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Rifles, Reloading, Optics, Equipment
Reloading
Polishing a body die?
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<blockquote data-quote="Bart B" data-source="post: 837189" data-attributes="member: 5302"><p>If that's true, what holds the case back when the firing pin smacks the primer and fires the round? No extractor I know of does that; there's too much clearance between the bolt face and extractor lip.</p><p></p><p>All of my .308 Win. over all case lengths got shorter each time they're fired.</p><p></p><p>They're driven hard enough into the chamber shoulder by the firing pin that sets the shoulder back a couple thousandths. (.30-06 case shoulders have been set back .006" or more by hard bolt closing or firing pin impact.) Then the primer fires and burns the powder while the case head is a few thousandths clear of the bolt face and the primer gets pushed out a few thousandths by chamber pressure.</p><p></p><p>As pressure builds, the thinner front part of the case body expands grips the chamber wall drawing brass from the neck and shoulder back in into the body area. The thicker back end of the case then stretches back against the bolt face seating the primer flush with the case head. The end result is the length from case head to case mouth is shorter than before it was fired.</p><p></p><p>I used to think that's the way things were with rimless bottleneck cases. But I've done a lot of tests about this and with loads reduced too much, the case doesn't stretch back against the bolt face and the primer's sticking out past the case head. There wasn't enough peak pressure to fully expand the case in the chamber. Loads more than 12% below max for the .308 Win. have often had this problem.</p><p></p><p>Belted magnum case belts stopping against the chamber belt headspace ridge from firing pin impact and having a bit of clearance between the case shoulder and chamber shoulder when fired, will stretch forward until the case shoulder ends up against the chamber shoulder. But the overall result is the case overall length is shorter after firing.</p></blockquote><p></p>
[QUOTE="Bart B, post: 837189, member: 5302"] If that's true, what holds the case back when the firing pin smacks the primer and fires the round? No extractor I know of does that; there's too much clearance between the bolt face and extractor lip. All of my .308 Win. over all case lengths got shorter each time they're fired. They're driven hard enough into the chamber shoulder by the firing pin that sets the shoulder back a couple thousandths. (.30-06 case shoulders have been set back .006" or more by hard bolt closing or firing pin impact.) Then the primer fires and burns the powder while the case head is a few thousandths clear of the bolt face and the primer gets pushed out a few thousandths by chamber pressure. As pressure builds, the thinner front part of the case body expands grips the chamber wall drawing brass from the neck and shoulder back in into the body area. The thicker back end of the case then stretches back against the bolt face seating the primer flush with the case head. The end result is the length from case head to case mouth is shorter than before it was fired. I used to think that's the way things were with rimless bottleneck cases. But I've done a lot of tests about this and with loads reduced too much, the case doesn't stretch back against the bolt face and the primer's sticking out past the case head. There wasn't enough peak pressure to fully expand the case in the chamber. Loads more than 12% below max for the .308 Win. have often had this problem. Belted magnum case belts stopping against the chamber belt headspace ridge from firing pin impact and having a bit of clearance between the case shoulder and chamber shoulder when fired, will stretch forward until the case shoulder ends up against the chamber shoulder. But the overall result is the case overall length is shorter after firing. [/QUOTE]
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Polishing a body die?
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