Adapting RCBS Precision Mic for 6BR and Similar Cases

The $35.00 RCBS Precision MIC is a well-made and useful tool for measuring cartridge headspace and bullet seating depth. The Precision Mic measures from a datum point on the case shoulder to the base. Unfortunately the Precision MIC is not specifically made for the 6mmBR Norma, 22BR, 6XC or 6.5×47 Lapua cases. Don’t despair. Reader Caduceus has devised a clever way to adapt a .308 Winchester Precision Mic for short cases that match the .308 Win in rim diameter and case body diameter. He simply creates a spacer out of a pistol cartridge. He trimmed a 9mm case to 0.511″ and “found this to be a perfect fit which gave a zero micrometer reading when the FL-sized 6BR case was placed in it.”

Caduceus explains: “I can use the 308 version of the RCBS Precision Mic to compare brass which has been fully sized in my 6BR body die with brass which has been fired in my chamber. With the spacer inserted, FL-resized cases mic 0.000” at the datum point on the shoulder. Using the same set-up, fire-formed cases measure +0.005”. In other words, my chamber has a headspace of +0.005″ above minimum dimensions. This is fairly typical of a custom rifle set up for switch-barrel use. If I were to FL-resize my brass down to minimum spec each time, this excessive working would shorten its life-cycle and might lead to case head separation. Now that I know the headspace of the chamber, I can substitute the standard shell holder on my press with a Redding +0.004” competition shell-holder. This ensures that my cases only receive 0.001” of shoulder set-back.”

Click HERE for a full article explaining how to adapt an RCBS Precision Mic for use with a 6BR. You can do the same thing with a 6XC or 6.5×47 case–just cut the spacer to a shorter length (for an 0.000″ mic reading). Note: You can also use this procedure with an RCBS .243 Winchester Precision Mic.