I suspect it is a chamber that the "go" gauge closes on but not a "no go" gauge. The bolt would be the one that the "go" gauge closes on too, so together (bolt & barrel) they are a pair.
Their "no go" gauge for a match chamber may be .001 over minimum SAAMI specs. The only trouble is that if your ammunition is within SAAMI specs but over .001 minimum SAAMI specs, it won't feed in the rifle.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!