[Bart B.]

While "matching ammo to the chamber" (or other words meaning the same thing) are often stated as the path to best accuracy, with bottleneck cases headspacing on their shoulder, only two "matching" dimensions are critical. Most important one's the head clearance, or space between the bolt face and case head when the round fires. 2 to 3 thousandths is about perfect. Put another way, if the .308 Winchester round is at hand and the chamber headspace is 1.631", the cases should be 1.628" to 1.629" from case head to shoulder reference. This is easy to control with proper sizing die position in the press and a gauge to measure case headspace after firing and after resizing it. Diameter differences between chamber body and case body are not critical; no part of the case body touches the chamber except at its back end at the pressure ring. And a .001" difference in the case at that point means a .0005" difference in the bullet's tip in the barrel; an insignificant amount.

Such ammo centers at the front in the chamber by the case shoulder being perfectly centered in the chamber shoulder as it's pushed there by the firing pin when it fires. It doesn't matter how much clearance there is around the case body or case neck to the chamber walls. The back end of the case is pressed against the chamber wall opposite the extractor anyway. But up front, where it counts, even a .243 Win case centers its neck and bullet dead center in a .308 Win. chamber; the shoulder's are identical on each.

Best results typically occur full length sizing fired cases in a die whose neck diameter is 1 to 3 thousandths smaller than a loaded round's neck diameter. Without an expander ball (noted for bending necks too much coming up through the sized down case neck still inside the die), the case neck's diameters are only sized down then spring back naturally and are typically better centered on the case shoulder as the full length sizing die keeps everything aligned on the same axis.

Too many people have won matches and set records with the .308 Win. with ammo neck diameters about .336" in standard SAAMI chambers with neck diameters at .344". The .004" clearance around the case neck to the chamber is quite uniform all the way around; bullets are well centered in the bore. Sierra Bullets' .308 Win cases fit their test barrels that way and their match bullets shoot 1/4 MOA in their 200-yard test range.

Therefore, the bushing in a full length sizing die should be a few thousandths smaller than a loaded round's neck diameter. Or have a standard full length sizing die's neck honed out to the desired diameter and don't use an expander ball. Doesn't matter what the barrel's chamber diameters are. A much more important diameter to get correct is that of the bullet for best accuracy. They must be at least .0002" bigger than the barrel's groove diameter. Even a full thousandth larger (yes, a .3090" diameter bullet in a .3080" groove diameter barrel) will drive pins (they're smaller than tacks) at all ranges.

The other critical dimension's how far the bullet jumps to the rifling. Doesn't matter if it's zero. 5, 10 or 20 thousandths; whatever's best for accurate ammo. But it needs to be the same so variables don't change the pressure curve. For cases headspacing on their shoulders, the case shoulder is the reference to set the bullet to. The bullet's contact point on its ogive has to be the same distance forward of the shoulder, not the case head. Virtually all cases have a couple thousandths or more spread in this measurement and that spread gets transfered to the bullet's contact point with the rifling. I don't know of any bullet seating die that uses the case shoulder as the stopping point relative to the bullet's contact point in the seating stem. Unless you seat bullets so they're set back by the leade when chambered, a couple thousandths spread in jump is not a big issue; several thousandths is. And you have to keep seating bullets shallower in the case neck as the chamber throat erodes away.