OK, to get an SBR in MO, you have to have some type of FFL and maintain it. This comes from Missouri State Law not the Federal Law. MO states that only "dealers and Mfg's" can have NFA, this is where the C&R gets you around this law. You must maintain your C&R otherwise you are in violation of the MO law. A form 1 built NFA (SBR or AOW) require that you have atleast a C&R as well, not just purchased weapons. MO allows only C2 and C3 dealers to have suppressors and Post 86 Dealer Sample MG's. Well that is the "short" version of the NFA laws in MO.
CBE
Metalman: The components to make a SBR, barrels. etc, are not regulated so anyone can purchase, they could be used on an AR pistol. The ATF's view is that if you have an AR and a seperate SBR upper it is what is called "Constructive Posession" which means that you "could "put it together and make an illegal SBR. The same goes with MG parts, even as small as a M16 carrier in an AR, in posession while you have an AR.