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Old November 19, 2012, 11:54 PM   #45
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
'Twas proved decades ago by Chreighton Audette that cases whose heads were out of square with the case body axis didn't shoot as accurate as cases with square heads. This happens most visible with bolts whose face ain't square with the chamber axis. Two-lug bolts show this problem the most.

When the high point of the case head aligns with the high point on the bolt face, that point's where the force of the case head slamming into the bolt face is centered. It's off center from the bore axis and the barreled action will whip more at an angle aligned with that off center high point.

Out of square bolt faces make new cases end up with an out of square head. This compounds the problem.

Squared up bolt faces tend to square up even the slightest out of square heads on new cases. These squared up cases tend to shoot more accurate from that chamber and bolt face 'cause their force axis is near perfectly aligned with the bore axis and the barreled action whips the same direction all the time for each shot.

Three or four lug bolts have the least problem with out of square case heads. Which is why folks shooting International Palma matches with their .308s using new cases (only thing allowed, either commercial or arsenal ammo) prefer 3 or 4 lug actions. They shoot these slightly out of square case heads the most accurate. And benchresters like 3 or 4 lug actions for the same reasons.

A big contributor to this is cases with quite uneven case wall thickness. When fired, all cases first grip the chamber right behind the shoulder; the thinnest part of the case body. As pressure builds, the case body expands more and the case body presses like a wave on the chamber walls working backwards. The case also stretches back; moreso on the thin side. This tends to make cases banana shaped. The side that stretched the most tends to push the case head aligned with that further back and it meets the bolt face first. There's no way to fix this problem with a case. There's been tools to measure case body wall thickness so the bad ones can be identified before they're shot.

Call those bad shots bad ones. Or wide ones. Or back if that'll turn 'em around and head back to you.
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