So  i finally got the last bit of the stuff i need to put my 10.5 upper together and when i went to put the barrel nut on, it wouldnt go past the area right before the extention.  i pulled out my micrometer and the barrel is .984-.985 and the I.D. of the barrel nut is .978.  ive tried heating up just the barrel.  then just the barrel nut.  then both. still nothing.  im thinking either sand the barrel or nut down some, or should i try freezing the barrel and heating the barrel nut?

The I.D. of the barrel nut should be 1.01" I believe. I'd have to find my print to verify. Edit: checked print. 1.01" +. 007

A lathe is the way to go. Turn the barrel nut to the proper I.D.

Not worth the trouble, what's a barrel nut, $10? Just get a proper replacement.

Well, i ended up able to get it on and torqued down.  I put the barrel in the freezer for 30 mins and took the torch to the barrel nut.  once i did that it slide right on.  i did have to heat up the barrel nut again to get it to get the gas tube lined up (was just off a hair, ended up just over 55 lb tq).

Reason I went through this "trouble" is because its a POF upper/barrel nut but the barrel was SAA.  i know POF is somewhat propriatary/fine line of "mil spec", mostly to match up to their own barrels.  The pain in the butt part was making sure the barrel nut was hot enough i was geting a correct tq reading.

Water under the bridge now but you should have enlarged the center of the barrel nut to allow it to fit over the barrel OD.  It's easy to do with a drum sander in a drill press.  You may have problems when you try to remove that nut for maintenance later.

yeah, i figured if/when i ever have to take the barrel off, its going to be "fun".  prob have to REALLY heat up the barrel nut.  guess i will find out 1 day  

Use a "cheater bar" with your barrel nut/armorers wrench. I slide a piece of pipe (extension from my floor jack) over my armorers wrench to get more leverage to remove problem barrel nuts. Just be careful, don't caveman it, and it shouldn't take much effort.

Worse comes to worse, you can actually pad the jaws on a pipe wrench and get it off in the future.