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Old March 24, 2009, 11:53 PM   #2
F. Guffey
Senior Member
Join Date: July 18, 2008
Posts: 7,257
Head space, a necessary thing, in the perfect world the difference between the perfect commercial/military case is .005 shorter from the head of the case to the shoulder than the chamber length from the face of the bolt to the shoulder of the chamber. Measurement are said to be taken from 'the datum line' the datum is not a line, it is a circle or hole, the 308 Winchester circle/hole is .400 in diameter, the distance from the datum (circle/hole) back to the head of the case is 1.634, the chamber in the perfect world is 1.634 + .005 or 1.639, I make gages, I make up DAT-UMS, for the 308 Winchester 3/8" (.375) works.

This does not work for everyone, I do not grind shell holders, I do not grind dies, when I size a case with the die adjusted down to the shell holder with an additional 1/8 turn, I size a perfect case that is .005 shorter than the perfect chamber.

Minimum/Maximum, I do not have one, I adjust my dies when I size to avoid sizing, in the perfect world the no-go gage is .004 thousands longer than the go-gage (which is .005 longer than the perfect case), If the perfect case is fired in a chamber that is no-go gage length, the head space will be .009. Beyond that there is the field gage, the field gage is .014 longer than the perfect case, when the perfect case if fired in the field gage length chamber there is .014 head space, I have one 30/06 rifle that has .016 thousands head space, when I fire it there is .000 head space, I make gages, I size cases to fit the chamber.

F. Guffey
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