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Old September 21, 2012, 11:55 PM   #11
Creeper
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Join Date: September 9, 2008
Location: Woooooshington
Posts: 1,797
Quote:
In regards to the "shoulder setback" I have to be completely honest and tell you that I'm confused now... I'm not sure I even understand which part of the casing is the shoulder.
Go back to the SAAMI drawing. See the tapered area just behind the neck that starts at .253" and tapers to .3542"? That's the "shoulder". The .330" in between is called the "datum line".
Headspace is the measurement from the bolt face to the location of the datum line on the case shoulder. There are inexpensive measuring fixtures designed to make this measurement accurately, with a dial caliper, for practically every cartridge.

As you can see, the measurement to this line is 1.4666″ maximum. Now look at the chamber drawing... note that the minimum dimension at the datum line is 1.4736"... or a difference of .007". If you measure a fired case with a headspace gauge, you'll have an idea of the headspace measurement in your chamber. Remember that after a cartridge fires, the case expands, in both length and diameter, to the chamber dimension, then shrinks back a small amount... sometimes nearly unmeasurable unless you have the appropriate length .0001" micrometer, and sometimes as much as a few thou.
The idea is that you want your resized case headspace value to be less than the chamber value. What amount is arguable and may depend on the individual shooters needs, knowledge and the intended use of the gun. Until you understand the difference, stick to the SAMMI drawings and the recommendations provided in your reloading manual.

Cheers,
C
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