Precision Mic 338 Lapua Mag by RCBS RELOADING PRODUCTS A few twists of the Precision Mic and you'll know your chamber deadspace and bullet seating depth to 0.001 of an inch.The Precision Mic measures from the datum point on the case shoulder to the base and giving you spot-on SAAMI tolerance readings. An indispensable tool for safe and accurate loads that extend the life of your rifle brass.