WD-40 will destroy your airsoft replica.
You should only use silicone oil and white lithium grease on or around your airsoft replicas.
WD-40 was designed in 1953 as "Water Displacement formula 40." It is designed to be a penetrating oil that loosens frozen bolts and clean surfaces of any water residue.
It contains:
50%: Stoddard solvent (mineral spirits, somewhat similar to, but not the same as, kerosene)
25%: Liquefied petroleum gas (presumably as a propellant, carbon dioxide is used now to reduce considerable flammability)
15+%: Mineral oil (light lubricating oil)
10-%: Inert ingredients (trade secrets)
This means that if you apply it to a plastic or rubber part, it will react with the material (penetrate it) and change it's molecular composition.
(Pics and description pending)