Lot's of choices. Even more if you're willing to spend in the 500's
http://www.pkfirearms.com/
Rock River Arms CAR A4 - R4 Chrome Lined, Lightweight Complete Upper Rock River Arms A4 flat top receiver, 16" M4, 5.56 Wilson Lightweight profile, 1/9 twist rate, chrome-lined bore and chamber. Steel front sight tower with bayonet lug.
Complete with bolt carrier assembly and forged charging handle.
Our Price: each $489.25
Rock River Arms Standard A4 20 inch Upper RRA Standard A4 Flat Top, 20" H-Bar. 4140 steel, 223 Wylde chamber, 1/9 twist rate, 1/2x28 RH threaded muzzle.
Includes RRA Bolt Carrier Group and Forged Charging handle.
Our Price: each $465.50
Daniel Defense MID Length With 16 Inch Hammer Forged Barrel Daniel Defense 16 inch Cold Hammer Forged barrel installed on a CMT Flat Top receiver. 5.56 chrome lined bore and chamber, 1/7 twist, F marked front sight, A2 flash hider, M4 ramped extension and receiver. Standard handguards.
Includes high quality, Bolt carrier assembly and forged charging handle.
These are assembled and test fired by PK Firearms.
Our Price: each $585.00
Sabre Defence 16 inch Mid Length Basic Upper Features a Sabre Defence CMV 16" govt. contour, 4150 grade Mil-Spec per MIL-B-1159E ( CMV ) chrome moly vanadium barrel with mid-length gas system, 5.56 chrome lined bore and chamber, 1/7 twist, M4 feed ramps on barrel extension, F marked front sight tower. Built on a Mil-Spec A3 flat top with M4 ramps.
Includes AR15 Bolt Group Assembly and forged Charging Handle.
These are assembled and test fired by PK Firearms.
Our Price: each $590.00