For truly accurate bore dimension, you can always "slug" the barrel. This gives you something to hold in your hand to measure. Mil surp Russian rifles are notorious for having widely varied bore diameters, and by slugging them you know what bullets you need when reloading.
Use lead sinkers. Get some slightly oversized, the softer the better. On a really clean barrel, lube well and using a wooden dowel start knocking them down the bore. You'll feel spots where the bore is narrower than other spots. That is fine. What you are looking for is to identify the smallest portion of the bore.
What drops out the other end is an exact representation of your bore, ready to measure. I keep the sinker, identifying which rifle it goes to.