Look up Clark Customs, DPMS carbon fiber, Carbon Arms (spendy), PRI (very spendy), Lancer systems (spendy), and Loki Vented hand guards. Some are cheap, but the barrel nut sucks to put on and get off, and the spendy ones might be more then you want to spend. I went with JP modular which gets very cold in the winter compared to the carbon fiber, but the feel and strength of them are awesome.