Uhm, it must be, or it's only using AP shells...
Used it today more then normal, Il2's attacked my PT so I opened fire at 3k range. Scored first hit at 2k in the wingtip of the IL2, no dammage, not even an aileron shot off. This was weird enough, so he came closer, in a shallow dive, hit him twice more at 1k both in the nose/cockpit. He started smoking then I died. Asked him what dammage he had, answer was "engine oil".
Now, this may happen one time. However, I hit 3 different Il2's at 2k range, all in the wing tips with the Bofors 40mm gun. (starting to learn to aim it

). I also hit one at 1k right after he took off, result that time was a fuel leak. No dammage at all on any of those I hit in the wingtip.
This just can't be right, I succeded in killing one (not sure, I might have gotten more then one) Il2 with the 40mm, that was after two hits in his right wing at a distance bellow 1k.
Now, this is a 40mm gun, that doesn't mean that it just makes twice as big hole then a 20mm, or 33% bigger hole then a 30mm. It means that it makes one hell of a bang when it goes off. One 40mm should be enough to down any plane, no matter where it hits more or less. I know the Il2 was tough, but not 40mm tough. Specially not when you can take it down quite easily using 50 cal guns in a single high speed pass.
I have film of the PT boat vs Il2 encounter if anyone wants to see.