Yes, I know I can find some directions and where to buy the tools.  I've never done this, I've put lowers together, but a special needs monkey can put an AR lower together.  

I've currently got paperwork in for a 12.5" SBR.  So, I've got a while to learn before I can buy all the parts and assemble.  Anyway, I've got my eye on Denny's Operator 12.5" barrel.  Comes with barrel nut and F marked front sight base already attached.  My big fear is the headspacing.  I know it can be dangerous if I screw up on the headspacing.  In addition it'll be on an NFA registered lower, any damage to that would majorly such, suck, suck!!  So, if this is something that an average guy who doesn't have a whole lot if build experience can do, OK.  If not, I'll probably be bettor sending all the parts to a pro to assemble.  

What say you?  Also, how many and which headspacing gauges would I need?

Let me know when you find out because I am one of those special needs monkeys.  I just built a lower for the first time ever and I thought it was real easy.

The barrel should already be headspaced, when the barrel extension is torqued on and pinned. You won't be doing that part

You can check it with a gauge, but I haven't encountered a problem yet.

A special needs monkey with a quadruple thumb amputation can put an upper together. After two bottles of Night Train. The only "special" tool you need is the barrel nut wrench. i got the Smith Armorers Wrench and like it a lot. It does barrel nuts, flash hiders, CAR castle nuts, and about everything else on an AR. Good piece of kit. Upper vise blocks are unnecessary unless you're doing A2's. And a torque wrench is useless. Snug it up 3 times then line up the next notch. Done.

Headspacing is done when the extension is installed, so you don't have any control over that. If you're really worried you can get a "matched" bolt when you buy the barrel, but it really doesn't matter unless you're trying to eke out every last iota of accuracy.

Thanks for the info. Duh!  How ignorant am I that I didn't know that the barrel would come headspaced already (provided that it has the extension installed).  Yes!  I'll grab that wrench (that one from Browenells looks perfect) and I'll slap it together...once the paperwork goes through.  

If I were to want to test the headspacing, what would you recommend?

You can get a headspace gauge. I wouldn't worry about it though. When I was building FAL's I used the WECSOG method. If the bolt closed on a Fed GMM round and didn't close on the GMM round with a bit of cellotape on the base it was GTG. You need to measure the thickness of whatever tape you're using, though. If you don't have good calipers then it's sort of tough and involves more purchasing. If you get your barrel from a reputable source, it's fine. Just go shoot it.

My first AR, I built the upper myself.  I like to build things so it wasn't a problem for me.  I just used the general tools you should already have (wrench, screwdrivers, nail punches, etc...), so those extra tools to help your upper assembly aren't neccessary.  If I was to do it again, I might be willing to buy them for the time savings.  As for the barrel nut wrench, that is only neccesary depending on your upper.  I bought the VLTOR VIS upper and all I needed was a wrench (because the upper came with the special wrench attachment for this upper), so for certain uppers the nut wrench isn't needed.

I recommend mock building the upper first (before you do the real assembly) so you can see where you might run into a hitch.  For example, my upper receiver has the forend permantely attached to it, so after I had put the barrel in the upper then got the gas block set I had yet to put the gas tube in there.  I didn't think the loctite I used on the gas block would be some damn permanent (it dried in 20 sec and wouldn't budge when hit with a hammer).  So it took me 45 min. and 3 sets of needle nose pliers to finally get the gas tube in.

Putting together an upper yourself is like playing with an erector set.

Which means?

^^ Which means really easy. And fun. Like an erector set.

Hey thanks  i had same question.

I wouldn't say "really" easy.  But it's not hard by any means, as long as you pay attention to what you're doing.  Erector sets can be screwed up if you use the wrong part in the wrong place.

Besides the armorer's wrench, get an upper vise block.  

A good vise block costs less than a good upper...

Shane

Probably the most difficult part can be getting the barrel nut aligned so that the gas tube passes through it...lots of tighten and loosen activity on mine after a thorough thread cleaning.

Good parts make life easier.  recently had a barrel where the extention was not torqued tight and it seemed that the gas tube port on the FSB was tapered.  I now check both before assembly.

If you can't finish it a gunsmith can do it for under 50$. my smith assembled an upper for me a couple weeks ago for $35. he warrantys his work too. You won't be taking alot of risk by buying the parts you actually 'want'. Nothing better than shooting a custom upper tailored to your taste

Home Depot has a great 4" vise for $19.50

Another thing that I thought about...

Everything should go together EASY.  You shouldn't have to force anything.  Yes, you have to torque the barrel nut, and you have to drive the sight pins, but if anything is REALLY hard to do, then something is wrong.  

Shane

Carefull some of you are showing your age. If you don't know what erector sets are you are either really old or really young.