Brass maufactures

I mix brass, and never weighed any of them, also be it noted Im not a "match shooter", and I have plenty of nice groups downrange with mixed brass, especially 30-06, my rifle doesn't seem to care as long as it's getting shot.;)
 
I'm picky about brass and used to spend a lot of time uniforming, weight sorting, and just overall prepping it. Then I just made the switch to premium brass and don't spend a lot of time doing case prep. It made me feel better that I was so picky and had things shooting so well, but just on a whim I loaded up a lot of mixed case brass (for my 223). Some cases were prepped and some weren't, some were about worn out, some were premium, and some were just run of the mill stuff from most of the makers. Gotta tell you...the mixed brass didn't shoot as well as the premium and prepped stuff, but the accuracy difference would not have mattered to a hunter. It was 10 shot groups. I was surprised, but that's what I found. Rifle was an over the counter Ruger Hawkeye in stainless.

But....I'm still picky about brass.
 
Because .308 has more brass capacity variation than most other chamberings except .300 Win Mag, the American Rifleman did some tests awhile back that found about 6,000 psi peak pressure variation with different makes of brass they tested with the same load. Same with swapping primers around.

As mentioned, brass is made from different alloys. This article on x-ray spectrometry of different brands of brass shows this. So even if the outside dimensions were identical, you could still get a 3% variation in case capacity due to alloy. The densities of the various brasses used are below.

Copper:Zinc / Density at 68°F / Notes

60:40 / 8.39 gm/cc / (aka, Muntz Metal)

70:30 / 8.53 gm/cc / (aka, Cartridge Brass)

80:20 / 8.67 gm/cc / (aka, Low Brass)

It needs to mentioned that the outside dimensions of the case head, the exact extractor groove angle cut and the rim and rim chamfer and extractor groove tolerance can account for some weight variation that doesn't affect the internal capacity. Other than the primer pocket, only the brass that is forward of the head affects capacity in proportion to the brass weight. If the head is light, for example, a case would have less internal capacity than an identical weight case with a wider, thicker rimmed head. You can see the .308"s head dimension tolerances in the SAAMI drawing. The last time I weighed and measured water overflow capacity of several different headstamp .308 and 7.62 cases, I found weight difference predicted capacity difference ±20%, so the head dimension variation is real. Even the difference in headstamp impression volume has some part of the effect.
 
Thanks Uncle Nick. I got a headache thinkin' about all this stuff so I'm gonna just pretend all my brass is identical down to the .01 CC case capacity, and attribute my misses to phase of the moon or earth rotational variations...:p
 
many reloaders purchased primer pocket swages and rendered them scrap. Many reloaders do not know the 308 W case is larger in diameter than the 30/06 case. many reloaders have no clue why the 308 does a good job keeping up with the 30/06 with 150 grain bullets.

F. Guffey
 
There are other reloaders who don't have a clue why the 308 doesn't do a good job keeping up with the 30/06 with 150 grain bullets with SAAMI spec barrels and pressures. Those from a .308 are about 100 fps slower. That, to me, is not keeping up.
 
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