According to the new interpretation, all so-called "Assault Weapons" are illegal but the State Attorney General has decided not to prosecute at this time. Which leaves local and county District Attorneys the ability to prosecute...
Also per the new interpretation, purchasing a stripped lower receiver is verboten as the AG has decided that a stripped lower is now an "Assault Weapon". Massachusetts law states that a "firearm" is only a firearm when capable of firing. Of course the state definition varies from the federal definition of what a firearm is, yet the AG has taken it upon herself to define a stripped lower receiver as a firearm - creating law on a whim. Three separate definitions, with only two being codified into law, yet all three having felonies associated with them. Confusing yet?
An 80% lower that is/was turned into a firearm is also verboten and defined as an illegal "Assault Weapon" - whether you finished and built it before or after July 20, 2016. Being that both are an illegal "Assault Weapon" via the AG's interpretation, they both carry the same felony.
Except that in MA you must register your firearm within 7 days of it being capable of firing. If it is not registered with the state you bought yourself a felony.
Let's look at some examples:
You bought an 80% lower in 2012 and finished it into a stripped lower in 2013. This meets the federal definition of what a firearm is, but not the state definition, hence you never registered the lower because it was never mated to a fully functional upper receiver and therefore not capable of firing. If today you are caught with the lower, you could still be prosecuted for a felony by having what the state AG considers an illegal "Assault Weapon".
If you decided to say F' it and build it into a functional AR15 prior to July 20, 2016, you are in still in possession of an illegal "Assault Weapon", but now you have an added charge of not registering your firearm within 7 days - another felony.
If you have this lower which is finished and has been functional and capable of firing a shot with its upper mated to it, and you feel some pangs of guilt and decided to register the firearm (because hey, it was legal when I finished the 80% lower back in 2013), you have now incriminated yourself for an illegal action and will be facing a felony for creating an assault weapon.
In Massachusetts you can neither build, assemble, buy, or possess any type of semi-automatic firearm (except a mini 14 or few others listed by name as acceptable). This includes stripped AR lowers.
Your best option is to leave the state and not look back.