I have the book from Fulton Armory on building AR15s, and find it interesting that after going over all the torque values and how to measure them, they then say that virtualy no professional builder uses a torque wrench, a) because its so arkward to do so, and b) because you don't get fine enough control to accurately line up the gas tube hole.
I now just use the gas tube wrench, and make the nut tight, not worrying if its only 28 ft-lbs, and being pretty certain that I won't easily exceed 80 ft-lbs with just the wrench (at least not without being fully aware that I am making it very tight!).
Works for me ...
Actually, think about it - the barrel doesn't have to be held down ultra tight since when the bolt is locked, everything is locked up solid to the barrel -- the barrel only has to be solid enough to not flap around in the breeze.
The torque applied simply has to be sufficient to stop the barrel nut working loose over time.