Like some of you experienced guys, I have installed a bunch of AR barrels over the years. I found that my most accurate assemblies were with nothing added to the barrel extension. Loctite em if you want, but that would be the only thing I'd use, being dissimilar metals and all. Many of the factory installed uppers that we swapped barrels on, including a few of the big name brands, showed no sign of any kind of sealant or other magic juice.