Old Old technology....the specific cause of many ammo failures due to penetration of primers AND gums up actions in guns over many years.
We often have to disassemble and tank gun parts in solvent tanks and then scrub with acetone to get old dried WD-40 residue off gun parts.
To clean/remove rust try KROIL www.kanolabs.com
for lubrication try militec www.militec1.com
for all the info.
Wiping down a blued firearm with kroil will wipe off surface rust, remove finger printing and protect the metal.
WD-40 just creates work for gunsmiths....which is not all bad !