I wouldn't use WD-40 on firearms except maybe as an emergency rust preventative, and that's only if my favorite products weren't already available. There's just far too many better (and affordable) products, and there's just as many stories of WD-40 polymerizing and gumming up delicate parts as there are alleged uses of it. You'd almost certainly have to soak the guns in another cleaner or oil (like Ballistol, as eth0up suggests) before you could ever use them again...

In fact, I've used Ballistol to clean up and lubricate rollers on sliding doors that were previously gummed up from excessive use of WD-40!