I'm looking to build an AR15 that will push out to 500 plus.  I built my first AR15 about 2 years ago, learned a lot about the weapon system as a whole and the individual systems inside.  I've always enjoyed shooting distance, my first rifle is a Remington 700 VTR in 308.  Anyways, I built my first on the cheap with a ATI Omni Hybrid Lower and a radical firearms 16" upper.  I've had my share of issues with my first build which lead to my further understanding of the lego type platform the AR is known for.  I just acquired another lower and put it together last night, it's an LRB receiver with the Stag Arms LPK.  Due to my, excuse me, ignorance; I had an extra buffer tube assembly laying around, so I put that on there.  I married it to the upper off my other AR and the functions check/ test went fine.  I'll be going out to  pretty soon, I'm just waiting for my CA bullet button to arrive (my wife makes me live here).  My basic understanding of rifles is longer barrel equals better accuracy.  I understand that ammo, glass, etc, factor in as well, but what components would you use to build a rifle that will reach out too 500 yards and beyond.  I'd like to keep this discussion to AR15 rifle parts but if you wanna gimme your $.02 about scopes I'm all ears.  I'd include some pictures but I don't see an easy way to do that without uploading my pictures somewhere else first.

there is more to it than longer barrels.  Quality trumps length.  Spend your money on a good barrel and good trigger.  They will have the greatest impact on accuracy when it comes to components.

The big three accuracy factors are your barrel, trigger, and bcg in that order.
Barrel-I have a 16 inch, midlength gas, stainless h bar barrel with 1:8 5r rifling. Puts them touching at 100 yards. Id say barrel wise look for one like what i have, maybe an 18 incher and a 1:7 twist so you can run the really heavy loads to overcome wind deflection.
Trigger- Any reputable trigger maker will do, really depends on preference. I have a timney with a flat face and overpowered hammer spring
BCG- I have a failzero in mine and love it but there is so many out there that its really up to you.

Since you haven't mentioned a particular caliber just "AR-15".  I would like to suggest going with the 6.5 grendel in caliber.  I'm finishing up a 6.5 build (waiting on optics) that is identical to what a friend has done.  He's had it out easily to 800 yds.  He's planning on taking it to a 1000+ yd range after the 1st of the year.  It's a Spike's lower with Alexander Arms 18" upper.

Barrel length is almost irrelevant.  Barrel uniformity and precision in profile, bore and rifling do matter.  A lot.   Stiffness (thickness) matters up to a point.  You want a heavier, stiffer barrel, but it need not be a "bull" barrel.

If you are serious, look at barrels from Lilja, Kreiger, Shilen, Bartlein and other custom barrel makers.  I have a Lilja 16" Recce profile barrel in Wylde .223 chamber, 1:8 twist that has the accuracy you are seeking, if used with match grade ammo.

There are many 16" and 18" sub MOA AR15 rifle barrel options.  The only benefit of length is that it can give you a little more velocity so that the bullet does not go transonic and destabilize at longer range.  With a 16" barrel that might happen out at around 700 yards,  a longer barrel will stretch that somewhat, but a .224" bullet in a .223 case, loaded in a manner that it will cycle through an AR15 magazine is not very good for trying to shoot to 1,000 yards.  You really need a different caliber for that.

A good match grade trigger and a free float rail (just about any free float will do -- your preference), are also essential.  FWIW, the Lilja barreled rifle has a Geissele SSA-E trigger, which has a light 1.2 pound second stage.  This is a "Recce" build so it commonly wears a QD mounted 3-9x40mm Leupold Patrol scope.  But for precision shooting I use a higher powered QD scope.  In this instance a now discontinued Burris Black Diamond 6-24x50 with 1/8 MOA per click turrets and a fine 1/8 MOA target reticle.  That scope is useless for general use, but set at 24x or dialed back a little as needed to address mirage, becomes superb for shooting tight groups or smaller targets "way on out there".

GOOD STUFF!!  Thanks for the feed back guys!  I just got back to my computer and will need to reread all your posts.  Common theme seems to be barrel and trigger.  I have Mosin I put into and Archangel stock, scout scope, and a Timney drop in trigger, I love my Mosin!  Also, I have a Vortex 6x20 with BDC on my VTR. Would something like that work for this build too?  I was planning on staying .223, but not decisions have been made so I will check out other one mentioned too.  I reload my own so any suggestions about ammo?

Adrian:  Just to add to the advice from the others:  NRA High Power shooters routinely use the .223 cartridge in AR-15 rifles for the National Match course of fire:  60 rounds total at 200, 300 and 600 yards.  For the longest distance, they like 77 grain Sierra Matchkings and similar bullets with slower powders like Varget and Reloder 15.  Sometimes they load these pills long for more powder space and single load them into the chamber for each shot (600 yards is strictly slow fire in the National Match course).

As for the rifle, a longer barrel is heavier, which helps maintain consistency of your hold, also some additional velocity which helps at the longer ranges.  My latest build is based on a Compass Lake "varmint" barrel, using a Douglas blank, 22" long, Wylde chamber, 1in8 twist.  The finished barrel weighs 4 lbs, cost was about $435 with a matching bolt.  I picked 1in8 twist because I typically shoot at shorter ranges with lighter bullets.  For the 77 grainers, I believe you should get a 1in7 twist.  

FWIW, my heavy 22" rifle routinely prints 1/2 inch, 5-round groups at 100 yards with 53 grain Matchkings, Vit N133 powder and BR-4 primers.  Velocity is about 3240 fps. I'm still working on load development, my objective is 10 rounds into less than half an inch at 100 yards.  good luck - CW