While I won't use WD-40 to clean my guns, I do use it to wipe down the barrel of my hunting rifle after it's (with me) been sitting in the rain or snow.

I'm on the fence about the whole issue. On one side WD-40's website says it's safe for all sorts of surfaces and I've used the stuff for years in other applications. On the other side, if I'm not mistaken, WD-40 is a degreaser which may be counter-productive to applying any grease or lube in the future. And it dries to a film that helps lubricate. Is there a build up after years?

Since I don't know and there are conflicting opinions, don't feel I save that much money not to use products that were made for the job.