Whether you are setting headspace for .308 .243 or, .223 or 450BM (it does not matter), I would expect the go gauge to give you a little more headspace than the actual factory ammo you are, ultimately, intending to use. If that wasn't the case, you would likely find some factory ammo that would not fit when closing the bolt. Headspace needs to be set neither too tight nor too loose. If you wanted, you could set it to match your customized ammo, but then it might be too tight for the next guy who is using factory ammo. Therefore, back to Darkker's point, if the go-gauge is wrong, don't use it. It's a bad gage. If it's right, don't mess with it. If you have read something here to the contrary, I would be suspect of it, no matter the source.