2 rail. I have shot 18s with rifle gas like how smooth and accurate These are. I'm able to set extra money into general performance parts then clone accurate areas similar to a ops brake and collar that I don't have a can for. This is known as carrier tilt. https://kac-m4-rail45913.techionblog.com/29558830/5-essential-elements-for-knights-armament-quad-rail