FiveInADime, those folks I mentioned are not using benchrest style tight neck chambers; they hold rifles to their shoulders in 3 positions shooting high power matches which are different than benchrest matches where rifles are fired in free recoil untouched by humans except for a light finger touch on their 2-ounce triggers. Chambers in the most accurate ones are SAAMI spec or close enough to not make a difference; some tested in free recoil have shot groups smaller than benchrest records. Sierra Bullets uses the same loose-neck chambers and fired cases so sized testing their bullets for accuracy getting sub 1/4th MOA at 200 yards with their best match bullets. They also used factory sporting rifles with sloppy factory chambers to develop accurate loads for their reloading manuals. Sierra's first ballistic tech had a saying "Sized rifle cases have to fit the chamber like a **** in a punch bowl." In other words, tight necks ain't best for accuracy; a loose fit's best. Even commercial .308 Win. match ammo shot under 4 inches at 600 yards in sloppy, mil spec chambers in 7.62 NATO service rifles when they were popular; about 1/4 inch at 100 yards which ain't too shabby for an M1 or M14.
You may not have your full length sizing die down far enough in the press. If you set your full length sizing die to move the fired case shoulder back only 2 or 3 thousandths, the bolt will close easily. If there's any binding when the bolt closes, it won't be at the same position in the receiver for every shot; that's a detriment to accuracy. Using a case headspace guage such as the RCBS Precision Mic will enable you to see exactly how much you're setting fired case shoulders back. Both too much and too little hurts accuracy; 2 thousandths is about right for most applications.
I lapped out several dies as follows. Strip the die then chuck it in a lathe headstock bottom out. Check it for runout on the inside as you may have to shim it at one or two headstock jaws a thousandth or so. Split the end of a wood dowel then wrap 400 grit emery paper on it for a tight fit in the die's neck. Spin the die at a couple hundered rpm, wet the emery paper then run the "lap" in and out a few times. Use a hole micrometer to check the neck diameter and repeat until it's the size you want.
People measuring how well case necks are centered on case shoulders after sizing learned that full length sizing dies center the neck better on the case shoulder. All bottleneck cases headspacing on their shoulder center perfectly up front in the chamber when fired. Doesn't matter if the case body and case neck are several thousandths smaller than the chamber at those places. The case shoulder angle matches the chamber shoulder's angle. When the firing pin drives the case hard into the shoulder, it centers perfectly; therefore so does the neck holding the bullet. The bullet in .243 Win. case will center perfectly in a .308 Win. chamber neck when its fired. And as there's no such thing as a perfectly round case or chamber, when neck only sized case bodies get too big, they start interfering with the chamber body.
Besides, full length sizing fired cases straightens out quite a bit those that are banana shaped from stretching back more on one side than the other when fired because wall thickness on one side's thinner than the other. And the back end of all cases are pressed against the chamber wall by the extractor pushing sideways on the groove in the case head; they ain't centered at all at their hind end.
