the rumor at the time was that the AG was unhappy with the way the MA AWB was written, and interpreted. The AWB defines a "Assault Rifle as a firearm with a barrel of 16 in or longer". SBR's have a less than 16" barrel there for they do not meet the definition of an Assault Rifle. This does not mean the AG will not try to jam you up if they want to, but Most SBR's are not set up as Mass compliant.