Quoted:

He said he had never exceeded 60, not that the manuals are wrong.  You can do a lot of things, like lapping or shimming, to make the barrel nut fit with less than 80 ft-lb of torque.  80 foot pounds of torque is the maximum allowed in the TM because beyond that bad things can happen, like making it even harder to change out the barrel, and potentially damaging the upper with the excessive stress applied to its threads.

On the other hand, with an in-spec upper, an in-spec barrel, and an in-spec barrel nut, it's out of the ordinary to get to 80 ft-lb if you follow the TM procedure properly, including using an appropriate grease on the threads.  The TM's "torque three times" procedure helps distribute the grease on the upper's and nut's threads, while smoothing over any potentially rough spots on the threads.  You can demonstrate to yourself how this process lowers the final torque by seeing how far around the barrel nut goes each successive time you torque to 30 - it will be farther around, sometimes by a couple of notches, each time you get to 30 ft-lb.  

If you can't get a notch to line up by the time you get to 80 ft-lb, "use a different barrel nut" is the TM's fix.  Or, you can look into the problem and maybe find some irregularity on the face of the upper which you can remove by lapping it.  Or, it's a unique type of nut and you're stuck with shimming to get things to line up.  ALL OF THESE are valid assembly methods, though only the first is allowable for a military armorer.

Quoted:

I agree with you on that.  In-spec uppers should be able to handle over 80 ft-lb fine, though I wouldn't expect to be perfectly safe with going a lot over that consistently.  In for what uppers "don't like" about more than 60 ft-lb of torque...