Custom building:
AR Performance: Barrels and Bolts
Wilson Combat Barrels- Stainless
Geissele SSA-E Trigger / National Match DMR
Budget Complete Uppers
Stag Arms
Rock River new uppers converted with MI free float hand guards can be bought cheap off Gun Broker.
Budget Budget Uppers Built to your Specs:
Model 1 Sales- in 6.8 SPC II chamber.
I have a 16" carbine and it shoots under 1 inch at 100yds with 90gr Speer TNT's at 2830fps- good frag bullet for varmints.
Burris 2-7x35mm Fullfield budget glass, nikon M223 mount. It is nothing fancy - a working man's stick.
I like Model 1 Sales over DPMS in this budget category, YMMV.
DPMS no longer makes 6.8 SPC II rifles, but Brownells sells some of their surplus remaining 16" carbine 6.8 SPC II barrels with a pinned carbine front sight gas block and barrel nut ready to directly swap out with a pawn shop bought 223 AR15. Bushmasters are good for this as "black rifles" are back on the shelves.
Brownells also sells 6.8 SPC LWRC bolts and Daniel Defense Bolts at very reasonable prices.
There was some uppers testing done awhile back and this illustrates some basic accuracy: Rock River did pretty good as a budget upper.
http://www.ar15chatterbox.com/viewtopic.php?f=25&t=5320