I have a UTG drop in rail mounted on my carbine and it has been perfect for my use. I'm going to upgrade to a troy-mrf unless, somebody can suggest a free float rail that uses a standard barrel nut. I don't want to remove my pinned gas block.

P.S. the UTG rail has served it's purpose and I don't understand why everyone is so disgusted by them...this one will be saved for a designated .22 build in the distant future.

http://www.hitechtactical.com/VLTOR-CASV-EL-Free-float-Handguard.html
you can find it for less somewhere else but it had pics

Midwest Industries make a 2 piece free float that utilizes the standard barrel nut. I've never had a problem with them.
You can also use the DD Omega with a standard barrel nut.
Dave N