I finally got my stamp back. I was trying to assemble the upper using a 12" Seekins MCSR hand guard. The barrel nut that came with the hand guard doesn't have a shoulder on the inside to cinch down the barrel. It is threaded all the way through. Is this a defective nut or am I missing something?

http://i.imgur.com/NDiSr2U.jpg

That is the newest style which has a second piece, a barrel retainer "nut" with wrench flats that threads into the piece that you have pictured.  It doesn't come with shims like the previous style with a shoulder.  You have to thread your piece onto your upper receiver, time it with the gas tube hole, make sure it's not too far back that your hand guard hits the upper receiver, insert your barrel, slide the other piece with wrench flats on, torque the barrel retainer "nut" while not losing your gas tube alignment, install gas block, hand guard, etc.  Click the link below to see a picture of what you're missing.

Seekins Barrel Nut

HTH,
Okak

So I'm missing a piece. Thanks, Okak.

Also make sure when assembling it, use a appropriate size drill bit shank through the big part of the barrel nut and into the gas port to keep it from rotating. Then tighten and torque the smaller lock nut.

Thanks for the drill bit tip.

I talked to Seekins this morning. They are sending me a new barrel nut. Very good customer service...