I think its also called an RIS, or handguard.
Basically I am looking at a Dboys M4 RAS II, and the RIS/barrelshroud has a riser built into it, which I dont mind, but plan on changing in the future. So I was wondering how they were mounted, and if I would have to buy a specific type to replace it.
Here's a pic:
So do they all work like that^, or is that manufacture specific.