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View Full Version : Measured headspace is different from fired cases?


d7cook
February 23, 2009, 00:10
I built an Izzy HB w/ a DCI receiver and an Imbel bolt/bolt carrier. The headspace is a little generous at 1.638 (the bolt will barely lock up on a 1.638 HS gauge). When I use my case mic on the fired cases they measure to .015 over (1.645). I resized some fire cases at different lengths to recheck and a 1.640 case will not lock up and a 1.638 case will just barely lock up. Why would the fired cases measure 1.645 when HS measures 1.638?

Thanks!

W.E.G.
February 23, 2009, 00:48
Fired cases from a semi-auto .308 will mic MUCH larger than the actual chamber dimensions.

Just for shits and grins, stick one of those fired cases in the chamber of your rifle right now.

Now let the bolt slam on it.

:devil:

Actually,.... don't do that. You'll hate me for it, 'cause you'll play hell trying to ever get the case out of the chamber. That sucker will be stuck eight ways to Sunday.

The case is still pressurized just enough during the extraction cycle that the fired case is still expanding as it is extracted. For this reason, the fired case will measure way larger than your chamber.

Fire a round with the gas turned off. If you can get the fired case out of the chamber without tremendous violence, you will have a fired case that is closer to a realistic "mold" of your chamber.

As an aside, 1.638 is excessive headspace. You really should replace the locking shoulder with one that is the right size.