The throat diameter range for achieving maximum accuracy is extremely narrow, at 0.004 inches. Deviating from this range, either smaller or larger, will result in reduced accuracy. By inserting the plug gauge into the barrel throat, users can easily determine when they are nearing the correct diameter. This tool provides a warning before exceeding the optimal range. Additionally, it serves as a valuable diagnostic tool for firearms with reported accuracy issues. It is specifically designed for chamfers cut with our tools and may not provide accurate readings on firearms with different chamfer angles. Each gauge is self-piloted, precision-ground, hardened and stepped to indicate the minimum and maximum optimal diameters.