From Armalite FAQ's....
ANSWER: At the factory, the barrel nut is torqued by feel. But, studies have shown the torque for the M15 should be 35 - 80 FOOT lbs. and for the AR-10 45 - 90 FOOT lbs. The torque for the gas keys for both the M-15 and AR-10 should be approximately 55 INCH lbs.