Especially using the tools that are available to the military, it's very difficult to torque much over 30 ft-lbs. The wrench usually slips and strips teeth on the barrel nut if you try to go much beyond 30 ft-lbs - at least that's been my experience over the last 30 years or so. Normally the barrel nut lines up pretty close at that point anyway.
The -23&P TM specifies both the moly grease and the 'tighten 3 times' routine. FWIW, Colt doesn't apply moly grease on M4s at the factory - at least none of the ones that I've seen.
As far as what grease to use, I bought a pound of wheel bearing grease at Wal-Mart for right around $2, and that's what I've been using for several years.
"This ain't Dodge City and you ain't Bill Hickok." Matthew Quigley, Quigley Down Under
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