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Old January 20, 2013, 03:15 PM   #6
Senior Member
Join Date: December 4, 1999
Location: WA, the ever blue state
Posts: 4,678
Per the SAAMI drawing:

The belt on the case is between 0.212 and 0.220"
The belt in the chamber is between 0.220 and 0.227"
That would mean the the firing pin would push the cartridge forward between 0.000 and 0.015" for the cartridge to be stopped by the belt.

The shoulder of the case is between 2.1092 and 2.1162"
The shoulder on the chamber is between 2.1253 and 2.1353"
That means the firing pin would push the case forward between 0.0091" and 0.0261"

The shoulder should then grow between minus 0.0059 and plus 0.0261"
So with a random distribution of case and chamber dimension tolerances, the shoulder should grow 82% of the time.

But the brass and chamber distribution is not random within tolerances.
All my 7mmRM brass is between 0.213 and 0.0215" to the belt.
The chambers from factories are mostly 0.221" to the belt.
That means that the case moves forward between .006" and .008".
The factory chambers are ~~2.127 and the brass is ~~2.111.
The shoulder grows ~~.008 to .010" most of the time.
Then the case stretches .006" back to the breech face.

To get around this, when I ream a 7mmRM chamber for myself, I set the chamber belt space at 0.215".
My cases don't have to stretch.
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