"I know it when I see it"
IMNSHO those in Massachusetts seeking to buy AR or AK style rifles have essentially no guideline by which to determine if the firearm they contemplate acquiring is legal. This is due to the current disconnect between the articulated state law and the capricious directive from the AG's office. Apparently, in the AG's eyes, the law was compromised in its inability to suppress ingenuity and creativity.
Now that Ms Healey's directive has been announced things are less clear than they were. Certain combinations of cosmetic and ergonomic features were used to qualify a gun as banned. The new directive 'Healeys' the sicknesses inherent in that law. 
Analogous divisions over porn festered through the courts during the 50s, 60s and 70s where the subject line originated for this OP originated from Justice Potter Stewart. It is in that spirit that I suggest checking with the AG before your purchase. Send an email, letter or just stop by Ms Healey's office and inquire about your purchase since, apparently as far as 'assault weapons' are concerned, she knows one when she sees one.
= new reply since forum marked as read
I know it when I see it.
Only those too dangerous...
Weapons of war...
Military STYLE???
Because I said so.
