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Old February 8, 2005, 09:00 AM   #1
BigSlick
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Join Date: January 25, 2005
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Same headstamp and lot - big variance in weight

Hi guys,

I was working on a new 10mm load last night. PMC and Winchester brass. Both brands were bought at once (marked with the same lot number on the Winchester virgin brass, PMC was factory ammo from the same case, now once-fired brass) and have never been loaded/reloaded.

I was working up small batches in increments for fine tuning a load I have been working on. I got one round from the lighter charge group mixed in with the next heavier load group. I figured the easy way to find it was to weigh the rounds - obviously the lighter one would be apparent.

I weigh the Winchester batch (now loaded ammo) and found there to be a 10 grain variance in the group from lightest to heaviest. I hand loaded, hand weighed each charge, the bullets and primer are the same, both from the same original packaging.

I weighed the bullets, 180gr on the nose +/- two tenths.

Primers weighed the same or varied less than a 10th of a grain.

I measured some of the remaining virgin Winchester brass and found the discrepency. Case weight varied between .5 and 11 grains. I thought maybe this was a batch that got past QC or something. I measured the PMC. Much closer in weight, but still varied up to about 4 grains.

I figured the difference in weight, might mean a difference in capacity, thus affecting pressure and probably accuracy, so I checked the capacity with water. Capacity was all within .1 grain.

OK, great, the capacity, case OAL, lot, brand etc.. are exactly the same. Where does the weight variance come from ?

Someone educate me here...

Thanks

BigSlick
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Old June 12, 2005, 08:51 AM   #2
pearson1662
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My understanding is that case weight variance is a function of case wall thickness and does effect capacity and therefore pressure as well.
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Old June 12, 2005, 07:07 PM   #3
Kamicosmos
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+1 on what pearson said.

Plus, I think that the Brass alloy can fluctuate in density when the cases are made, but I'm not sure about that. This would explain why the wiegh different, but have the same dimensions sometimes.
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