These tubes come with either an aluminum or steel barrel nut. The aluminum ones appear to be approximately 1-1/4" on the flats. The steel ones measure somewhere between 1.263" to 1.264", or approximately 33mm. 

I can get a 1-1/2" crowfoot wrench pretty easily at an auto parts store. But to torque the steel barrel nut, there really isn't a reasonably priced solution, other than an adjustable crowfoot wrench. And the flats on the steel nuts are narrow, so wider wrenches won't fit.

I could use a regular adjustable wrench, but since I don't do barrel nuts more than once every couple of years or so, I don't have a "feel" for the proper torque range. 

What are you guys using to get the Guntec barrel nuts tight and timed right without exceeding 80 ft-lbs?

Guntec sells a wrench made to work with the aluminum version, but nothing for the steel version. I am considering just cutting, drilling, and filing a piece of 1/4" thick aluminum to make a crow foot, but I'd rather not have to spend a few hours doing that.

Will a std barrel nut wrench fit.
I believe most contain a 1/2 socket reciever you can attach a torque wrench to.
Torque so the between tube is timed between 50-80 ft/lbs
Use a torque extention formula to determine the corrected torque setting when using an extention.
http://www.engineersedge.com/manufacturing_spec/calculator/torque_adapter_extension.htm

Thanks for the link to the calculator.

A regular armorer's wrench will not fit, unfortunately.

i think i just used a big crescent wrench and got it to about what felt right for all the uppers i have built and then just snugged it a bit.

I called guntec because I ordered one as well and installed it, they said between 30-40lbs of torque max. What I did it I figured how much it needs to turn with the old barrel nut and marked it using 30lbs then marked it at 40lbs. I marked a starting point on the upper so I can use it as a point of reference on the new barrel nut.  A crescent wrench was the only thing that fit so then I just toruqued it until it aligned the holes on the rail and was between or close to the 40lbs mark, I think it was close to the 40lbs. May not be the right way but its snugged up pretty good and my armores wrench didn't fit soo....

I used a vice grip style Cresent wrench I bought from sears it's a actual crescent wrench with a lock so it us snug. It was narrow enough to fit between the flats. The barrel nut torque needs to be between 35-80 lb-ft. That's a big gap. I tightened it and loosened it a few times to season the threads so you will get a feel for it. Mine needed 1 shim to get to clock right. Give it a tighten and don't worry about it. I must say I have been impressed with the guntec rail. I have the mlok octagonal one and bought it off Amazon trying to make a purchase for a certain amount to get a rebate. I had a MI rail for the rifle but tried the guntec first. It's still on this build. It has been a great rail.

I think this is going to be how I end up doing it. I really wanted to get a proper torque on it but I guess it will have to be a guestimate. I might clamp my torque wrench to another vise and use it to get a feel for the settings.

Someone should make a crescent wrench with a torque handle. 

Ok, after puzzling over this for a bit I think I have a solution that will only require a cheap trip to the hardware store.

The supplied screws are too short and hanging on by a thread, but you get the idea. This will allow me to snap a torque wrench into the armorer's wrench and torque it to the desired values.

I just totally bypassed using the stupid wrench flats altogether. This will probably mar the threads of the screws, but it's better than guessing, or buying a 33mm crowfoot wrench from overseas.

Genius.  You can always buy replacement screws at Ace.

Thanks! 

The screw threads turned out to be 10-32 in case anyone needs to know.

Installed mine tonight, used a regular crescent wrench and tightened it down pretty tight. Didn't need a shim and went right in, ill post a pic up tomorrow.

Looks like it mates up with the receiver pretty close.

It is up against the reciever with no gap, it's a DPMS LR-308 so when I looked at the mounting holes I knew the base was different shape and the color is a bit diff but as long as the rails mate and its one fluid rail I'm happy. Once thing I noticed, its SUPER LIGHT, I was impressed.