There is a very narrow, (.004") throat diameter range where maximum accuracy is obtained. Go smaller or larger and accuracy will suffer. By simply slipping the Plug Gauge into the barrel throat, it�s possible to tell at a glance when you are approaching the correct diameter. Warns you before you go too far. Also valuable as a diagnostic tool for guns brought in with accuracy complaints, (designed for chamfers cut with our tools - may not give a correct reading on guns with other degrees of chamfer). Each gauge is self-piloted, precision-ground, hardened and stepped to show minimum and maximum optimum diameters. 11� and 18� Gauge measures both chamfer angles. Sizes .22RF; .32; .38/.357; .38-40/.40; .41; .44; .45. 5� Ruger-Style Gauge available in .38/.357, .44 and .45 caliber only.