This gun did just fine for the first 250rds, then the trouble began. When a round is fired the action seems fine. The empty is ejected about 20ft and a fresh round picked-up. When the bolt closes, though, it doesn't lock fully and pulling on the trigger gives no joy. If I cycle the action by hand it works fine, even if I do it very slowly. Today at the range I fired 20rds manually cycling. The unfired empties ejected easily except two that required banging the butt. I've tried different gas settings, removing and reinstalling the gas block, lubing the bolt and carrier, different mags and hotter ammo. Terra checked the bolt and pronounced it OK. I just installed the heavy hammer spring, and the problem may have gotten worse. Ideas?