I have been reading about the problem of stuck barrel nuts ,barrels etc.  Early on with my 15-2 I decided to use a copper bearing anti seize compound on the threads, currently I am using a stick made by Loctite, it also comes in brush on cans in automotive stores. 

It is used in many industrial sites  on bolts that have to be removed for maintenance .It can be quite messy so care is required but it will allow a tighter barrel nut that can removed easily  when needed It should be placed on the threads that enter the receiver, and threads at the barrel nut end and where the barrel nut contacts the shroud as this spot could gall if left un lubricated and all the push pressure is concentrated on this one spot. If you look inside the shroud where the metal on metal contact occurs there will be bright spots get some anti seize on this area. it will smoke a little if the barrel gets heated. get a good wrench such as an EWK.

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