If you want to measure the bore, use Cerro Safe. It is a soft metal that melts easy. You plug the bore ahead of the chamber, melt the cerro safe in the bore and when it cools knock the cerrosafe plug you just cast and mic the throat. That will give you the exact diameter of the bore next to the throat. I get my CerroSafe from Brownell's. You can read the directions better there this is just what I remember. JMO Hope this helps.