I'm guessing you don't have a factory extension (but I could be wrong) since it extends out past the barrel and the clamp doesn't fit. Although I don't wanna sound like a smartass but did you try flipping the barrel clamp the other way 'round and try screwing the barrel extension all the way down (it screws down onto the magazine male end and the extension itself screws into the nut. I wouldn't even attempt to shorten it any, just get a ported choke or something.