Gunsmithing Applying proper Torque with a Strap Wrench

You don't need a torque wrench for that, you will need an armorers wrench. Most standard barrel nuts require indexing to line up with the gas tube, so no torque value other than good n' tight.
 
Take a strip of emery cloth long enough to go around the nut twice. Fold it in half backwards, abrasive side out. Wrap that around the nut and then apply your strap wrench and snug the nut down. 40 ft/lbs is your starting torque and then just enough additional torque to line up for the gas tube. Don't go over 80 ft / lbs. The emery cloth will bite into the strap and nut enough to get it pulled up snug and although it will leave some minor marks on it it's not enough to note.
There is no really good way to get a torque on it so "Kentucky Torquage" comes into play.
If the nut has a bunch of holes in it for the gas tube a PRI wrench usually works well.
 
I have a buddy who used to lift a lot of weights. He used to just go by what felt like the weight the fastener needed. We called him Torque arm Bob. With practice this can actually get pretty close.

Human operators can be remarkably precise in this regard; we are quite good at producing consistent forces. Accuracy is another matter altogether, but I've seen statistic studies that show well-trained human operators beating low-quality torque wrenches (not that this sets the bar all that high, but it was interesting nonetheless).