From "the Ar-15 Complete Assembly Guide" by Walt Kuleck, Vol 2, page 118:
"Secret: To "fuse" the barrel and upper receiver, clean and degrease the inside of the upper receiver extension and the outside of the barrel extension. Coat them with LocTite 271 ("Red Loc Tite") before inserting the barrel extension into the receiver extension. Be sure to coat the barrel extension's flange and the very front of the receiver as well. When properly executed, the very substantial increases in the bearing surfaces of the OD's and ID's of the barrel and receiver extension adds a trememdous additional "capture" and support to both. The real beneficiary is the rather frail aluminum receiver, which must support the weight of the barrel, handguards, etc."
It should be kept a secret. The "rather frail aluminum receiver" does not take much stress. This is BS. Ignore it.