Manson 6.5 Creedmoor GaugesWe offer headspace gauges for use in fitting replacement barrels to bolt action rifles. - The "Go" Gauge is used to make sure that standard factory loaded ammunition will fit reliably. Insert the Go gauge in the chamber and close the bolt. If the bolt won't close or closes with resistance, you may have the barrel screwed in too tight. Your headspace is too tight and won't fit factory loaded ammo reliably.- The "No-Go" Gauge is used to set your headspace when installing a new barrel. Manson NO-GO gauges allow for a .006" headspace. If your action closes on this gauge it doesn't mean you have excess head space... Only that its more than the gauge allows. SAAMI spec allows up to .010" headspace. Finish you barrel installation a little closer.- The "Field" Gauge (Not Offered Here) is used make sure that headspace on a factory installed barrel is not too long to support factory loaded ammo. SAAMI spec for maximum allowable headspace is .010". If your action will close on the Field gauge, it probably has excess head space. Actually anything between the "GO" and the "Field" dimension is within SAAMI specs. We don't offer the Field gauge primarily because these gauges are intended for customers who are installing new barrels on Savage rifles and getting the install close to the NO-GO dimension is the best approach.