I used a Starrete depth micrometer, measuring from the face of the receiver to the bolt face, the way that you would to find correct headspace. It doesn't have tenths on the barrel, but I will guarantee my measurements within .0001. I've spent the majority of my life using such instruments.
This one is a magnum bolt face, but I've seen a .223 Predator bolt face with the same deviation. It's hard not to form an opinion when the few I've seen are like this.
