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Rifles, Reloading, Optics, Equipment
Reloading
Known Loads too hot now..why
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<blockquote data-quote="Pdvdh" data-source="post: 385132" data-attributes="member: 4191"><p>Could you use 7mm WSM brass? Neck it down to .277 with your resizing die, and set the shoulder back to a snug headspace fit at the same time. My LoadBase 3.0 ballistic software shows a case head to shoulder measurement of 1.702" for 7mm WSM brass, versus 1.664" for both the 270 WSM and 300 WSM. If the 7mm WSM brass is ~0.038" longer from case head to shoulder, then it seems like this approach would also work. But it you can easily reset chamber headspace, that should cure the headspace problem also.</p><p></p><p>Differences in the brass life you described could be both in the properties of the brass (softer versus harder), the production headspace measurements of the brass, and even the friction coefficient of the case surface.</p><p></p><p>The effective differences between slicker than snot (low friction coefficient) brass/chamber interfaces and rough (high friction coefficient) brass/chamber interfaces - as related to case head thrust and case head separations - are as follows:</p><p></p><p>The slicker the surface the more the cartidge case head <strong>and</strong> sidewalls are able to slide back towards the bolt face. After they are set back firmly into the bolt face, the brass that then blows out to conform to the chamber dimensions is the thinner brass located at the shoulder area. This thinner brass can blow outwards in a gentler manner because the expansion of the brass is spread out over a larger surface area of the casing. ALSO, the brass in the shoulder area has been annealed, and it is able to stretch to fit with less chance of tearing than the harder brass back in the case web area. Little harm is done to the casing. And the case head strikes the bolt face with greater force.</p><p></p><p>With a rough, unlubricated, high friction coefficient case/chamber interface, the <u>forward portions of the case wall</u> are blown out and tightly grip the chamber with. So firmly that the <u>forward portions of the case wall</u> are locked to the chamber - immobilized. Tremendous pressures are generated and blow the case head back into the bolt face. The forward side walls are immobilized yet the case head is moving back into the bolt face. Something has to give. The brass yields just forward of the case head, where case web stretches (permanently thinning) and case head separations occur. </p><p></p><p>The reason the brass stretches at this location is because this is the first location moving from the shoulder toward the case head where the case wall thickness becomes rigid enough to prevent its being pressured out and locked onto the chamber walls. The thinner case wall is easily pressured out with enough force to tightly grip the chamber. At the case web, the web thickens to the point that the pressue isn't sufficient to lock the web onto the chamber. This is first location where the brass is able to shift/move along the chamber. So this case wall thins out at the location where it's too thick to bond to the chamber, and too thin and weak to survive the tensile force trying to stretch it apart. </p><p></p><p>Lastly, how does this stretching of the case web area reduce case head thrust (force) against the bolt face? The case wall stretches at a location where is starting to get thick. A fairly substantial tensile force is required to stretch, thin, and permanently deform this thickened case web. And this tensile force is restraining (trying to pull the case head away from bolt face. It's not a strong enough tensile force to prevent case head setback, but it is substantial enough to reduce case head thrust.</p><p></p><p>So if you want to accelerate case head separations and reduce case head thrust, roughen up your chamber to increase the coefficient of friction between the case & chamber. (Don't really do this!)</p><p> </p><p>If you want to slow the progression of case head separations and increase case head thrust, polish your chamber and lube your casing exteriors to reduce the coefficient of friction. BTW, this is what we all should want. The lugs on the bolt are designed to survive the additional case head thrust against the bolt face. The brass is the weak link, and the component that needs the tender loving care.</p><p></p><p>Hope this helps provide a better, conceptual understanding. If you understand these consepts, you will be better able to toubleshoot and resolve issues related to case head thrust/setback, case web stretch, and case head separations.</p><p></p><p>Back to the reloading room...</p></blockquote><p></p>
[QUOTE="Pdvdh, post: 385132, member: 4191"] Could you use 7mm WSM brass? Neck it down to .277 with your resizing die, and set the shoulder back to a snug headspace fit at the same time. My LoadBase 3.0 ballistic software shows a case head to shoulder measurement of 1.702" for 7mm WSM brass, versus 1.664" for both the 270 WSM and 300 WSM. If the 7mm WSM brass is ~0.038" longer from case head to shoulder, then it seems like this approach would also work. But it you can easily reset chamber headspace, that should cure the headspace problem also. Differences in the brass life you described could be both in the properties of the brass (softer versus harder), the production headspace measurements of the brass, and even the friction coefficient of the case surface. The effective differences between slicker than snot (low friction coefficient) brass/chamber interfaces and rough (high friction coefficient) brass/chamber interfaces - as related to case head thrust and case head separations - are as follows: The slicker the surface the more the cartidge case head [B]and[/B] sidewalls are able to slide back towards the bolt face. After they are set back firmly into the bolt face, the brass that then blows out to conform to the chamber dimensions is the thinner brass located at the shoulder area. This thinner brass can blow outwards in a gentler manner because the expansion of the brass is spread out over a larger surface area of the casing. ALSO, the brass in the shoulder area has been annealed, and it is able to stretch to fit with less chance of tearing than the harder brass back in the case web area. Little harm is done to the casing. And the case head strikes the bolt face with greater force. With a rough, unlubricated, high friction coefficient case/chamber interface, the [U]forward portions of the case wall[/U] are blown out and tightly grip the chamber with. So firmly that the [U]forward portions of the case wall[/U] are locked to the chamber - immobilized. Tremendous pressures are generated and blow the case head back into the bolt face. The forward side walls are immobilized yet the case head is moving back into the bolt face. Something has to give. The brass yields just forward of the case head, where case web stretches (permanently thinning) and case head separations occur. The reason the brass stretches at this location is because this is the first location moving from the shoulder toward the case head where the case wall thickness becomes rigid enough to prevent its being pressured out and locked onto the chamber walls. The thinner case wall is easily pressured out with enough force to tightly grip the chamber. At the case web, the web thickens to the point that the pressue isn't sufficient to lock the web onto the chamber. This is first location where the brass is able to shift/move along the chamber. So this case wall thins out at the location where it's too thick to bond to the chamber, and too thin and weak to survive the tensile force trying to stretch it apart. Lastly, how does this stretching of the case web area reduce case head thrust (force) against the bolt face? The case wall stretches at a location where is starting to get thick. A fairly substantial tensile force is required to stretch, thin, and permanently deform this thickened case web. And this tensile force is restraining (trying to pull the case head away from bolt face. It's not a strong enough tensile force to prevent case head setback, but it is substantial enough to reduce case head thrust. So if you want to accelerate case head separations and reduce case head thrust, roughen up your chamber to increase the coefficient of friction between the case & chamber. (Don't really do this!) If you want to slow the progression of case head separations and increase case head thrust, polish your chamber and lube your casing exteriors to reduce the coefficient of friction. BTW, this is what we all should want. The lugs on the bolt are designed to survive the additional case head thrust against the bolt face. The brass is the weak link, and the component that needs the tender loving care. Hope this helps provide a better, conceptual understanding. If you understand these consepts, you will be better able to toubleshoot and resolve issues related to case head thrust/setback, case web stretch, and case head separations. Back to the reloading room... [/QUOTE]
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