If you look at this Public Safety Notice about 3D printed weapons, and you replace the phrase "3D printing" with "completing an 80% lower", there does not seem to be any reason that a Mass resident
with an FID could not complete his own 80% lower. He would NOT need to register it. But he would not be able to sell or transfer it, unless it was serialized and registered.
https://www.mass.gov/files/documents/2018/08/08/Public Safety Notice re 3D Printing 8 9 2018.pdf
Of course he would have to possess it before 7/20/2016 to avoid the Enforcement Notice: "The Guidance will not be applied to possession, ownership or transfer of an Assault weapon obtained prior to July 20, 2016."
https://www.mass.gov/files/documents/2018/11/13/assault-weapons-enforcement-notice.pdf
But then he is in the clear.
Anybody have a statute citation to indicate otherwise?