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Rifles, Reloading, Optics, Equipment
Reloading
How often do you bump the shoulder back?
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<blockquote data-quote="Bart B" data-source="post: 1138090" data-attributes="member: 5302"><p>Bottleneck cases headspacing on their shoulders of the right dimensions don't have their heads against the bolt face when fired. The firing pin drives them forward pushing their shoulder hard into the chamber shoulder (often setting that shoulder back a thousandth or more). There's a few thousandths clearance (head clearance in SAAMI glossary) between the bolt face and case head. That's needed on chambering the round so the bolt head goes into battery the same for all shots fired.</p><p></p><p>Extractors typically push the case back end against the chamber wall on chambered rounds. So the back end of the case is a thousandth or two off center in the chamber at that point. Not a problem.</p><p></p><p>When the round fires, the bullet's gone before the back half of the case expands against the chamber wall and its head against the bolt face. Greatly reduced loads have case headspace shorter after firing than before; proof the case body expands fully back only at close to normal peak pressure; but the bullet's gone proving low pressure loads don't push the case fully against the chamber limits. And their primers will be backed out a few thousandths because pressure wasn't enough to push the case head against the bolt face to push them back in from being pushed out a bit long before peak pressure is reached.</p><p></p><p>I've shot dozens of rounds with reduced loads down to 15% or more below max to observe this stuff.</p><p></p><p>Martin James (Jim) Hull, Sierra's first ballistic tech, was a good friend I shot many matches with. He may be the one most responsible for proving full length sized bottleneck cases fitting the chamber "like a t*rd in a punch bowl) [his other moniker] produced best accuracy back in the 1950's. Minimally sized so the fired case shoulder's bumped back a couple thousandths is all that''s needed for long case life and best overall accuracy. When Sierra finally made their first hollow point 30 caliber match bullet (The 168-gr. International), he was finally able to get 1/10th inch 10-shot groups frequently in their 100-yard indoor test range with any bullet larger than 24 caliber. No other fired case resizing tool or technique did that but the Lyman or RCBS standard full length sizing dies with their necks honed out to a couple thousandths smaller than a loaded round neck diameter. Sierra's tool and die shop modified their dies to do that. About 10 years ago, the benchrest folks finally started doing the same thing with dies custom made to size bodies down only .001" instead of a bit more that standard dies do; I've no idea why they took so long to do that.</p></blockquote><p></p>
[QUOTE="Bart B, post: 1138090, member: 5302"] Bottleneck cases headspacing on their shoulders of the right dimensions don't have their heads against the bolt face when fired. The firing pin drives them forward pushing their shoulder hard into the chamber shoulder (often setting that shoulder back a thousandth or more). There's a few thousandths clearance (head clearance in SAAMI glossary) between the bolt face and case head. That's needed on chambering the round so the bolt head goes into battery the same for all shots fired. Extractors typically push the case back end against the chamber wall on chambered rounds. So the back end of the case is a thousandth or two off center in the chamber at that point. Not a problem. When the round fires, the bullet's gone before the back half of the case expands against the chamber wall and its head against the bolt face. Greatly reduced loads have case headspace shorter after firing than before; proof the case body expands fully back only at close to normal peak pressure; but the bullet's gone proving low pressure loads don't push the case fully against the chamber limits. And their primers will be backed out a few thousandths because pressure wasn't enough to push the case head against the bolt face to push them back in from being pushed out a bit long before peak pressure is reached. I've shot dozens of rounds with reduced loads down to 15% or more below max to observe this stuff. Martin James (Jim) Hull, Sierra's first ballistic tech, was a good friend I shot many matches with. He may be the one most responsible for proving full length sized bottleneck cases fitting the chamber "like a t*rd in a punch bowl) [his other moniker] produced best accuracy back in the 1950's. Minimally sized so the fired case shoulder's bumped back a couple thousandths is all that''s needed for long case life and best overall accuracy. When Sierra finally made their first hollow point 30 caliber match bullet (The 168-gr. International), he was finally able to get 1/10th inch 10-shot groups frequently in their 100-yard indoor test range with any bullet larger than 24 caliber. No other fired case resizing tool or technique did that but the Lyman or RCBS standard full length sizing dies with their necks honed out to a couple thousandths smaller than a loaded round neck diameter. Sierra's tool and die shop modified their dies to do that. About 10 years ago, the benchrest folks finally started doing the same thing with dies custom made to size bodies down only .001" instead of a bit more that standard dies do; I've no idea why they took so long to do that. [/QUOTE]
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How often do you bump the shoulder back?
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