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Old May 5, 2013, 07:00 AM   #13
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
Sure Shot Mc Gee claims:
. . .first fired brass memorizes its very first chamber's profile. That other rifles chamber may have had excessive headspace issues and other weird things going on. Maybe its brass grew in length beyond suggested reloadable tolerance.
I disagree.

When first fired, a new .308 Win case is a couple thousandths inch smaller in diameter than the chamber. Its headspace (head to shoulder) is a thousandth or so shorter than chamber headspace and is usually greater than when new. It springs back from hard contact with the chamber walls.

It's also a couple thousandths shorter in case length; head to mouth. It gets shorter as it expands against the chamber then shrinks back a little, but never to original diameters.

How much that first-fired new case changes size depends on the difference between its dimensions when new and the chamber dimensions. The metalurgy properties of the case brass also has an effect on how much its size changes. It will not fit in a chamber whose diameters in the body area are smaller than its new body dimensions.

Full length sizing that fired case makes it grow in length about a thousandth longer than when it was a new case. Its headspace is determined by how much the sizing die allows it to move. If the die doesn't set the shoulder back, sizing down the body diameters may push the fired case shoulder forward; sometimes enough to not allow it to be chambered back in the rifle it was first fired in. The sizing die's dimensions relative to the fired case dimensions effects the amount of sizing that fired case.

And if the "other" rifle had excessive headspace, that fired case fired in a normal headspaced chamber would easily fit in a chamber with more headspace; diameters allowing it.
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Last edited by Bart B.; May 5, 2013 at 07:14 AM.
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