The RCBS Precision Mic is designed for only one caliber and the Hornady cartridge case headspace gauge will fit any cartridge.
I prefer the Hornady gauge used with a digital vernier caliper with big numbers for old eyes.
Below a Colt 5.56 Field gauge at 1.4736
Below the same headspace gauge in a adjusted Hornady gauge at 1.4735
Below a "fired" Lake City 5.56 case from my AR15 that give me a idea what the actual headspace is minus brass spring back.
I then use this reading to adjust my full length die for .003 shoulder bump.
I prefer the Hornady gauge because you can spin the case with your fingers and get more accurate readings.
With the RCBS Precision Mic you can't spin the case to center it and you get variations in the readings.
Below my RCBS Precission Mic and you can only use it with the .308 and .243. You have to open and close the gauge several times to get the case to center and get the shortest reading.
Below a fired .308/7.62 case in a Dillon case gauge, this case is a once fired Lake City case and fired in a military chamber.
Below the same case in a JP Enterprise case gauge, and this gauge is closer to minimum SAAMI diameter.
Meaning if the resized case fits in the JP Enterprise gauge it will fit in any chamber. And the Dillon gauge is larger in diameter and only checks shoulder dimensions.
Bottom line, if you buy bulk once fired Lake City brass that was fired in larger diameter chambers the JP Enterprise gauge checks case diameter. After sizing you are dealing with brass spring back and range pickup brass or once fired military brass may not chamber in your rifle.
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