Point was, you will find small variations in these dimensions between manufacturers, so in the interest of consistency it is best to keep to the same one.
I received a call from a smith in North Texas that was starting to build one of his custom rifles, he had a no go gage but did not have a go gage. I explained to him he did not need the go-gage. I explained to him he could measure a short chamber, a go-gage length chamber and a no go-gage length chamber with a no go-gage. Variations and inconsistencies between gages and manufacturers is not a problem if the person using them can verify, I am the fan of verifying, I am the fan of standards and I am the fan of transfers.
That was not my point. "Measuring" the exact length to the gauge datum is not so simple (but please enlighten me if you know something I do not) but can be done with reasonable accuracy with a comparator- but even that is not exact as these are not precision ground for extreme accuracy.
I do not make excuses for not being able to do it. Like I said, the collector building the Rock Island 03 had 20 30/06 head space gage, not one of the 20 would measure the length of the chamber; but I could use any gage in his collection to determine the length of the chamber. Some of the gages went back to the beginning when the datum was on the case body/shoulder juncture.
F. Guffey