[tobnpr]

No real need to headspace an AR barrel where the barrel extension was installed by the factory- any more than you would headspace a brand new, off-the-shelf Remington 700...

As P.P. said, barrel headspace is set at the factory, as are bolt dimensions. Only an egregious factory error would result in a headspace issue.

In bolt guns that might use factory ammo- or fireformed cases from another rifle, I use go gauges.

Usually though, I use an average of fireformed cases to set the shoulder bump on my FL sizing die. This means the rifle might have problems chambering factory ammo ( but I don't use it)- but I'm working the brass less, specifically to the chamber to that rifle and maximizing brass life.