^^^^ This. And make sure its not a polymer sight.You need to do a search for front sight for railed gas block. Unless the gas block is rail height, which most of them aren't, you need a taller front sight.
Amazon.com : Midwest Industries Flip-Up Front Sight Gas Block Height, Black : Airsoft Gun Sights : Sports & Outdoors
Amazon.com : Midwest Industries Flip-Up Front Sight Gas Block Height, Black : Airsoft Gun Sights : Sports & Outdoorswww.amazon.com