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Old October 16, 2012, 03:07 PM   #1
NWPilgrim
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Join Date: September 29, 2008
Location: Oregon
Posts: 2,153
Question about ACTUAL case capacities...

I am wondering about the actual case capacities based on headstamp, and if I am measuring correctly.

I have read in numerous places the guideline that military brass is thick and less case capacity so download y 1-2 grains. I understand that case weight is not a good indicator of capacity because the extra brass can e in the head and not the case wall, etc.

I finally got around to actually measuring case capacities and was surprised by the results. I had my brass sorted by headstamp (NOT year for military brass as it seemed consistent at least in weight) and had found the approximate average weight for each headstamp.

Method I used
So I took a case weighing near the average for each headstamp and seated a primer and trickled in water to fill it completely. I then dumped the water from the case into a pan on the zeroed electronic scale (+/1 0.1 gr). All cases were sized and trimmed to the same dimensions.

SEE UPDATED RESULTS IN POST #6

.30-06 Results in Grains of Water
Military
- LC = 67.9
- SL = 67.7
- WCC = 67.6
- HXP = 67.5
- TW = 66.3 (only significant variance)

Averages about 67.7 gr. for all but TW.

Commercial
- R-P = 67.5
- Win = 67.5
Might pull some Federal ammo just to measure the cases while I am thinking about it.


Now in QuickLoad it states to measure the capacity of a fired, UNsized case, but I would think at least for headstamp comparisons it is valid to use sized cases. QL using the fired case size for computing the actual pressure.

Questions
Is there any definitive reference for case capacities, that is, with actual measurements? From my measurements I cannot see ANY significant difference between R-P and Winchester commercial cases and military cases except of course TW. Do I have unusual cases, measured incorrectly, or is the old adage wrong or outdated? My military cases range from 1953 through 1970.


So then I did the same thing for .223, except due to the water surface tension I did not seat a primer but just held my thumb of the primer hole and released it to pour out the water.

.223/5.56 Results in Grains of Water
Military
- LC = 30.0
- WCC = 30.0
- MAI = 30.0
- CJ = 29.9
- TW = 29.5 (again, the only outlier, slightly)

Commercial
- FC small = 30.4
- FC large = 30.2
- PMC = 30.1
- Win = 29.7
- Fiocchi = 29.7
- R-P = 29.6
- S&B = 28.7 (significant outlier)


In the case of .223 versus 5.56 it appears the military brass has less spread and is near the top of capacity (contrary to popular guideline), and the commercial ranges from the military capacity to much less.

I am thinking that except for a couple of headstamps the military and commercial brass has practically the same capacities even though the weights vary widely. These were single samples taken from the weight averages from hundreds of cases (at least 50 of any one headstamp and some were hundreds of cases).

Any problem with my method, results or tentative conclusion?
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Last edited by NWPilgrim; October 16, 2012 at 07:37 PM.
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