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. It also ruins the finish on wood stocks. Also, as penetrant, a small quantity of WD on bullets can neutralize the primers.
Ive always used Napier and Bisley and never had an issue, if it works don't fix it i'd say.