Anyone have recommendations for what grease to uses for the barrel nut threads?
@Rxrider aeroshell 33 is the mil spec. I personally use a lithium grease I got at autozone. You want it because of the differences in metal between the nut (steel) and the upper(aluminum).
I use Aeroshell or Weapon Shield Grease
White Lithium will work fine, but Aero is top of the line
Molybdemum disulfide is the important ingredient. It has anti-galling properties, and is used for high pressure service. The barrel nut threads do experience reasonably high pressure when properly tightened. Aluminum-on-steel also tends to gall. The presence of grease also excludes moisture from the spaces between the mated threads, and the lubricity enables proper and repeatable torque values to be set during assembly. Any decent MDS grease will work just fine. This is a stationary application. Spending any extra money or time to find Aeroshell 33 just because it’s Mil-spec is a waste of time and money.
I just noticed the Aeroshell 33 MS64 and it says it’s synthetic. Is this the same one that they’re referring to?
This is getting confusing, but Aeroshell 33 appears to be Lithium complex thickened, while Aeroshell 64 is Molybdenum Disulfide thickened. I am not a tribologist, but here’s a link I found to the different Aeroshell grease products: Aeroshell Grease | Aircraft Spruce
I’ve heard the Loctite copper-based grease is really good too - always have liked the Loctite company 
I use white lightning lithium from autozone
Sort of. Aeroshell 33 has been reformulated to contain molybdenum disulfide, and is now MS64. The simple fact is that virtually any decent moly grease will work just fine for barrel nuts on AR platforms. Running all over and spending a bunch of money to find Aeroshell specifically is just silly. It is a static application!
welcome to the forums @CubaLou
I bought 2 small containers of Umbrella Corp AS 33 about 10 years ago for about $5 each and I don’t think I’ve even gone through one. And I have set up many uppers, changed handguards/barrel nuts, etc. Good investment.