The questions about ARs since 20 July 2016 tend to get boring, tedious and obnoxious. We only have 500 threads on the topic. If you think something actually changed on 20 Jul 2016 then it is not possible to give you an answer. Since the LAW did not change, any answer people provide is just plain made up.
Read MGL chap 140 sec 121 for the definition of "assault weapon". Read MGL chap 140 sec 131M to understand legality of "assault weapon". Read MGL chap 140 sec 129C to understand your obligations as LTC/FID holder.
Don't read
https://www.mass.gov/doc/enforcement-notice/download and expect to gain any clarity on anything (other than the fact it actually enumerates "federal Public Safety and Recreational
Firearms Use Protection Act, 18 U.S.C. section 921(a)(30) as appearing in such section on September 13, 1994" which can be hard to find)
Don't read any part of MGL chap 140 sec 121-131Y expecting it to actually be consistent, logical, make sense, or to answer any of your hard questions. Ask your lawyer since you will rely on case law and that will frequently be in direct contradiction of the law itself. As the marsupial courts of MA apply the doctrine of "GUNS", logic and the written law is not relevant.
Get an AR. limit it to one evil feature. Buy lots of ammo. Go shoot it.
If you want more clarity, move to a free state where there are not pages and pages of gun laws that confuse you or AGs that hold press conferences to reinterpret that gun laws. You will never be happy with the answer in MA because it is frequently going to be "who knows?" because on any given day in any given MA court you can be convicted of a violation of pretty much any gun law on the books.