The M16A4 is most often equipped with the Knights Armament Corp (KAC) M5 RAS.

Personally, I prefer a good free-floated handguard instead.  The LaRue is probably the best, followed very closely by the Daniel Defense and KAC FF-RAS.  These are all much more secure, stable rail systems and give you a free-floated barrel, so that the torque from a FVG or bipod, or the weight of accessories being installed or removed, aren't going to affect the rifle's accuracy.

Installation of a FF handguard is a little more involved, yes, but absolutely worth the effort, especially since the rails I listed cost about the same as the M5 RAS.

-Troy