Top tier rail - Daniel Defense M4A1 FSP Rail Interface System RIS II

Rock solid lockup, aesthetically pleasing, and exactly what you need if you're doing a block II build! Its built like a tank, lets you retain your FSP if you already have an upper or barrel with one, however bear in mind quality comes at a price, and the RIS II is pretty dang heavy. But if you want a guarantee that anything like a PEQ or what not will hold zero, this is the rail for you.