I was told by the feds if you can unscrew the extension, whether the barrel end is threaded or by removing screws such as sight screws, it doesn't make it. They were quite forceful about welding or soldering, but they didn't close the door completely on some type of permanent adhesive. They want to make sure that you don't make a short barreled rifle that can be hid under a coat and snuck into the bank you want to rob. That you can have the same gun with a pistol stock and it is legal with a short barrel doesn't make much sense, but the feds are the feds, and the law is the law.