Here's my 2 cents on WD40 since I've been through the topic extensively. Should help wrap up the topic.

As most people know but often forget, the "WD" refers to water displacing. The reality is that the product is very good at doing its job- displacing water- but tends to fail when other uses are assumed. It's major ingredient- fish oil- is not a synthetic designed for long term storage and rust inhibition.

If your firearm gets wet, WD40 will help you prepare the surface for rust inhibition. Beyond preparation, there are other products to use.