Lets you know instantly if a revolver barrel and cylinder are in perfect alignment and if cylinder crane or yoke has been sprung, thus throwing axis of each out of line with axis of barrel bore . . . for timing - “accuracy jobs” . . . locating cause of lead splatter . . . professional service inspections. An excellent tool to use when demonstrating to a customer “why”! The handle with knurled grip end is threaded to take any of the interchangeable heads, with sizes carefully selected to indicate very small misalignments. The .38 caliber heads are available in two diameters, one for Match guns that must be dead on for best performance and one for Service guns which aren’t always thoroughly clean and don’t have to meet quite as fine an accuracy standard. And, there’s a special head for the S&W K-22 which has a slightly undersized bore.