This is what I did for my DPMS bolt and Rainier barrel. 1) Removed bolt from carrier, took out the ejector and extractor. 2) Put a GO guage into the bolt. Holding the bolt upright, lowered the barrel over it and confirmed that the bolt would turn(lock) 3) Did the same procedure with the NO GO gauge and confirmed that the bolt would not turn. 4) Replaced the guages with unfired 7.62mm (Surplus) & .308 (PRVI) cartridges and noticed that the .308 was a much tighter fit. 5) Put the extractor back in the bolt and tried to repeat steps 1) to 4) again, but the bolt wouldn't turn on any gauge or cartridge. Checked the extractor and noticed that the lower section of the extractor groove stood proud of the bolt face, tilting the gauge/cartridge ever so slightly so that it wouldn't sit flat in the bolt. This slight alignment wouldn't let the bolt lock up. With a Dremel cutting disc, I slightly ground down the lower bit of the extractor grove so that it was flush with the bolt face. Tested the guages/cartridges again and confirmed proper function. 6) Put the ejector back into the bolt, the bolt assembly back into the carrier. Assembled the rifle and checked headspace again with the guages, first slowly lowering the bolt carrier over the guages and using the forward assist to fully close, and then checking again by letting the carrier fly forward under the power of the main spring. As a side note. Rainier barrels are chambered very tight, barely closing on the GO gauge with my bolt and should hopfully be a consistent tack driver.