Shoulder line or Datum line

[Metal God]

I'm trying to understand how to directly measure the chamber's headspace to the shoulder datum line

You use a go gauge and place custom cut feeler gauge shims on the base of the GO_Gauge or on the bolt face . If you have the correct measuring tools you can use any thin none comprisable custom cut shim . Some guys use scotch tape . Just find something that is firm and REALLY thin that you can measure and cut reliably .

For 308 you start with a 1.630 GO-Gauge and chamber it with a stripped bolt , it should chamber . Now throw a 1.634 NOGO gauge in there , It should not chamber . Great now you know your chamber is between 1.630 and 1.634 . Now start adding .001 shims to the 1.630 GO-Gauge and chamber one at a time . When it closes with slight resistance . Pull the gauge and add all the shims sizes to the 1.630 GO-Gauge measurement .

EXAMPLE-ISH , I don't feel like stripping the bolt just for the pic ;-)

If you used a .001 and .0015 shim that means you add .0025 to the 1.630 for a chamber size of 1.6325" from bolt face to shoulder datum

 

[ebb]

If only R Guffey were here to straighten us all out on this. He is the only one in the whole world that knows this stuff.

 

[Coyotefurharvester]

The shoulder datum line is a reference point for chamber tooling and cartridge case uniformity(in SAAMI cartridges) and measured externally. Since the chamber is an internal structure, an actual measurement would be very difficult(why the go no go). What method you use to size fired brass to properly fit/function safely in the firearm is what works best for you.

 

[Metal God]

Omg , I thought he was back haha - screen shot

 

[Ray123]

The datum head space line is a diameter on the case shoulder .400 dia on the 20 deg shoulder back to the bolt face 1.630 min to 1.640 max on the 308 SAAMI print...as an example...When you reload a 308 the die pushes the 20 deg shoulder back along its full length including the head space datum line at its dia. So you push back or shorten the 308 case 20 degree angle back about .002 along its full length not just at the datum line dia. from its expanded fired condition. Brass spring back will affect how far down to adjust the die, annealing the area will most likely help keep the spring back to a minimum, and be more consistent.

 

[Ray123]

Totally easy to measure it all with an optical comparitor with .001 graduations. Or build a gauge in your lathe. Example 308... drill and finish bore .344 hole slip fit for your brass, use or make a .400 dia gauge pin...drill then bore to .400 over the .343 dia. Easy to grind your boring tool with a 20 deg angle or set your compound rest. Face off to the angle after gaugeing the accuracy of the .400 dia hole ...add a .145" +.030" radius between the bottom of the 20 degree angle and the .344 neck diameter and you have your 308 Win .400" dia. datum line gauge. But ya still gotta move the whole 20 deg shoulder back .001 or .002 the datum line is a measurement line where no radius or variations are supposed to exist ...except the 20 degree angle could be slightly off... reamer wear, small metal chip slightly gouging the 20 deg angle on the finish reamer...nothing is perfect. So ya set the complete case shoulder back just enough to chamber your cartridge freely. Easy...