There are several ways.
One that gives you the most information is to make a chamber cast. There are some metals (alloys) that melt at very low temperatures (180dF). Brownell's markets one called cerosafe.
You plug the barrel just in front of the throat. Melt and pour in the alloy. It will harden, you punch it out with a ramrod after a specific time of cooling and measure it at the various diameters of interest at another specific time after removal.