Re: Accuracy Systems uppers, anyone used them.
Actually, their guarantee isn't quite what you are making it out to be, bjl333.
They guarantee SOME of their rifles to sub 1/2 MOA, others to 1/2 MOA accuracy, others to 3/4 MOA and others only to 1MOA. The guarantees on JUST UPPERS vary from complete rifles that they build. Also, they only use 3-round, 100yd groups, shot with what they call "factory selected ammo." I don't put a lot of faith in 3-round groups as the litmus test for a rifle's true capabilities. Before you (or any of your buddies) place an order, please read the fine print on exactly what you are buying and what they are guaranteeing you.
Now, with that said, I have had the chance to shoot some of their AR-10 uppers and complete rifles. All were sub-MOA capable out of the box with match ammo and quality reloads. The best of the ones I have fired was one of their UltraMatch complete build rifles in 308 which would hold 3/4MOA 5-round groups all day with good ammo and if the driver was doing his part. Reliability was also excellent. In shooting several hundred rounds through various rifles and uppers that ASI built, I don't recall experiencing a single malfunction.
I don't have any hesitation recommending them to you for a quality-built, accurate AR-platform rifle or upper. Just know what you are buying, talk to them about the extent of their warranty/guarantee and your expectations from the rifle/upper before you drop the coin.
A few other sources to consider for a custom AR-build are (in no particular order):
1) WOA
2) CLE
3) Dave Lauck/D&L Sports
Good luck!