Quote:
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...headspace is the distance from where the head of the casing meets the breechface, to the point in which the shoulders of the case meet the chamber of the barrel.
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That's roughly correct. "Headspace" is nothing more than the extra room that a cartridge has to rattle back & forth in the chamber when the bolt is closed. Every rifle's chamber dimensions are slightly different, but all are within a +/- spec of a few thousandths to the SAAMI standard.
For the 30-06, the chamber must measure
- At
least 2.0487" long (must "GO" Gauge)
- Preferably
less than 2.0547" long (factory standard: "NO-GO" Gauge)
- ...and
absolutely less than 2.0587" (Maximum safe "FIELD" Gauge
All commercial ammo is sized approx to/under the GO dimension to fit in all rifle chambers for that cartridge (even "tight" chambers having minimum dimensions). First firing will stretch the case to fill the chamber. Upon extraction (theoretically), the case will now have the actual headspace dimensions of that specific rifle. If left at that dimension (and the rifle had long headspace) that case might not fit
other rifles having smaller chambers -- even though those other chambers were within "spec".
The
re-sizing die solves that problem as it swages the case back down to (as much as) minimum headspace again so that it will fill
all rifles.... not just the one it was originally fired in.