I was a WD-40 champion for years, now I still think it's a great product when you need to remove moisture from a tool. But I do not use it as a lubricant.
Where WD-40 fails is that it turns gummy after a short period of time. And it also has poor cold weather performance. That's not what you want in a gun lubricant
Note that this was only a rust test, not a lubricant test and note the surfaces where sand blasted not polished as most in a gun would be.
I'm pretty much coming around to ATF for most of my gun/knife lubricant needs these days.