First you need to check local and state laws. Some states like Missouri require a Curio & Relics license to own a SBR. Other states like Alaska, no license is required.
Then you need to fill out BATF Form 1 (Application to Make and Register a Firearm), then you need to have the Chief Law Enforcement Officer (CLEO) of your area sign off on your application. Sometimes that is the toughest part is getting someone to sign off for you. Then you send it to BATF along with your hard earned cash ($200) and wait. Could be a month or could be 5 months. Wait till you get the papers back before you buy the barrel. If you already have an AR they could slap you with a conspiracy charge for having the components to manufacture an unregistered SBR so you're best off to wait. If you have Adobe Acrobat, you can download a Form 1 pdf from ATF at [url]http://www.atf.treas.gov/forms/pdfs/f53201.pdf[/url]