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Spit will work in a pinch too.
The point I was making was the grease that was called out on the drawing is different from the grease in the TM . . . Don't get to wrapped up in "which grease should I use".
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I agree with this, mostly. I disagree with finding the cheapest crap-tastic axle grease you can find to assemble a rifle with. Yeah, it'll probably work, but...
I was an "Aero Shell Elitist" for quite some time before I actually found enough information to understand the whole thing about barrel nut grease.
My first build (in 1984) was almost literally part-by-part, and I got "the real deal" GI grease from a crew chief friend who'd been teaching a bunch of newly minted crew chiefs how to do a brake job on an RF4-C, and wound up with extra. Later, I bought a cartridge (14 oz) of AeroShell 33MS because it was "MIL-G" correct.
Since then, I've learned something about what the barrel nut really does, what MIL-G-21164 greases do, and most importantly WHY you need some sort of grease between the nut and the upper.
The barrel nut clamps the flange of the barrel extension in place (DUH!), applying tension by stretching the aluminum upper. The grease allows the threads to slide easily enough that the torque applied is really the torque applied and not altered by threads dragging on each other.
Ideally using a grease that has a viscosity similar to a -21164 grease should give you the most consistent torque, while the other characteristics of these greases allow them to stand up to the application.