Rail Interface System (RIS) and Rail Adapter System (RAS). Both of these units use MIL-STD-1913 and MIL-STD-1913 Update Notice 1 "Picattinny" rails incorporated into innovative forend rail assemblies to replace the factory handguards of the host SR-15(AR-15), M-16, and SR-25 weapon systems and provide attachment points for a variety of weapon system accessories.
The RIS and RAS units are similar in that they consist of two major components: a top component consisting of top, left, and right quadrants, and a bottom component consisting of a bottom quadrant. Each quadrant contains a MIL-STD-1913 rail that can be used to attach a variety of weapon accessories. Differences between these two units occur in the way that they attach to the handguard cap and delta ring/barrel nut, how the units are numbered, and the types of barrels that they can be used with.
---------------------------------------------------------------------------------------
Rail Interface System (RIS) (KAC P/N: 94297)
The RIS is the older version of the two systems and is numbered T-L-R-B (top- left-right-bottom) in odd numbers 1 - 13 beginning at the front sight and ending at the upper receiver. The RIS system works with a wide variety of barrel contours and mounts like a conventional handguard except for an adjustable metal tab, at the handguard cap, that squeezes the two halves together. At the rear, the RIS unit is held in place by the delta ring.
The biggest advantage with the RIS system is that it is able to accommodate a wide variety of barrel profiles while the disadvantage is that the RIS doesn’t clamp to the weapon as solidly as the RAS system. Just the same, the Navy SEALS reportedly prefer the RIS over the RAS system for the advantages described above.
-----------------------------------------------------------------------------------------------
The R.A.S. system is the newer of the two systems and uses the same basic layout with a different system of attachment to the AR-15 and M-16. Knights manufacturing has a patent for the Rail Adapter Handguard System (US Patent# 5,826,363) and this patent contains an excellent description of their invention and the details about it. I also have scanned and converted my M5 RAS owners manual to a PDF file which you can see here (CLICK HERE).
The RAS system clamps to the handguard cap via a clip on the top half while the rear is attached by a clamping mechanism that fits over the M-16 gas tube and fits into a recess in the M-16’s barrel nut. This rear clamp is then tightened via a hex-head screw producing a very tight fit that prevents most movement as experienced in RIS units. The RAS also has a tighter profile at the front sight that limits the use of the RAS to barrels that measure 0.835" - 0.855" diameter just after the front sight tower.
The RAS is also different from the RIS is the way that they are numbered. The RAS units are numbered T-L-R-B 14 - 28 from the upper receiver forward. The Army and Marines use the RAS as it is sturdier for use with an M203 grenade launcher and other accessories that can be mounted to these units.
From here:
KAC RIS and RAS
RAS & RIS and level across the top Rails?
Linear Mode
