Here we go again. Sorry that I cannot quote the source but from a reply in a gun magazine publication from a few years past a company rep stated unequivocally that the use of WD40 will NOT harm gun metal in any way. He stated that the only exception was firing rounds thru a bore that was not first swabbed and cleaned of its carrier.
Because no matter how much care is excercised in keeping WD40 away from ammo primers, the possibility of neutralizing them is there. For that reason alone I do not use it on my business pieces. Hope this helps.
"To earn a million is easy, a real friend is not."