The RCBS Precision Mic is an indispensable tool for safe and accurate loads that help to extend the life of brass. The Precision Mic measures from the datum point on the case shoulder to the base of the cartridge giving the reloader the exact SAAMI tolerance readings. Just a few twists of the Precision Mic and the chambers headspace and bullet seating depth to 0.001" will be determined. Function: Gives the reloader the rifles chamber headspace and bullet seating depth to 0.001"Notes: Measures from the datum point on the case shoulder to the base of the cartridge