Unexpected Results using RCBS Precision Mic

DrSteve

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May 20, 2020
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Location
Virginia
I've been trying to figure out the readings I'm getting for my Ruger American 7mm-08. On some loads I did full-length sized, the reading is -2 thousandths before firing and between minus four to five thousandths fire formed. On some Hornady Americen Whitetail factory ammo, the rounds measure 0.000 before firing and between minus .001 and minus .002 afterward. Aren't the cases suposed to get longer so I can bump the shoulder back. I'm confused what would cause this.
 

DrSteve

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May 20, 2020
Messages
48
Location
Virginia
I’m using the RCBS Precision Mic which measures from the base of the cartridge to the datum line on the shoulder.
 

MagnumManiac

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Feb 25, 2008
Messages
3,150
Have you removed the primer cup from the fired cases?
I use the Precision Mic, if there are ANY high spots like craters on your primers it will give misleading results.
However, getting negative readings after firing leads me to believe you may be over tightening it on the first reading. It is a very light touch on the case you take your readings from.

Cheers.
 

Bob Wright

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Jan 23, 2018
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1,337
Location
Litchfield Park, Az.
You're thinking of a different tool. I'm using the RCBS Precision Mic.
I would say your brass isn't going to shrink at the shoulder.
I'm wondering if there is a calibration fault somewhere or your not getting a good feel for the tool. If you have someone that has the Hornady tool and calipers or this very tool, maybe cross check the measurements.
 

ShtrRdy

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Jan 14, 2012
Messages
3,464
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High Plains
I know the tool you're using. It seems many others aren't familiar with the RCBS Precision Mic.

But I don't know why you're getting the measurement you're getting. I've never tried measuring a new loaded case before firing.
 

misterc01

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Feb 15, 2019
Messages
598
Location
Florida Panhandle
So far in my experience with the Precision Mic, it does take a soft touch when using. You can over tighten it and get a "false" reading if you crank it just a bit hard. I have used it to check new factory ammo. new brass, fired brass and re-sized brass. I worked with it on each type of brass and practiced until I got consistent readings, which is how I found the "touch" for using it. It also helps to use a headspace comparator as a cross check. I have had ocassionally had brass that was only one or two thousandths longer or shorter after firing, with the majoiry being longer. Next mystery - why do some cases not fit in a case gauge when they measure the same as those that to do?
 

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