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Weighing cases acceptable variation?

 
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  #8  
Old 02-24-2008, 06:13 PM
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Quote:
Originally Posted by davewilson View Post
just take 10 cases that weigh exactly the same and then measure how much powder they hold. with my RUM case i've seen as much as 2 gr of powder difference with cases that weigh the same. i almost hate to agree with ole Boss because we usually argue enough to be married, but on this one we're on the same page.


"MARRIED"????? Send me some MONEYThen ASAP !!!!! So little time so many guns to build!!!!!
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Old 02-24-2008, 06:40 PM
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Sludge-----Sorry but I was just helping you to avoid wasting time. Anything you do with respect to weighing the entire case is meaningless with respect to the interior volume of the case. This means that you are making the assumption that by having 2 cases of the exact same weight that they have the identical internal volume. This is not a valid or logical assumption however; those two cases may just by happenstance have the same volume. Now we know that the internal volume of the case is what determines powder capacity and if the cases are different volumetrically, they will develop different pressure curves which will translate into different velocities. This will include among other things possible POI changes as well in addition when shooting at distances that are long enough the vertical dispersion caused by the different muzzle velocities will be very noticeable.


If you in fact want to check the interior volume of your cases, then start with clean cases and fill each one with a powder that flows readily such as 748 to the top of the case and then weigh the charge. This takes more time plus you have to use water but fill each case with water (eye dropper or bulb syringe) and put a few drops of surfactant in the water (i.e. 1 gallon jug) to break the surface tension to ensure no voids (tiny bubbles) on the inside of the case. If you add surfactant or dishwashing soap, make sure you rinse them well when finished.


Hope this helps.

How did I do Dear and send MONEY???? LOL!!!!!!
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Old 02-24-2008, 07:39 PM
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Well I guess I didn't get the response that I thought I would but that is how you learn. If you do the volume method, should you fireform first, clean and then what?
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Old 02-24-2008, 08:09 PM
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Well, for some reason I still cant see how if the external dimensions are the same (fire formed, same length etc.) that with the same weight there can be a great volume difference... However, im gonna have to play with that now.

2 gr of difference eh??

Darn it Boss :P ya had to start me down the road to more experimentation...
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  #12  
Old 02-24-2008, 08:28 PM
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On the one hand... they are correct. The weight could be other places than just the case walls... like the extractor groove, or case head.

On the other hand... people have done some tests in the past comparing weight and volume, and the results seemed to show (not 'prove') that for some brands... the weight did seem to be fairly
indicative of the case volume in some instances, like Lapua .308 Win cases. Others, like Winchester .308 win... there was no correlation whatsoever.
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  #13  
Old 02-24-2008, 09:49 PM
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sorry Dear, i gave all of my money to my girlfriend!

i think the main problem with weighing as a comparison is there can be areas of the metal which are more dense than others.the softer the metal, the more it is prone to differences in density.
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