as you can see from the table, the length between the shorter "go" gauge and longer "no go" gage is anywhere from .003" to .006" depending on the calibre in question. .003" is approximately equivalent to the thinkness of a sheet of paper.

for a rimless case, the length is measured from the back of the case to a datum line on the shoulder of the case.

a field gauge is longer that the "no go" gauge and should only be used to measure headspace on a firearm used in very dire circumstances.

no firearm used by for sporting purposes should not accept a "no go" gauge, let alone a "field reject" gauge.