That purpose is to exclude salt water from the sea, rain water from the sky and the constant exposure of the inner workings of the turret to humid, salt-air.
In truth and fact, those covers (or 'joints' if you prefer that term), were never actually referred to by "real Navy" and Coast Guard sailors - as what most uninformed ship modelers now seem to be calling or referring to those covers as "BLAST BAGS."
Those covers are in reality, generally referred to as "bloomers" - refering back to the days when women wore long-legged underwear that was billowly and extra large under their dresses and were seen only when the winds did not behave or when they were climbing stairs.
Technicially, those joint covers between the turret-face and the gun barrel, are actually called "gaskets," and thus provide the same benefits that any "gasket" in the civilian automotive world - or elsewhere in other civilian applications - would supply.
Just an "FYI"
-Bill C.
wcraik@sbcglobal.net