A Countersink Gage quickly and easily measures the top diameter of an internal or external chamfer (depending on the model) while also verifying countersink angle. 

A countersink gage has a cone ground to match the angle of the countersink it was inteded to check. 30°, 60°, 82°, 90°, 100°,110°, 120° are typical angles.  However a countersink gage does not display an angle, it displays the top diameter of the bore being measured.