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Rifles, Reloading, Optics, Equipment
Reloading
Concentricity - setting up dies - runout
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<blockquote data-quote="Bart B" data-source="post: 512346" data-attributes="member: 5302"><p>Well said; reality's often what we don't want to believe, but it's always best to believe reality. Best accuracy happens when all the real contributors are the same from shot to shot. Which is why I prefer to measure bullet runout by mounting the cartridge in the measuring tool the same way as it fits the chamber.</p><p></p><p>Bottleneck cases that headspace on their shoulder fit the chamber as follows:</p><p></p><p>First, they're pushed as far forward as their headspace limit allows by the ejector in the bolt pushing them there. Extractors have enough clearance between bolt face and case rim front edge to allow this. The case shoulder stops and perfectly centers in the chamber shoulder. It doesn't matter how much smaller the case diameter at the shoulder is from the chamber shoulder nor how much out of round both are; the front end of the round centers at that point. With ejectors external to the bolt (Mauser and pre-'64 classic Model 70) and in-line ejectors in the bolt, the case is driven forward hard into the chamber shoulder when the firing pin puts it there usually setting the case shoulder back a thousandths or two. In either situation, the front of the round's well centered up front.</p><p></p><p>Second, the back end of the case is pushed sideways by the extractor. The pressure ring on the case bears against the chamber wall from the extractor's force. So the back end of the case is a thousandth or two off center relative to the chamber. As both chambers and cases are not perfectly round at this point, the same case will be a different amount off center depending on how it's indexed in the chamber. </p><p></p><p>Third, because of the first and second conditions, how crooked the bullet is relative to the chamber axis depends on how well centered the case neck is on the case shoulder and how far off center the back end of the case is relative to the chamber. So any round whose diameter at the pressure ring's smaller than the chamber at that point's going to be off center at the back end of the chamber by half the difference in diameters.</p><p></p><p>So I made a cartridge spinner that has a nylon washer at the front end with a .400" diameter hole in it and a V block at the back end where the case pressure ring touches it. The dial indicator's put a tenth of an inch back from the bullet tip. Rounds are put in the tool pushing the case forward against the nylon washer and holding it down in the V block. I'm measuring bullet runout relative to how the round fits the chamber.</p><p></p><p>Note that if the distance between the shoulder datum point and bullet tip minus 1/10th inch is twice that of shoulder datum to pressure ring. when the round's chambered there'll be half as much bullet tip offset from chamber/bore center as the pressure ring's off center in the chamber.</p><p></p><p>I always full length size my belted cases such that their shoulder's set back no more than two thousandths. I don't want those cases headspacing on a belt after they've been fired. After using a standard full length sizing die with its neck lapped out to a couple thousandths smaller than a loaded round's neck diameter, I use a body die (made from the middle part of a full length die) to resize them all the way to the belt which traditional full length sizing dies don't do. That step right in front of the belt needs to be eliminated else the case won't chamber the same each time it's loaded.</p></blockquote><p></p>
[QUOTE="Bart B, post: 512346, member: 5302"] Well said; reality's often what we don't want to believe, but it's always best to believe reality. Best accuracy happens when all the real contributors are the same from shot to shot. Which is why I prefer to measure bullet runout by mounting the cartridge in the measuring tool the same way as it fits the chamber. Bottleneck cases that headspace on their shoulder fit the chamber as follows: First, they're pushed as far forward as their headspace limit allows by the ejector in the bolt pushing them there. Extractors have enough clearance between bolt face and case rim front edge to allow this. The case shoulder stops and perfectly centers in the chamber shoulder. It doesn't matter how much smaller the case diameter at the shoulder is from the chamber shoulder nor how much out of round both are; the front end of the round centers at that point. With ejectors external to the bolt (Mauser and pre-'64 classic Model 70) and in-line ejectors in the bolt, the case is driven forward hard into the chamber shoulder when the firing pin puts it there usually setting the case shoulder back a thousandths or two. In either situation, the front of the round's well centered up front. Second, the back end of the case is pushed sideways by the extractor. The pressure ring on the case bears against the chamber wall from the extractor's force. So the back end of the case is a thousandth or two off center relative to the chamber. As both chambers and cases are not perfectly round at this point, the same case will be a different amount off center depending on how it's indexed in the chamber. Third, because of the first and second conditions, how crooked the bullet is relative to the chamber axis depends on how well centered the case neck is on the case shoulder and how far off center the back end of the case is relative to the chamber. So any round whose diameter at the pressure ring's smaller than the chamber at that point's going to be off center at the back end of the chamber by half the difference in diameters. So I made a cartridge spinner that has a nylon washer at the front end with a .400" diameter hole in it and a V block at the back end where the case pressure ring touches it. The dial indicator's put a tenth of an inch back from the bullet tip. Rounds are put in the tool pushing the case forward against the nylon washer and holding it down in the V block. I'm measuring bullet runout relative to how the round fits the chamber. Note that if the distance between the shoulder datum point and bullet tip minus 1/10th inch is twice that of shoulder datum to pressure ring. when the round's chambered there'll be half as much bullet tip offset from chamber/bore center as the pressure ring's off center in the chamber. I always full length size my belted cases such that their shoulder's set back no more than two thousandths. I don't want those cases headspacing on a belt after they've been fired. After using a standard full length sizing die with its neck lapped out to a couple thousandths smaller than a loaded round's neck diameter, I use a body die (made from the middle part of a full length die) to resize them all the way to the belt which traditional full length sizing dies don't do. That step right in front of the belt needs to be eliminated else the case won't chamber the same each time it's loaded. [/QUOTE]
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Concentricity - setting up dies - runout
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