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Firing from the blimp on a submarine that you spotted with radar in the dark night, what sort of gun can you penetrate it with? And I mean punch holes in the pressure hull that prevent it from diving.

20mm Oerlikon?
25mm (e.g. Hotchkiss, it's terrible as AA, but might be adequate for anti-submarine?)?
Something around 30mm?
37mm or similar anti-tank gun?
2-pounder pom-pom?
40mm Bofors L/60?

40mm Bofors is about as high as I reckon is plausible to carry on a blimp, unless we get into the short "derpy" infantry support howitzers with reduced charges, which would serve what later became grenade launcher duty in infantry support.

IIRC the Bofors had armour penetration of something like 65mm RHA, and using AP rounds (APHE by our standards) I think it might be able to crack a submarine's pressure hull. Can anything lighter plausibly do it?

But the 57mm with Molins autoloader (used on motor torpedo boats) might also work on the blimp... pretty sure it would be overkill against a submarine hull though.

I ask because you can carry more ammo for a gun than just 4 depth charges, and I doubt a 12.7mm Browning (found on K-class blimps) can punch through a submarine's pressure hull.

EDIT: Penetration of the pressure hull is the ideal requirement. However, inflicting other critical damage such as rendering the submarine unable to dive or surface effectively is also adequate, provided the weapon's modified rate of fire (which the blimp gondola must be able to withstand the recoil of over time) is enough to inflict such damage well before the submarine can crash dive.

In addition to AP possibilities, I also inquire about the calibre of HE shell (mortar type for thinnest walls, a howitzer type analysis is also welcome) required to "befriend" a surfaced submarine into flat-out surrender instead of trying anything else.
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