SBR's are legal in MO .. so are suppressors .. NO need for the C&R FFL anymore - they changed that this year.
I would recommend you look up the member "spook" or "tp555" .. both are competent smiths and could steer you in the right direction for a trouble free set up. OR .. you could just plop down the $$$ for an AAC 300 BLACKOUT upper and be one of the "kool kids on the block"!