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.
Unexpected Results using RCBS Precision Mic
- Thread starter DrSteve
- Start date
Help Support Long Range Hunting Forum
Are you using a headspace comparator or just measuring oal of the case?
MagnumManiac
Well-Known Member
- Joined
- Feb 25, 2008
- Messages
- 5,557
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.
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.
MagnumManiac
Well-Known Member
- Joined
- Feb 25, 2008
- Messages
- 5,557
Is this brass being fired the first time?
Bob Wright
Well-Known Member
Are you zeroing your caliper with the headspace tool, especially after tightening the screw? Check it numerous times....?
You're thinking of a different tool. I'm using the RCBS Precision Mic.
The factory ammo was fired for the first time. The handloads were also fired for the first time after I full-length sized them.
Bob Wright
Well-Known Member
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
Well-Known Member
I know the tool you're using. It seems many others aren't familiar with the RCBS Precision Mic.
www.rcbs.com
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.
Precision Mic | RCBS
RCBS Precision Mics determine chamber headspace and bullet seat depth by measuring from the datum point on the case shoulder to the base of the case.
www.rcbs.com
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.
jimbires
Well-Known Member
neck size only until the brass starts to get tight to chamber , this could take 3 or 4 cycles . then measure the brass and bump the shoulder about .002 . new brass never fully expands on the first fire .
the precision mic is a good tool for this .
the precision mic is a good tool for this .
misterc01
Well-Known Member
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?
