We believe the Sinclair Concentricity Gauge is the easiest to use and most accurate gauge of its kind on the market. Precision made for a lifetime of use, the Sinclair Concentricity Gauge will accept cases up to 50 BMG. The Sinclair Concentricity Gauge is used to measure runout by rotating the case or loaded round on two sets of bearings that are captured in anodized aluminum blocks. These bearing blocks are adjustable longitudinally to allow for different case lengths or varying lengths of loaded rounds. The handloader can measure sized cases on the neck, and measure loaded rounds on the neck or out on the bullet. The bearing blocks ride in a milled slot in the gauge base plate. Thi Mfg: Sinclair International