So a chamber is created with a reamer, that reamers geometry is fixed, the reamer wont change, so youre stuck with the shape, all you can really control is how deep you insert the reamer into the barrel during the process of chambering the barrel. Headspace is basically a measure of how deep the reamer was inserted into the barrel indicated by the shoulder (which is at an angle).
Saami is a governing body and they publish some charts, in those charts are specs. In this chart they published for the 6.5 creed you will see that the red line is on basically the middle of the shoulder. That red line is on the datum plane where the shoulder diameter is exactly .400" (datum explained below). So basically we are measuring from that datum plane at .400" all the way back to the bottom of the case. Which should be a max of 1.5438" and up to .007" below that meaning anywhere from 1.5438" to 1.5368" deep is in spec.
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So to measure from the angled surface of the shoulder back to the case head is tough. What we are really wanting is to measure the distance between two flat parallel planes. The first datum plane of reference is the base of the case which is nice and flat and easy, the second datum plane of reference will need to be at some point on the shoulders. Because the shoulders are a cone, a circle tool with a given diameter will allow us to settle into exactly the same location on the shoulders evenly all around consistently each time. So we create a datum diameter (.400" as example for a bunch of chamberings) and that will allow us to measure the same spot on the shoulders consistently back to the case head which is basically how deeply the reamer was inserted.
What a go gauge does is act as a depth gauge. It is preciously cut to a certain distance between the case head where the bolt touches the case and the chambers shoulder. When you insert the go gauge you want it to close on the go gauge so that you know the chambers is cut adequately deep/long. When you insert the no go gauge, which is slightly longer than go, you want the bolt to stop short of closing so that you know the chamber isnt cut overly deep/long. This guarantees that you are firmly inside the spec designed for that ammo.
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Being slightly long isnt the worst thing in the world and actually guarantees it will function with dirt and grime etc, AI is a great example if this as they cut their chambers slightly long so the inspec ammo will always chamber in the rifle during war even if the rifle falls in the sand and mud and ice.
But there is one more gauge that is called a field gauge which is even longer still and is designed for the actual safety check. I fit passes on the field then you need to get the case checked because its wayy too deep and can lead to brass cases cracking and stretching and just not great stuff at 60k psi.
But for a rifle that Im not doing war with Id rather have a tighter chamber that falls inside the go and no go parameters and limits the amount of deformation that is imparted on the case during firing, too much stretch can split brass and do all sorts of nasty things but it can also stretch a fair amount safely if you dont over do it, its how ackley improved chamber brass is made and its called fire forming.
