Originally Posted by Waltech Jim
... please share with us the scientific method you used to get from the above to your statement below.
There is no scientific method. Just proper use of ordinary full-length sizing dies that are set in the press correctly. I'll list my (and others who've put 20 or more consecutive shots into under 2 inches at 600 yards and 3.5 inches at 800) techniques used for .308 Win. cases.
First, the full-length sizing die gets its neck lapped out to .002 - .003-inch smaller than a loaded round's outside neck diameter. This does away with using the expander ball (also known as a "neck bender") and lets the case neck be sized only down; not back up again which works the case neck excessively. Depending on the neck wall thickness of your various case lots, you may need several dies each with their necks at different diameters. Fired cases sized this way have much straighter necks than when an expander ball is used.
Second, the full-length die's set in the press such that the case shoulder's set back only about .002-inch. This means minimal case stretching as the round fires with its shoulder pressed hard against the chamber shoulder and its back end stretches back very little to the bolt face, then shrinks back a tiny bit.
Third, the full-length sizing die's body diameters at the pressure ring and shoulder should not reduce fired case diameters at these points more than about .003-inch. Sizing dies vary a bit in their dimensions, so use one that just does this.
Full-length sizing means reducing the fired case body and neck diameters as well as setting its shoulder back a bit. Body and neck diameter reduction limits aren't as important as shoulder set back. New cases can shoot groups virtually as small as resized ones providing their necks are straight, all necks have the same tension on the bullet and case headspace is no more than .003-inch shorter than chamber headspace. It doesn't matter that the case body's .004 - .005-inch smaller than the chamber. They all align themselves exactly the same way when they're fired anyway.
Sierra Bullets Head Ballistician, Martin Hull, used this method to test their bullets in their California plant. With WCC-58 7.62mm NATO cases and very good lots of 30 caliber HPMK bullets, their rail gun test bed produced 10-shot groups in the ones at 100 and 200 yards. I don't think Sierra's changed anything these days.