WD40 is not only a *lubricant* but an effective *penetrant* (read the label). It is the penetrant properties that are damaging. While these work well in removing rust and corrosion, they can be *devastating* to well kept metal surfaces.
Also, as penetrant, a small quantity of WD on bullets can neutralize the primers. Test this by putting a few WD40 coated rounds in a zip lock bag for a week. You're bound to get one or two "sizzlers" at the end of that time.