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It is a test of correct chamber dimensions (namely headspace) and fit of the bolt and barrel extension which is generally a standard practice by manufacturers. The gauges are meant to check whether the chamber is the correct depth to fit a cartridge of the SAMI specification size. If a go gauge does not fit in the chamber it has been reamed out too short and will not chamber and fire ammunition reliably if at all. If a go gauge fits in the chamber but a no-go (longer) gauge does not then dimensions are correct. If a no-go gauge does fit, the chamber is too deep which can cause split or ruptured cases, excessive parts wear, and a potential safety hazard. In this case they are saying headspace has been verified as correct with gauges and the upper receiver and bolt should work correctly together - generally not a selling point but more of a basic expectation.

Hi Steve and welcome. Head Space can affect accuracy, chamber pressure and rifle reliability greatly. Full details here : https://criterionbarrels.com/media/what-is-headspace-and-why-should-it-be-checked/?v=7516fd43adaa ----------------- Cheers, stay safe and enjoy!