Assembled the gauge and got 2 measurements from my 2 Krags. First photo is setting the zero on the gauge.
Second is my sporter bolt (Assembled) measures .450 plus the indicated .0175 or .4675 total. (small dial reads .450) Ser #410XXX
Third  is my fathers relatively unused bolt (Stripped) it reads .450 plus .0278 or .4778 total. ser #397810
The markings are similar on these 2 bolts.

Stripped bolt on the back has sideways V underneath S12 and a punch mark, bottom flat has a 0 (zero).

Assembled bolt back S12 underneath 0 and a punch mark.
Bottom flat F.

The wear on the assembled bolt left a small unworn patch that caught on the gauge, the pen in the photo was lifting it so that patch was not touching. Indicating the wear to be about .005. These 2 bolts with similar marking and serial numbers were probably .005 different when new. That is almost the whole head space tolerance.

The rifle with the assembled bolt was probably last used by my father in 1940 as he was mobilized in 41 and hunted with his 1917 Enfield afterwards so it has very little wear.
I plan on milling a small pocket on the gauge to clear the unworn portion of a bolt so it will give a closer to effective measurement.

2 bolts is not enough to mean anything yet, but indicates some caution be used when swapping bolts, especially if wear is evident.

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