I say NO to WD 40 or other ad hoc lubricants for firearms. WD 40 will turn gummy in firearms after a few rounds are shot through it. It would be better to completed clean and put very little gun oil on inner workings of shotgun or rifle than to put WD 40 or other lub not specifically made for firearms. I also stopped using gun lub that has teflon in it, because it'll make the action of a shotgun or rifle very stiff in cold weather 20 F or less.