Dannyboy, the case neck's dimensions have nothing to do with how a rimless bottleneck case centers its neck in the chamber neck when loaded and fired.
When chambered, if the bolt's got an in line ejector, that pushes the round full forward into the chamber where its shoulder centers in the chamber shoulder. Its back end gets pushed where the extractor presses it against the chamber wall, usually. When the firing pin smacks the primer, that further drives the case shoulder hard into and well centered in the chamber shoulder before the firing pin dents the primer enough to crush the pellet against the anvil and finally fires the round. And there's typically a bit of clearance all around the case before and when fired except at its back end that's pressed against the chamber wall. Rounds do not lay in the bottom of the chamber except when they're in it and the bolt's open and not touching the case head whatsoever.
So, all that's needed is to have the case neck well centered on the case shoulder after resizing that fired case. It floats in the chamber neck untouched by anything except the bullet in it; there's clearance all around it. Full length bushing dies center sized necks on shoulders better than neck sizing dies. Doesn't matter if the neck's .001" thicker on one side than the other. The bullet will well align itself in the rifling when it gets there. Or seat bullets out far enough to jam gently into the rifling when loaded.
All this aside, one will get better accuracy if their barrel's groove diameter is right for the bullets used (.0003" smaller or more than the bullet diameter), very uniform dimensions and twist rate and the receiver's properly epoxy bedded in the stock; totally free floating the barrel. Sloppy handloads will shoot very accurate in such rifles. If the rifle's not up to par, really tiny groups may well be a dream.
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Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
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