I've seen the RAS, but I've never seen a RIS.  what exactly is the difference?  Do these replace the handguards?

You may want to consider the Advanced Armaments Rail...for $99 you get both the rail and the vert grip...they don't recommend using it for aiming devices though.

My rail attached to the bottom of my forestock. Looks sweet.

One is made for the heavy barrel profile. The other is made only for the light barrel profile. I think the RAS is the one that could fit only the light barrel profile.  In addition, the locking system to the upper is different.  The RIS uses an adjustable front tab to squeeze the two halfs togethter.  The RAS uses two hooks in the back to catch the barrel nut.

what ar18 said, also a RIS's index numbers start from the front sight, a RAS's numbers start from the rear.

Can someone post a pic pointing out the differences? Also doesn't the Navy/Marines use one style will the Army use the other?

Thanks,
Clinth

From here:

noradairsoft.tripod.com/Intel/M4RIS/M4RIS.html

"After the astoundingly successful release of their M4A1 AEG, Marui developed and released a modular 20mm rail system for that gun based on the Knight’s Armament M4A1 Modular Weapons System, including a vertical foregrip and a fake laser unit battery compartment. Following the failure of the Advanced Combat Rifle trials in 1995, this system was introduced into the US Army as the RIS (Rail Interface System) and then later renamed the RAS (Rail Accessory System) and is currently issued to special operations, airborne, and light infantry troops. The purpose of the MWS was to enable current M4A1 carbines to mount a large array of targeting equipment and structural modifications. Currently, airborne troops use the PAQ-4 laser and a vertical foregrip. "


Some RAS pics:

www.jtdistributing.com/ar15/knights_armament.shtml

Apparently you can still get the RIS for carbines with light weight barrels:

www.impactguns.com/store/kmc_mws.html

Personally I think the Fulton-Aromy solution is much better than the RIS or RAS.  Its a very sturdy unit and allows for true free floating.