This Madbull Noveske Upper Receiver allows you to customize your CA M15 so it doesn't look like every other M4 out there. Madbull made these upper receivers to match up with their Novekse free float rail, but it looks great with any other M4 front end. This piece is only compatible with the Old Syle Classic Army Lower Metal Body. You can tell if you have an old one if you have 2 pins, one left and one right, sticking out of the back of your upper receiver that is supposed to line up with 2 holes in your lower receiver. This is a great aesthetic upgrade with or without the Noveske Rail.