For the utmost in accuracy, the bolt face should be perpendicular (square) to the centerline of the bore and chamber. On production rifles, no special attention is normally given to the squareness of the bolt face as it relates to the chamber, which provides the gunsmith with another opportunity to improve the accuracy of a factory rifle. To use this tool, simply screw it into the receiver threads, apply polishing compound to the face of the mandrel and use a 3/8" electric drill to turn the mandrel and polish the bolt face square to the centerline of the bore. This must be done before final head spacing. Notes:Lap is .475" diameter to fit 243, 308, 30-06 case head diameter cartridgesLap center shaft length 4"