If you cut that barrel to less than 16" you will be creating a SBR (short barreled rifle) which is not allowed without special dispensation from BATF. You have to pay a tax and get special permit, but my memory is not perfect. Either way, if you do it without the paperwork, they can and will lock you up for possessing and illegal firearm. It may be illegal to even have a short barrel in your possession without the paperwork, even if not mounted.
