I have a KAC M14 RAS on my M14.
The KAC M14 RAS basically replaces your handguard.
It attaches to the reciever via the reciever scope mount and a threaded clip that fits in place of the handguard clip.The front of the rail has a spring loaded clip that slides under the barrel band.
Mine is the deluxe version that has a Picatinney rail section that replaces the stripper clip guide.
All in all, the set up is pretty solid and, abd there is lots of room above the chamber for cases to eject without the fear of them hitting or hanging up on anything.
If you are installing rails on an M4, you want the Free floating rail systems so that you don't run into the problem of "tweaking" your barrel.I have a KAC RAS II on my M4 and it is the cats ass!!!
SKBY.