlas once.

(I'm on that photo, too: the one with the binoculars, far left in the bridge wing, at the bearing compass.)