Thats one of the things about an SBR. An SBR is only an SBR when its assembled as an SBR. The lower might be the part you register but its only an SBR when its all together so having it only on the barrel is 100% kosher. Put a long barrel on there and its not an SBR at that moment and you can bring it across state lines without prior authorization. Ive even got a factory SIG lower with no caliber designation on it because it came as part of a complete rifle with a marked barrel.