Coat the threads of the barrel nut and receiver liberally with an anti-seize compound. This will prevent galling when we tighten everything in place.
The top of the barrel sleeve has an indexing stud that mates with a groove in the receiver, assuring the gas port is indexed to the top dead center.
Slide the barrel into the receiver......and screw on the barrel nut just until it's hand tight.
A barrel nut wrench is used to tighten the barrel nut into the final position, while a torque wrench is used to control the tightness. Proper torque is 30 to 80 pounds, according to the Marine Armorer's Manual.
Starting at 30 pounds, tighten the barrel nut until the wrench reaches torque. Then, using a breaker bar, loosen the nut and back it off slightly.
Repeat the tightening and backing-off process two additional times. This allows the threads on the receiver and barrel nut to mate properly. Now, we can tighten it to spec for its final positioning.
Using a small punch, align the openings in the delta ring and weld spring.
Now, using a gas tube alignment gauge, check to make sure the barrel nut is aligned properly to allow the gas tube to enter the receiver. The gas tube should not touch the barrel nut.
If the barrel nut is not properly aligned, increase the torque a little at a time until it is. But don't apply more than 80 pounds of torque.
Now, we can install the gas tube. Insert it five or six inches into the receiver. Then pull it forward inserting it in the front sight base. Be sure that the gas port is facing down, and the roll pin holes line up in the front sight base.
Install the gas tube roll pin in the sight base using a roll pin starter punch and drive it the rest of the way home with a standard roll pin punch.
Now, install the carrier assembly and check to make sure that it closes flush with the back of the receiver and the gas tube doesn't bind in the key. The gas tube should enter the carrier key freely and without any binding.