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Old October 16, 2012, 03:29 PM   #3
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,327
Fired capacity is closer to the volume the case actually gives the powder in your chamber because it expands to touch the chamber walls before the pressure peaks, unless you are loading below around 30,000 psi. Measuring the resized capacity is good to learn when your load will actually start to compress powder or to determine pressure when you fire the round in your sizing die instead of your gun (not recommended).

Figure to have to change powder charge between 0.06 and 0.07 grains for each grain of case weight difference, assuming outside dimensions are all the same.

Difference in military case capacity and commercial case capacity is only significant in .308/7.62. As you've discovered, .30-06 and, especially, .223/5.56 need little to no compensation for brand as far as powder charge is concerned. In .308/7.62 I've seen up to 3.5 grains water capacity difference in some makes. Thats about 30 grains of brass weight difference or around 2 grains powder charge difference to keep the same barrel time and in the same peak pressure range.
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