Look at Troy, Midwest Industries, Knights Armament, or any other big names. Alternatievely, the
VLTOR CASSV is a unique option that utilizes the flat-top picatinny rail to attach to the rifle.
I have a
Midwest Industries SS rail on my 18" build. The modular design lends to lighter weight and variety in accessory placement than a standard 12/3/6/9 o'clock quad rail allows. The newer keymod and Magpul M-LOK type systems are easier to attach accessories.
MI M-LOK rail example:
http://vtsupply.com/mi-midwest-indus...handguard.html
TFL member Plouffedaddy has some info on the M-LOK and some handguards on Youtube (Mr. Guns N Gear).
https://www.youtube.com/user/Mrgunsn...ch?query=M-LOK