The Satern barrels with an AA licensed reamer are too accurate for many people to believe. I've shot them and watched others shoot them on the IPSC flagger at 700yds, and get bored. Basically shooting a 10" plate repeatedly in shifting winds at 700yds, ringing it time after time, to include new shooters.
If you can't find an AA complete upper...
Precision Firearms makes some top-end Grendel uppers with barrels from Krieger, Bartlein, or Lilja.
Lilja makes a drop-in Grendel barrel now as well.
JP has been making Grendel uppers/barrels for years and years.
J&T/Doublestar sells complete uppers.
The only parts that are different are:
Barrel
Bolt
Magazine
That is based on the premise that we are dealing with quality AR15 parts made to the Mil-spec or TDP, then if you want the most accuracy you can get, you will have to look at several other areas of the components to ensure consistency. The main parts I pay more attention to with a Grendel project are:
* Bolt: The bolt needs to be made by a machine shop and kiln who know what they are doing, not like what we've seen with most of the after-market 7.62x39 bolts. AA, Maxim, and Underground Tactical all use 9310 high grade steel bolts, made to certain dimensions. For starters, the bolt is longer at 2.810", with a .136" (-.003") recess. The extractor usually needs some tuning, and should be made from tool steel if you can get them that way. Some are tuned, de-edged, then nitrided. I chamfer/radius the ejector face so that you don't get right side magazine feed failure to feeds.
* Upper receiver: Face trued, billet if using heavy barrel (7075 T6 and Type III are givens no matter what I'm doing.) I also like tighter extension tunnels for accurate builds. JP uses a thermal fit extension to a very tight upper.
* Barrel extension: The extension needs to be held to much higher standard of dimensional uniformity and material specs. I then blend and polish the extension to the upper, removing any sharp edges that normally scrape your brass. Lilja uses BAT Machine extensions, which are then tumble-polished and de-edged significantly. Precision Firearms uses polished extensions with feed ramps that are ideal for the Grendel, and then has them Black-nitrided. I suspect JP has something peculiar for their extensions as well. AA is very particular about what kind of extensions they use, and theirs are made specifically for them according to their requirements.
* Gas system and recoil system: Depending on what barrel length and gas system length you use, there are certain gas port dimensions and carrier/buffer weights that will work better together, depending on the loads you use. With 123gr shooting medium-slow powders, there is a certain gas volume and impulse imparted on the bolt carrier group and recoil system. For accuracy, High Power smithing techniques are common throughout the shops that know how to make accurate guns, which means Red Loc-tite is going to be used to secure the gas block, and the extension will be bedded with Blue Loc-tite or some other compound. Someone here complained that they couldn't remove the barrel from their Les Baer rifle, even though it was shooting bug holes.
For my 16" MLGS with .078" gas port, I'm using a full M16 carrier weight, with an ST-T2 buffer (H2 weight), and a correct carbine spring.
For 18" MLGS, you might consider an H3 buffer and M16 carrier, an MGI RRB, or a JP SCS. Rifle buffer works well too there.
For 20" .094" gas port RLGS, the rifle buffer works fine with a rifle extension tube, and many of the carbine buffers work with the carbine extensions. The JP tuned flat wire springs work well too with the right buffer.
I can't see what region you live in, so I'm not sure where to point you in terms of barrel length for a 600-1000yd target AR15, but I would generally advise the 20" if that is all you really want it for. I've shot my 16" out to 1200yds with surprisingly predictable results, but I was at much higher altitude, with 80 F temps, and low barometric pressure. I really need to install a 20 MOA cant scope mount, since I figured I "only" had a 700yd capable 16" carbine that is hitting steel with 85% of the energy of a .308, but from a smaller AR15 package that has half the recoil.
