WWWJD, if your Forster bushing type neck sizing die caused case neck thickening in only a few reloads on a given case, I think there's something amiss in the die or how it's set in the press.
I mention this 'cause some folks have been using full length sizing dies without expander balls but proper size neck diameters on .308 Win. cases for decades getting dozens of reloads per case. And no case neck thickening or other causes of different amounts of case neck tension on bullets as they're seated.
Regarding best concentricity of case necks and bullets relative to the case, no sizing die puts the case neck, and therefore the bullet, best centered on a bottleneck case shoulder better than a full length bushing type sizing die that sets the fired case shoulder back 1 or 2 thousandths. Such dies keep the case neck concentric with the case shoulder while the neck's sized down; dies that only touch the neck when sizing it and the case shoulder and body aren't kept in line with the neck axis end up pushing the neck off center on the case shoulder.
It's the bottleneck case shoulder that centers the front of the round (and the bullet) in the chamber when the round fires, not the case body as the case body's typically not touching any part of the chamber (except one point at its pressure ring at the back end) when the round fires. If such dies didn't do this, then the benchresters would not have pretty much given up on neck only sizing and moved to full length bushing dies some years ago. While their smallest groups didn't get any smaller, their largest ones did shrink in size.
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Last edited by Bart B.; January 12, 2013 at 07:35 AM.