This is the method i use as well. I have also taken a sharpie and colored the inside edge of the gas key to see if the gas tube was rubbing slightly. Although the sharpie method may not hold true, I still do it.
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Loosen the barrel nut and repeat the process. I usually snug the bbl nut, loosen and repeat a minimum of 5. This allows the threads to "seat". Then after tightening to 30-35 ft lbs, if the gas tube hole is still blocked by a bbl nut tooth, I tighten the nut to next available groove.
If I'm wrong in my process, someone please correct me.
Brownells has two tools that are very helpful in this regard.
The first and one that I use on every barrel install is the barrel nut alignment gauge (080-216-014 $4.73). By slipping the gauge into the upper at the gas tube port you can tell instantly if your nut is aligned with the gas tube entry hole in the upper. Saves a lot of time.
The second is the upper receiver lapping tool (080-000-182 $26.95). If you are going for a specific torque number, you can change the barrel nut alignment by lapping the receiver face. It will also true the face for a more solid interface if the upper is not completely true.
I am usually not overly concerned about hitting a number unless building a precision rifle. Generally I aim for a torque range of 40-60 ft. lbs. on most builds and have had excellent results.
Adco sells a great barrel nut alignment tool. Rather long and fits snug that'll let you know if your gas tube is going to bind or not. Hell, they make several tools specifically for AR building that are great and priced pretty decent.
Having an extra barrel nut is not a bad idea. Over the years I have had one or two barrel nuts that came close to the 80 ft-lb torque limit in order for the gas tube to pass through without binding. I would prefer the torque was in the 35-45 ft-lb range.
Brownells has the alignment tool and everything else you need to build and repair AR-15s. That's where I picked up all of my tools.
Keep the tips coming Iraq Guns!
Ok the gas tube is made of stainless steel the barrel and barrel nut is steel and the upper is aluminum. Now when these different metals heat up and expand at different rates would that not cause the gas tube tube to be tweak even more when the rifle gets hot if the gas tube was not install correctly?
That hasn't been my experience. Consider how many variables determine the location in space of the open end of the gas tube;
Axial alignment of front sight rings to front sight gas tube tunnel.
Perpendicularity of the gas tube pin hole, and the gas tube locaiton hole for that pin
Bends made in the gas tube by the manufacturer to meet specs
Bends inadvertently added by warehouse, packer, other gear in shipment, storage on shelf and schlepping to your AO.
Inadvertent bending induced by assembly, as you wrestle it into place.
Alignment of barrel nut clearance cut.
Location of clearance hole drilled in upper receiver
Maybe I've worked on too many frankenguns, but I go into it assuming Ill have to at the very least check alignment, and probably adjust it to make it a perfect fit.
But a good tip, and one worth having in your bag of tricks.