What kind of configuration do you want? Fullsize 20", M4 carbine, varmint rig?
Personally I prefer Colt and Rock River. RRAs and Bushys are well within your price range. Most people here have had better luck with Bushy than I have so I'll keep my editorial on Bushmaster QA to myself (this time [:)]).
If you'd like an M4 the Colt 6400c M4 carbine can be had for around $900 if you shop around.
It has an M4 marked upper and lower:
[img]http://home.comcast.net/~cjan99999/M4_Lower_small.JPG[/img]
[img]http://home.comcast.net/~cjan99999/Upper.JPG[/img]
Heavy duty "H" buffer and heavy duty extractor spring for reliability:
[img]http://home.comcast.net/~cjan99999/Buffer.JPG[/img]
[img]http://home.comcast.net/~cjan99999/Extractor.JPG[/img]
Barrel has proper 1/7ROT, chamber and barrel are chrome lined:
[img]http://home.comcast.net/~cjan99999/Cmp.jpg[/img]
Upper is RAS numbered:
[img]http://home.comcast.net/~cjan99999/Ras.jpg[/img]
Side sling mount:
[img]http://home.comcast.net/~cjan99999/Sling.JPG[/img]
The 6400c has a standard pushpin lower, no protruding sear block, proper M4 handguards with dual heatshields, extended feedramps, and has a [b]lifetime service agreement.[/b]
There are [b]plenty[/b] of rifles available in your price range, and lots of merchants here who can hook you up with a great deal. To get an idea of what you want you can always check the manufacturers sites too:
[url]www.rockriverarms.com[/url]
[url]www.colt.com[/url]
[url]www.bushmaster.com[/url]
Good luck on your quest for the perfect black rifle [:)]
Edit: You could also build a complete rifle for less than $600. There is a complete lower assembly guide [url=http://www.ar15.com/content/docs/assembly/lower/]here[/url]