Thank you, Ken- your vivid and complete answer confirms my beliefs about the larger deck guns.
Toward the end of the war when Germany mounted the quad 20mm's and were ordered to duke it out with aircraft, the crew must have had a terrible time as the allies changed their tactics. They just stayed out of range till help arrived! The quad 20mm's must have been mounted permanently on deck. The 20mm cannons must have a more intricate firing mechanism than the heavy calibers.
I am familiar with the roller-lock design of the MG42 and know this design would be destroyed in no time with salt water immersion.
I can find no engineering design drawings or maintenance information that defines the mechanisms of the 20mm guns. Thats something that does not show up in the NRA take down manuals I would suspect that the breech locking mechanisms and firing pin assemblies would be vulnerable to heavy corrosion. I suspect that the bore would be a less troublesome device to maintain, possibly with a coat of grease.
If someone could guide me to a net discussion of the mechanical assemblies of the 20mm cannons and maintenance thereof, I'd appreciate it.

Richard