I have a KAC RAS on my Defender 2000 M4
its pretty nice and easy to install.

I was looking at the KAC RIS at OPERATIONPARTS.com

They look about the same

whats up?

The RIS is held, by tension, between the handguard cap and the delta ring.  This means that there will be some slight rotation of the rails around the barrel; not enough to make a difference for VFG's and lights, but I wouldn't trust it to hold zero for optics and lasers.  Naval Special Warfare is the only group still issuing the RIS because they like its ability to accomodate heavy barrels.

The RAS is the newer of the two and is issued to everybody else.  It is tremendously more secure because it clamps to the barrel nut as well as being secured between the handguard cap and delta ring.  It is rock solid and can hold zero for optics and lasers.  It's only caveat is that it will only fit on government profile (M4) barrels.

I've got a RAS on one of my rifles and I love it!  The RAS is the way to go.

IRC, the RIS is for the pencil barrels.

+1 for the RAS, its what you want.

Any info on the SAS lower rail version?

Quoted: Any info on the SAS lower rail version?

What do you want to know?
It clamps just like the upper RAS portion.
Downside- by the time you buy one, a FF of some sort would likely have been cheaper.
I noticed mine was slightly tougher to get a cover on, but that's better than too loose.

Quoted: Naval Special Warfare is the only group still issuing the RIS because they like its ability to accomodate heavy barrels.

They like it because it can be easily removed for cleaning after maritime operations.

The RAS is the newer of the two and is issued to everybody else....  It's only caveat is that it will only fit on government profile (M4) barrels.

Wrong

It will fit any standard HBAR or SOCOM weight barrel.

I do have a RAS on my 14.5 defender M4

It is rock solid

I also have a Larue cantilever and ML2

I like my aimpoint to sit as far as possible to the FSB

Do you guys really think the RAS is SOLID Enough for mounting  ML2/cantlever/

I use to shoot my shorty A1 with a goose neck,QRP,ML2
and FOUND it to be the best set up for a aimpoint

IMHO yes and no.

The RAS is solid enough for optics as long as you don't need fine precision.  Minute of man at 300 yards or less, yes.