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Old November 19, 2012, 06:27 AM   #6
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
Benchresters have known for centuries that best accuracy happens when the case neck's well (perfectly) centered on the case shoulder. As the firing pin drives a rimless bottleneck case shoulder hard into the chamber shoulder, that perfectly centers the case at that point. If the case neck ain't well centered on the case shoulder, it'll be off center in the chamber at that point. Even case necks that have a .001" spread in wall thickness are better centered on shoulders with full length sizing dies. Go measure neck runout on cases relative to the shoulder center (not the body as cases ain't perfectly round) and you'll see the difference.

Any fired case sizing tool and process that doesn't size a case such that its neck isn't centered on the shoulder won't produce best accuracy. Benchresters finally figured this out a decade or two ago and quit using neck only sizing dies of any type including the bump dies that sized necks and also pushed the fired case shoulder back a bit. The only way they could get those perfectly centered case necks on case shoulders was to hold the case body in place while sizing the case neck. Full length sizing dies do this better than neck sizing dies and these are what benchresters use these days to shoot the best groups. Competitors shooting shoulder fired rifles producing the best scores have been using full length sizing dies for over 50 years to do this.

These full length sizing dies have bushings in them a thousandth or so smaller than a loaded round's neck diameter. Their body dimensions are about two thousandths smaller than the fired cases from their rifle's chamber so fired case bodies get sized down about one thousandths. And the fired case shoulder gets set back about a thousandth of an inch, too.

While the benchrester's smallest groups shot still are about the same size, the largest ones that determine aggregate group average are smaller. This is evidence (proof?) that proper full length sizing fired cases produces the most accurate ammo.

One other thing; none of them use a collet die to size necks if they want to be competitive.

The above aside, if one insists on using a collet die to size their fired case necks, go right ahead and do it. Neither the ammo nor rifle knows about it. Otherwise, get a full length sizing die that uses bushings from RCBS or Redding and follow their instructions for picking the right bushing diameter for your cases.
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