I'm doing my first build and accumulated quite a few parts....enough for two builds actually. One I'll make into a pseudo M4. So in this process, I bought a UTG tri rail barrel mount, #002, but am now confused as to how it fits. My initial thought was that it'll naturally fit in the groove that is in the barrel right in front of the floating forearm. But the groove front indent is at a right angle (perpendicular to the barrel) and tapers in the back - like many barrels I've seen out there.....Does this groove have a name? A specific purpose? A special 'accessory' for which it's made? My UTG won't fit in it....... Thanks for the knowledge,
http://www.outbreakordnance.com/images/products/originals/504_725_20140424_112013.jpg
Heres a pic of what I mean.....

It's my understanding that the notch is for mounting an M203 grenade launcher.

FPNI

Makes the barrel a suitable diameter at that point to mount the M203 grenade launcher, the design of which is unchanged since the original M16/M16A1.  

The real question, is why bother with the rest of the heavy profile part.