Be very careful, once you own a pistol AR and a rifle stock, you are potentially in trouble. The ATF are not our friends, and never have been for four administrations. Personally, I don't get the attraction on an SBR (aside from the cool factor). I'd rather spend my $$$ on on a suppressor, but its your call. Just understand that its more of a minefield legally going from pistol to rifle than the reverse. Most folks choose to use a trust, rather than registering the guns in their own names. Basfu can help you here, PM him. Or try the AR15armory.com, or even the Ar15 forum.com, though you get a lot of idiots there. My advice is to take it slow and steady and make sure your I's are dotted and your T's crossed. You want to do it right. I wish you well with the build, just don't make an assumption that will land you in the hoosegow.
FQ13