Headspace gauges - Go Gauge is supposed to be made to SAAMI or CIP specified minimum chamber blueprint specifications. No-Go is some arbitrary number usually in the middle between blueprint minimum and maximum spec. A field gauge should be at blueprint maximum spec. The SAAMI specs is to establish safety dimensions for factory rifles with factory ammunition in a mass production scale, so for a cartridge, there is both the SAAMI cartridge and SAAMI chamber spec.
When you're sizing your cases, you want it to be smaller than your chamber (but not excessively smaller) in order for you to be able to chamber the round smoothly without issue. This would be under your chamber size. So if your chamber is at min spec, then your loaded cartridge needs to be smaller than chamber min but not smaller than SAAMI cartridge min spec.
The SAAMI cartridge specifications are designed so that the minimum sized cartridge spec will fire safely in a maximum SAAMI sized chamber without blowing up. Cartridge spec normally has a max spec that will still feed reliably in a min chamber even with a tiny bit of fouling. 6.5cm cartridge headspace is 1.5438 (max) and 1.5368" SAAMI min cartridge headspace.
Based on your comparator measurements of the 6.5cm headspace gauges, your comparator bushing is giving you a relative measurement of the headspace that is around 0.007" lower. When you're trying to compare SAAMI spec to what you measured with your comparator, you need to subtract 0.007" from the SAAMI blueprint numbers.
Use -0.007 and apply it to SAAMI min cartridge spec to obtain relative measurement (what you should see with your comparator). So for 6.5cm, 1.5368 (SAAMI cartridge min) - 0.007 =
1.5298"
1.5298" is the dimension for min limit sized ammo (measured using your comparator) that is still safe to fire in a SAAMI max chamber.
As a handloader, you don't have to follow SAAMI cartridge spec exactly since you can load to a specific barrel chamber. Since you have a min SAAMI chamber, you can even go below SAAMI cartridge min as long as you never shoot that handload in a chamber larger than SAAMI min.
SAAMI Max chamber (1.551) minus SAAMI min cartridge (1.5368) = 0.0142" difference. You use this number to maintain the same relationship with min chamber spec.
1.534" (your headspace gauge/chamber spec using comparator) - 0.0142" = 1.5198" is the shortest your case should measure with that comparator otherwise you blow up. In other words, plenty of room to bump the shoulder back without issue.
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