What RIS RIS? There's only one Rail Interface system. It covers the handguards, and the top of the receiver.
Need more?
The rail is typically placed directly on the weapon's receiver, in the position normally occupied by the rear sights. The rail's cross-section is shaped roughly like a wide T. Scopes are mounted on the rail either by sliding them on from one end or the other, or by means of a "rail-grabber" which is clamped to the rail by means of bolts, thumbscrews or levers. - Wikipedia.
NOW, in case you're too lazy to read that, here it is cut down:
The rail is typically placed
directly on the weapon's
receiver, in the position normally occupied by the
rear sights.
Where are the rear sights?
Why... on top of the receiver.
Proof
Sorry, I take great offense when people assume I'm a misinformed dumbass.