Here is a link to the BATF Frequently Asked SBR and SBS questions page. It has the answers to most of the questions asked about these guns and is fairly clear and readable. Reading this first may help you get the answers you need without asking a question that has been asked over and over for some time.
Hope this helps
Frank
ATF Online - Firearms - Frequently Asked Questions - National Firearms Act (NFA) - Short Barreled Rifles and Shotguns
Hope this helps
Frank
ATF Online - Firearms - Frequently Asked Questions - National Firearms Act (NFA) - Short Barreled Rifles and Shotguns