Which grease should buy for the barrel nut threads? This is the one part of AR building I've never actually done before. Aeroshell 33 is the original milspec grease for this purpose, but it's expensive, and as far as I can tell from their respective spec sheets, Lucas "X-tra Heavy Duty" (green), "Marine" (blue), and "Red n Tacky" (red) all meet or exceed Aeroshell 33's specs.