To be clear, I am not saying to use a threadlocker on the barrel nut threads. Use a dab of grease for that.

The issue is if there is a small gap between upper receiver and the barrel extension. This will allow the barrel to move around inside the receiver. Adding a small amount of Loctite 602 or similar, to the outside of the extension, then putting it into the receiver, will act as a bedding compound. When it sets, the Loctite will harden, holding the barrel in place.

Similar to putting two pieces of pvc pipe together. You apply glue to the inside of the coupling and the outside of the pipe, then press them together.