I just got my AR10 upper back from ADCO, threaded for the Armalite Flash suppressor.  I've never actually used a threeaded barrel before, so forgive my ignorance.  I was under the impression that the flash suppressor would simply screw off, but I cant get it to budge.  Any thoughts?

It should unless it was pinned or silver soldered in place to be "permanently attached."  Check with Adco to see if they did.  If it is not permanently attached, use a strap wrench to take it off.   Note that the crush washer is pretty much good for only a one time use to achieve that perfectly desired position, if it matters.  If you are putting on a new FS and have a new crush washer, it is easy to put on the new FS to the desired position.

P.S.  The upper will need to be in vice block or under someone's butt for both removal and application.  Getting a hold of the FS is the easy part, getting a hold of the barrel/upper can be the hard part without a vice block.

Quoted: I just got my AR10 upper back from ADCO, threaded for the Armalite Flash suppressor.  I've never actually used a threeaded barrel before, so forgive my ignorance.  I was under the impression that the flash suppressor would simply screw off, but I cant get it to budge.  Any thoughts?

I'm confused (not an unusual state of being ...)

Your AR-10 upper from ADCO is threaded - does it have a flash suppressor on it?

If yes:

Are you trying to take the ADCO flash suppressor off and put on another flash suppressor?

If no:

Are you trying to take the flash suppressor off another rifle? If so, details?

I had ADCO thread my barrel for the Armalite flash suppressor, which is the same pitch as the .308 Vortex.  They shipped it back to me with the Armalite and now I'm installing the vortex.  They appear to have threaded it in such a way that it indexes without a washer.

In that instance, they may have put a bit of threadlock on.  Thus, you may need to put in that much more effort.  I would still check with Adco first.

Unless it was permanently attached... or threaded with left handed threads.... it should just be righty-tighty-lefty-loosey.

That being said, the proper way to remove and reinstall any muzzle device is with the barrel firmly gripped in a vise with barrel vise blocks.  Sometimes it takes some torque.... depending on how it was installed (loctite, overtorqued... etc..)