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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Headspace -vs- Case Stretch
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<blockquote data-quote="Bart B" data-source="post: 669051" data-attributes="member: 5302"><p>For a .308 Win. case 2.005" long and charge giving normal, safe maximum pressure with a brand new case headspace at about 1.628" fired in a SAAMI spec chamber with 1.631" chamber headspace, impact from the firing pin will force the case shoulder hard into the chamber shoulder setting the case shoulder back a thousandth or two as the primer fires and starts the powder burning. The case neck and shoulder will expand first as they're the thinnest, then the body starting right behind the shoulder and working back until the pressure ring in front of the extractory groove expands against the chamber. The primer backs out of the case a little bit when this happens. The case head also moves back as the back half of the body expands until it stops against the bolt face pushing the primer back into the case flush with the case head. This case expansion pulls brass out of the case neck back into the shoulder and the fired case is a few thousandths shorter that when it was new; in this example, maybe 2.003" long.</p><p></p><p>Full length sizing that fired case makes it grow in length; how much depends on how much its diameters get smaller an how much the case shoulder's set back. It usually ends up a thousandth or more longer than when new; in this example, maybe 2.006". Neck only sizing the fired case lengthens the neck only a few ten-thousandths of an inch.</p></blockquote><p></p>
[QUOTE="Bart B, post: 669051, member: 5302"] For a .308 Win. case 2.005" long and charge giving normal, safe maximum pressure with a brand new case headspace at about 1.628" fired in a SAAMI spec chamber with 1.631" chamber headspace, impact from the firing pin will force the case shoulder hard into the chamber shoulder setting the case shoulder back a thousandth or two as the primer fires and starts the powder burning. The case neck and shoulder will expand first as they're the thinnest, then the body starting right behind the shoulder and working back until the pressure ring in front of the extractory groove expands against the chamber. The primer backs out of the case a little bit when this happens. The case head also moves back as the back half of the body expands until it stops against the bolt face pushing the primer back into the case flush with the case head. This case expansion pulls brass out of the case neck back into the shoulder and the fired case is a few thousandths shorter that when it was new; in this example, maybe 2.003" long. Full length sizing that fired case makes it grow in length; how much depends on how much its diameters get smaller an how much the case shoulder's set back. It usually ends up a thousandth or more longer than when new; in this example, maybe 2.006". Neck only sizing the fired case lengthens the neck only a few ten-thousandths of an inch. [/QUOTE]
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Rifles, Reloading, Optics, Equipment
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Headspace -vs- Case Stretch
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