There is probably a flash hider or fake suppressor available to bring barrel to legal 16" length but it will have to be pinned and welded on. Of course that defeats the SBR so that only leaves ATF paperwork, a $200 tax stamp and a long wait for their foot dragging.
Like the others have posted, don't take chances, the ATF isn't your friend and they have a heavy hand.
If you have to have it right now and even shoot it while waiting for your BATFE approval/stamp, buy a registered pistol lower and everything will be completely legal. I think a RR complete pistol lower is running about $300. BATFE is running at about a 5-6 month wait time. I'll post up a pick of mine tonight