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Rifles, Reloading, Optics, Equipment
Reloading
Case weight vs pressure
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<blockquote data-quote="DocB" data-source="post: 921593" data-attributes="member: 55312"><p>Just my thoughts on this...</p><p></p><p>Whether you are determining capacity by sorting brass in the same lot by weight or determining capacity between different head stamps this is what I use as a guide. </p><p></p><p>This is by Aaron Davidson, <a href="http://www.gunwerks.com/Long-Range-University/Learning-Articles/Powders,-Primers,-and-Long-Range-Shooting" target="_blank">Gunwerks - Powders, Primers, and Long Range Shooting - by Aaron Davidson</a></p><p></p><p><em>" A general rule of thumb for a large rifle case is +/-7 grains of brass weight means more or less case capacity equating to nearly 1 grain of powder. I have measured over 5 grains of weight difference in one bag of Winchester brass in 7mm Rem Mag. Changing a load by one grain of powder can cause a difference in velocity of 50 fps. That is well outside of our 30 fps minimum extreme spread criteria for long range precision shooting."</em></p><p></p><p>Consistency is the key in case weights and in loads. Case weight differences in the same lot or by headstamp equate to case capacity differences due to the internal capacity of the case. Think of it this way... cases are designed to have the external dimensions of the case match the standard rifle chamber. The external case dimensions are pretty much fixed due to the mass manufacturing process of commercial brass cases. So... if there are weight differences within the same lot of cases it stands to reason that since the external dimensions are pretty much a fixed property of the case, then the difference must be in the thickness of the brass which, in turn, will either decrease or increase the internal case capacity of the case. </p><p></p><p>Consistency being the mantra of long range, we want our case capacities within the same load to be as close as humanly possible. Why? Well, as described in the article, a powder weight difference of 1 grain can mean as much as a 50fps difference in your MV, more or less depending on your cartridge caliber. In this same example a variance of 30fps in MV will mean a point of impact vertical shift of 5" at 800yds and at 1000yds a vertical shift of 10" or 1 MOA. Remember this 1gr of powder weight difference was the result of a 7gr case weight difference.</p><p></p><p>This is why most folks who sort their brass they try to keep case weight within 2.0gr.s in the same load, closer if possible.</p><p></p><p>MV variances aside, as Mr. jfseaman cautioned, there are the ramifications of pressure variances at or near max loads to consider as well. </p><p></p><p>So for me.. I sort within the same lot by weight and with separate brass by headstamp and weight/capacity.</p><p></p><p>Just what I do, hope this helps.</p><p></p><p>DocB</p><p></p><p><em>Animo et fide "Courage and Faith"</em></p></blockquote><p></p>
[QUOTE="DocB, post: 921593, member: 55312"] Just my thoughts on this... Whether you are determining capacity by sorting brass in the same lot by weight or determining capacity between different head stamps this is what I use as a guide. This is by Aaron Davidson, [url=http://www.gunwerks.com/Long-Range-University/Learning-Articles/Powders,-Primers,-and-Long-Range-Shooting]Gunwerks - Powders, Primers, and Long Range Shooting - by Aaron Davidson[/url] [I]" A general rule of thumb for a large rifle case is +/-7 grains of brass weight means more or less case capacity equating to nearly 1 grain of powder. I have measured over 5 grains of weight difference in one bag of Winchester brass in 7mm Rem Mag. Changing a load by one grain of powder can cause a difference in velocity of 50 fps. That is well outside of our 30 fps minimum extreme spread criteria for long range precision shooting."[/I] Consistency is the key in case weights and in loads. Case weight differences in the same lot or by headstamp equate to case capacity differences due to the internal capacity of the case. Think of it this way... cases are designed to have the external dimensions of the case match the standard rifle chamber. The external case dimensions are pretty much fixed due to the mass manufacturing process of commercial brass cases. So... if there are weight differences within the same lot of cases it stands to reason that since the external dimensions are pretty much a fixed property of the case, then the difference must be in the thickness of the brass which, in turn, will either decrease or increase the internal case capacity of the case. Consistency being the mantra of long range, we want our case capacities within the same load to be as close as humanly possible. Why? Well, as described in the article, a powder weight difference of 1 grain can mean as much as a 50fps difference in your MV, more or less depending on your cartridge caliber. In this same example a variance of 30fps in MV will mean a point of impact vertical shift of 5" at 800yds and at 1000yds a vertical shift of 10" or 1 MOA. Remember this 1gr of powder weight difference was the result of a 7gr case weight difference. This is why most folks who sort their brass they try to keep case weight within 2.0gr.s in the same load, closer if possible. MV variances aside, as Mr. jfseaman cautioned, there are the ramifications of pressure variances at or near max loads to consider as well. So for me.. I sort within the same lot by weight and with separate brass by headstamp and weight/capacity. Just what I do, hope this helps. DocB [I]Animo et fide "Courage and Faith"[/I] [/QUOTE]
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