[vanir]

Dude, only a Heavy Cruiser can stand up to several anti-armour aircraft guns (pretty much anything from a MK-101 or Vickers-S or bigger). Converted PT-gunboats and MGB's used to run around the Pacific and Mediterranean sinking destroyers and light cruisers every chance they got, with twin 20mm deck guns and half a dozen .50's....

well, many interesting things happened in WWII... once P-39 wing has managed to disable steering and engine of (approx.) 5,000GRT tanker (Baltic Sea)... the vessel didn't sink though but grounded.
...and there was a story re-told zillion times by Black Sea Navy pilots about MBR-2 navigator who shot Bf-109 using flare pistol...
but when I imagine massive sinking of destroyers and cruisers by means of 20mm and .50's in real combat operations (not at firing range)... it reminds me famous discussion on UBI about Mustang-a-tank-buster... sorry. :wink:

Oops, oh sorry. By "sank" I meant put out of action wrt converted PT gunboats and yes of course in the Pacific. Mostly they attacked Jap barges afaik but still there are stories of leaving destroyers floating wrecks with small calibre fire. I was kinda economising this thought with the Tse-tse Fly's 6-pounder (Molins Gun) which has about 68mm case hardened penetration at 500yds from memory. In some spots that'd go through the deck of the Scharnhorst, let alone a destroyer which afaik this armament was intended capable of sinking (and destroyer-escorts).
I don't have armouring figures for destroyers off hand but I can say definitely most anti-armour guns will penetrate at least parts of several heavy cruisers just based on figures, turrets and decking for example. The MK-101/103 penetrates at least 30mm with AP rounds but over 50mm with tungsten cores (its muzzle energy is about that of the latest Mauser MK-30 using APDS shells). The Vickers-S penetrates around 50mm. But weapons designed for aerial combat like the MK-108 or the Oldsmobile 37mm of course couldn't do much to an armoured ship of any kind, penetration of case hardened armour for the 37mm T-9 is only around 25mm for example when fitted with AP rounds.

I simply wished to demonstrate that (small) ships of the line are definitely vulnerable to aircraft gunfire, considering the capabilities of some of the weapons carried. Generally of course in practise the targets were often transports and the objective to kill passengers and crew, hence the "strafer" variants of several US attack-bombers (though the B-25 did get a 75mm gun and I don't think it was for Jap tanks...).

Naturally however overall conclusions are symbiotic with those of the learned gentlemen here. Bombs really for sinking ships. Big ones or AP types, later rockets for things like transports. Big guns on aircraft aren't really very efficient for hard targets to begin with, and questionable in effect.
Just I think, a mistake to think one is invulnerable to aircraft gunfire in something like a destroyer, depending on the guns likely to be mounted.