20mm vs ships
Things to be kept in mind...
Ship hulls are NOT hardened steel unless it is the armoured belt. From memory the standard plate on a merchant is 1/4 inch mild...that's roughly equivalent to 4.5mm of hardened plate (depending on who's plate). A 20mm AP from the Oerlikon should put a hole in it quite easily at it's effective range of about 1000m. Has a chance out to around 2000. Haven't crunched the numbers on this yet...
Biggest danger is not fire, but ruptured boilers. The AP rounds will puncture a boiler. A destroyer (essentially unarmoured, with high pressure boilers and oil burners) could be very seriously hurt by 20mm fire, assuming the firer lived long enough to complete the act. Aircraft attacks on the beam were a worry. From memory even the .50cal could score mobility kills on Japanese destroyers.
Piers
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