I recently finished my first ar15 build.  I have a spikes upper, cmmg barrel, with a Midwest industries 12" free float rail.  After installing the barrel nut and completing the build I realized I didn't use anti seize or grease on the barrel nut/threads. Is there any reason for concern to installing it dry?  I would hate to take it all apart if it is not a huge deal.  Any help or opinions would be great.  Thanks guys.

You know the answer.

Maybe he doesn't, so he's asking?

You should disassemble, apply oil, and reassemble. Sorry man.

Thanks for the reply. I do not know the correct answer, just a feeling I should take apart and apply some. I will go ahead and take it off and apply so lube. Thanks man

Meh ... Most of the AR's on this forum will never see any inclement weather or a round count in excess of 2500. They'll go from under the bed to the truck to the range and back again.

During the assembly of your AR, should you use some grease, meh, wouldn't hurt. Wanna tear down your completed build and add grease ?  If it'll help you sleep at night, go for it.

The grease helps prevent galling the threads on the upper from galvanic corrosion.
If moisture gets in and has a chance to attack the aluminum, even worse.

Think of the initial assembly as a practice run.