Air Gauging
A pneumatic method that measures bore diameter along a barrel's length, using airflow to verify the bore is uniform end to end.
Air gauging measures the inside of a bore without any tool touching the steel. A plug carrying small jets is drawn through the barrel, and the rate at which air escapes around it changes with the surrounding diameter; tighter spots restrict the flow and looser spots let more through. Reading that flow at points along the length tells the maker exactly where the bore runs tight, loose, or true.
This is how a shop confirms that a blank, or a freshly hand-lapped bore, holds its diameter uniformly rather than tapering or bulging. A uniform bore is one of the marks that separates a match-grade barrel from an ordinary one. The same data guides later steps such as choosing the right barrel contour for a given blank.