As Robo said. Cerrosafe is a low melt temp material with unusual properties. After it cools it shrinks- so that it may be removed- and then- miracle of miracles- returns to it original size, so you can use a micrometer to accurately measure it.
Although in a pinch I HAVE plugged the bore, pounded lightweight aluminum foil into the chamber, pushed it out and measured it. But Cerrosafe does much better.