With few twists of the Precision Mic reloaders can determine chamber headspace and bullet seating depth to 0.001 inch. The Precision Mic measures from the datum point on the case shoulder to the base giving reloaders spot-on SAAMI tolerance readings. It's an indispensable tool for safe accurate loads that extend the life of rifle brass. - Determines chamber headspace and bullet seating depth to 0.001-inch - Measures from the datum point on the caseshoulder to the base giving reloaders spot-on SAAMI tolerance readings