I know a lot of Shooters who insist that Good Old WD-40 is all they need.  Maybe it does a lot of things well.  Okay, I know it does a lot of things well… But Firearms, fishing reels, locks, is not something you want to use it on, because it gets gummy after awhile. Read this.  All the way to the end.  Do you really want that in your gun?  Seriously?  You can think of nothing better?
Okay, we all know WD-40 doesn’t really have any Fish Oil in it.  But the fact remains that it’s a Water Displacement product that functions as a poor cleaner and an even worse lubricant.  I’ve been told by a lot of old school gunsmiths that they always use WD-40.  That’s fine.  But these guys were not really good gunsmiths.  The better gunsmiths that I know, will use Kroil.  It gets under the gunk, rust, and stuck parts because unlike WD-40, Kroil is a Penetrating Oil.  And it’s awesome for that.  I’ve used it myself working on several firearms… most recently one of my own 870’s. It’s a liquid tool and does it’s job well for what it’s for.  But again, it’s not a great Lubricant.  If you want a good Lubricant, use Slipstream.