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Old October 18, 2012, 04:58 PM   #10
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,452
NWPilgrim,

Yes. Head diameters, extractor groove depth, forward angle off the extractor groove, etc. These all have tolerances. The sizing die has a radiused mouth and it stops on a shell holder deck 0.125" above the breech end of the case, so the sizing isn't actually getting all the way to the head, normally. But the brass itself, AFAIK is all 70:30 cartridge brass that has a specific gravity of 8.53. I just don't know what the tolerance for that is. If you want to get really scienterrific about it, the water displacement of the case brass has to be measured.


Bart B,

That sounds about right. In .308 Winchester, if I plug a 3% brass weight change into a typical 170 grain commercial case weight shooting the 175 grain SMK over IMR 4895 to 2640 fps from a 24" tube, it only takes about 0.2 grains of powder charge change to keep velocity constant. So figure that if your brass weight is in the center of the range, that's like a ±0.1 grain powder charge difference as far as velocity drop on target is concerned. It was about 15 fps difference in the simulator. Still well within normal muzzle velocity extreme spread for a load, even if the charges were weighed.
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