The Winchester XPC .308 Winchester bolt-motion rifle was a late addition to the combination of guns for this report, and it carried out nicely with most of the loadings. (Photo courtesy of Steve Gash) Caliber is definitely the nominal bore diameter of the firearm. It is actually calculated in inches https://stephenbmwgp.ageeksblog.com/23608013/top-once-fired-308-brass-secrets