all of this is my opinion worth what you spent on it:
SBR in 5.56 is beyond dumb. SBR in 300 blk is also dumb unless one is running subsonic + suppressed. so overall AR SBR's are just stupid. the topic of filing paperwork to CLEO and ATF (presumably form 1) indicating an AR build at residence in boston...well let me just say I could certainly understand why one would not be inclined towards this.
if need an SBR build something in 9mm. a kriss vector, sig MPX or CZ scorpion all make damn nice 9mm SBR's. the up side to a vector is it's easy to acquire the parts, put through paperwork, then build and engrave all while avoiding AWB crap. in general it's easier to build up an SBR rather than convert a rifle to SBR.