I believe what's being talked about is the indexing of the barrel nut so the gas tube hole/notch lined up with the hole in the upper receiver for the gas tube to pass through.

Sometimes it's tricky to get the nut cinched down and have the hole line up right from the start... usually you need to make a judgement call and decide whether to crank the barrel nut down a tad bit more or back it off, to get the alignment.

Putting together an upper is not rocket science... remember, one of the purposes of the modular design of the AR was ease of assembly and repair, in a less sophisticated settings than a gunsmith's work bench with all of the attending precision tools and gauges.

One doesn't just slap one together in a few minutes (although some people I know have become so proficient that it seems like they do), but there is also no reason to get all OCD over it and over think the process either... unless you're talking about a precision match rifle.

Don't be intimidated... give it a try, take your time first time out, have some fun with it. I've built 4 AR's... you learn and improve, and you can always go back and fine tune what you did on the first build.

Also, all uppers are not created equal... quality and price points vary. That thread life stated, in my opinion has been overly generalized. Receivers machined from solid billet verses investment cast will stand up better.

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Conrad
SWCA #1830 SWHF #222