Two rail. I have shot 18s with rifle gas like how clean and accurate they are. I can place more cash into performance pieces then clone appropriate areas like a ops brake and collar that I don't have a can for. But don’t forget, Aero also has the reduce, LPK, https://kac-carbine-rail41740.shotblogs.com/the-definitive-guide-to-ras-m4-43578408