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Old March 15, 2013, 08:47 PM   #25
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 5,009
Lucas, some folks full length sizing fired cases testing them in rifles for accuracy have been equalling benchrest tiny group records, sometimes beating them, with ammo fired in loose neck SAAMI spec chambers since the 1950's. Chamber necks do not have to be "tight" for best accuracy. Sierra Bullets has proved that for decades.

I've got a .308 Win. headspace GO gauge and there's no neck on it. It's .4000" diameter datum on its shoulder matches the gauging ring's diameter in my RCBS Precision Mic. I know how its shoulder centers in the chamber; exacly like a firing case does. Even made some contact tests and evidence shows 90% or more of the shoulder makes full contact with the chamber shoulder but it's very even all around the case shoulder even before the primer fires. That tells me the case shoulder is well centered in the chamber shoulder when it fires.

And all my rifle's extractors push the back end of the case at its pressure ring against the chamber wall; some to the side, others to the top. That doesn't hurt bullet alignment with the bore enough for me (and many others) to be concerned about. It's easy to calculate exactly how much it changes bullet runout. Best part of it is that it's amount and angle is repeatable from shot to shot so it has zero impact on accuracy.
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Last edited by Bart B.; March 16, 2013 at 07:15 AM.
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