Air gauging is a volumetric measurement used to insure a concentric and consistent barrel. A perforated rod is inserted into the barrel blank and a float slowly travels the length of the barrel while air is bled through the perforations in the rod. A highly sensitive flow meter will react to variances in the rate of air flow to detect any tight or loose spots along the barrel's length as the float travels along the bore. Something as small as a tooling mark or a speck of dirt will be detected if present and the problem corrected before the barrel is approved. Ideally, an air gauge barrel will have a VERY slight internal taper towards the muzzle that will allow it to grip the bullet securely until it exits, thus insuring a good gas seal as well as positive contact with the lands and grooves. This is actually somewhat dated technology and there are more accurate methods currently in use by many barrel manufacturers. Hope this answers your question.