A question of rail systems

[Clutch]

Googling it found me this:

http://www.quarterbore.com/kac/kacrisras.html

If I'm reading the article right, the RIS system is a two-piece system, while the RAS is a single piece. The RIS also doesn't clamp on the barrel as tight as the RAS, allowing for different barrel profiles to be used in conjunction with it.