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Rifles, Reloading, Optics, Equipment
Reloading
Flattened Primers? Help?
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<blockquote data-quote="boomtube" data-source="post: 566374" data-attributes="member: 9215"><p>Bart: <em>"I strongly disagree with this premise. Here's why."</em></p><p> </p><p>You are correct on most points but I disagree with your disagreement and here's why.</p><p> </p><p>Yes, the firing pin impact drives cases forward and, with bottle necks, it's usually about 1-2 thou, somewhat dependant on shoulder slope and brass hardness/thickness. </p><p> </p><p>The detonation of the primer adds to that driving forward force and the explosion instantly drives the primer cup hard against to the breech face, there's no need for powder pressure to do so. Add that 1-2 thou to whatever slack we may have overly resized onto our cases and the primer will be that much proud of the case head. And, yes, case head stretch-back will normally reseat the primer (but it won't "suck the shoulder back, chamber pressure will normally keep the case fore part pinned to the chamber). But not always.</p><p> </p><p>With the primer protruded as far as it can go and chamber pressure rapidly rising, it is not uncommon for the very thin primer skirt to be locked as hard to the pocket wall as the case is to the chamber. That can easily prevent the primer skirt from sliding back into the pocket as the head moves back, the effect won't be totally consistant for a lot of reasons so there will be variations in how much flattening we may see from round to round. But, it's certain that when a primer cannot slide in the ONLY way the exposed primer can move is sideways, pushing itself into the radiused mouth of the primer pocket. Result varies from a mildly flat to full "muffin top" primer, and all done at normal pressure, no excess pressure is required. Proper FL resizing to achieve minimum shoulder set back is the cure.</p><p> </p><p>IF a case has been neck sized, OR properly FL sized so it's a snug chamber fit, the primers can hardly move back and there will be very little flattening even at very high chamber pressures; the primers in a blown rifle may look quite normal!</p><p> </p><p>Thus, reloaders expecting flattened primers to be a reliable pressure indicator are making a mistake.</p></blockquote><p></p>
[QUOTE="boomtube, post: 566374, member: 9215"] Bart: [I]"I strongly disagree with this premise. Here's why."[/I] You are correct on most points but I disagree with your disagreement and here's why. Yes, the firing pin impact drives cases forward and, with bottle necks, it's usually about 1-2 thou, somewhat dependant on shoulder slope and brass hardness/thickness. The detonation of the primer adds to that driving forward force and the explosion instantly drives the primer cup hard against to the breech face, there's no need for powder pressure to do so. Add that 1-2 thou to whatever slack we may have overly resized onto our cases and the primer will be that much proud of the case head. And, yes, case head stretch-back will normally reseat the primer (but it won't "suck the shoulder back, chamber pressure will normally keep the case fore part pinned to the chamber). But not always. With the primer protruded as far as it can go and chamber pressure rapidly rising, it is not uncommon for the very thin primer skirt to be locked as hard to the pocket wall as the case is to the chamber. That can easily prevent the primer skirt from sliding back into the pocket as the head moves back, the effect won't be totally consistant for a lot of reasons so there will be variations in how much flattening we may see from round to round. But, it's certain that when a primer cannot slide in the ONLY way the exposed primer can move is sideways, pushing itself into the radiused mouth of the primer pocket. Result varies from a mildly flat to full "muffin top" primer, and all done at normal pressure, no excess pressure is required. Proper FL resizing to achieve minimum shoulder set back is the cure. IF a case has been neck sized, OR properly FL sized so it's a snug chamber fit, the primers can hardly move back and there will be very little flattening even at very high chamber pressures; the primers in a blown rifle may look quite normal! Thus, reloaders expecting flattened primers to be a reliable pressure indicator are making a mistake. [/QUOTE]
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Flattened Primers? Help?
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