When playing with a new rifle I pour a chamber cast that includes an inch or so in front of the chamber, in addition to slugging the bore. If your bore mic's at .308, I would try the .309's, and be prepared to try some at .310 and .311 too. Most of my .30's respond well to .310 diameter so that is what I usually start with, and .311 for plain based soft lead bullets at low velocity.