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Rifles, Reloading, Optics, Equipment
Reloading
Making a datum line gauge
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<blockquote data-quote="bigedp51" data-source="post: 1387491" data-attributes="member: 28965"><p>Don't over think the Hornady gauge and the SAAMI datum point.</p><p></p><p>You are just measuring a fired case and a resized case for shoulder bump. The datum point is to make sure a headspace gauge measures chamber headspace at one point of the chamber.</p><p></p><p> And you are just "comparing" the two case lengths after sizing for proper shoulder bump.</p><p></p><p>Below a Colt 5.56 Field gauge at 1.437</p><p></p><p><img src="https://i.imgur.com/F81aB6g.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>Below the same gauge in a adjusted Hornady gauge. But it is still just a comparison reading because of brass spring back after sizing and you average a few cases to set the die.</p><p></p><p><img src="https://i.imgur.com/kkoU6og.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>Below a "fired" Lake City case from my AR15 in the Hornady gauge. And then the die is adjusted for .003 shoulder bump.</p><p></p><p><img src="https://i.imgur.com/OJqNmQH.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>And your shoulder bump is your head clearance when the cartridge is chambered. And the distance the rear of the case has to stretch to meet the bolt face. And .001 to .002 head clearance is well within the brass elastic limits and not stretch and thin when fired.</p><p></p><p><img src="https://i.imgur.com/HK76WCp.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p></blockquote><p></p>
[QUOTE="bigedp51, post: 1387491, member: 28965"] Don't over think the Hornady gauge and the SAAMI datum point. You are just measuring a fired case and a resized case for shoulder bump. The datum point is to make sure a headspace gauge measures chamber headspace at one point of the chamber. And you are just "comparing" the two case lengths after sizing for proper shoulder bump. Below a Colt 5.56 Field gauge at 1.437 [IMG]https://i.imgur.com/F81aB6g.jpg[/IMG] Below the same gauge in a adjusted Hornady gauge. But it is still just a comparison reading because of brass spring back after sizing and you average a few cases to set the die. [IMG]https://i.imgur.com/kkoU6og.jpg[/IMG] Below a "fired" Lake City case from my AR15 in the Hornady gauge. And then the die is adjusted for .003 shoulder bump. [IMG]https://i.imgur.com/OJqNmQH.jpg[/IMG] And your shoulder bump is your head clearance when the cartridge is chambered. And the distance the rear of the case has to stretch to meet the bolt face. And .001 to .002 head clearance is well within the brass elastic limits and not stretch and thin when fired. [IMG]https://i.imgur.com/HK76WCp.jpg[/IMG] [/QUOTE]
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Rifles, Reloading, Optics, Equipment
Reloading
Making a datum line gauge
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