After removing the receiver cover, operating handle, and bolt carrier and bolt, rotate the gas cylinder lock to its uppermost position and lift the rear of the handguard to remove the gas cylinder tube and the gas piston. The tube can be upended and the piston will slide out.
For the trigger group; Depress the catch located behind the loop of the trigger guard and pull the trigger group down and out of the rifle. The safety must be in the safe position before the catch can be depressed.
From Army TC 9-56.... I hope this helps.