used o-ring w/ enhanced extractor spring. heavier buffer also helps.

from twl in another thread:
"Heavier mass slightly retards bolt-opening timing, which will allow pressures to drop slightly further before the bolt can unlock and try to extract the round.

The higher pressure present inside the case, when the bolt opening cycle begins, is what causes this difficult extraction condition in carbines. In 10.5" barrels, the dwell time is so short that the gas port must be bigger than usual to force the bolt open quickly, and when the pressure in the case is high, the case is swelled against the chamber and doesn't want to come out easily. So what happens is, the extractor with normal-power spring slips off the case rim before the extraction is completed, thus this classic jam pattern. Added extractor spring tension and possibly heavier mass in the buffer can both help to solve this issue in carbines."