Briefly . . .
Mass Gen'l Laws says that he must have a MA LTC in order to possess anything gun related. Also it must be neutered to meet the MA Assault Weapons Ban & mags must hold no more than 10 rds (both same as the old Clinton Federal Bans that expired in Free America in 2004). Exception if it was mfd on/before 9/13/1994, those are grandfathered.
Now the MA AG spouted off in a news conference (in 2016), memo and FAQ that all ARs are illegal unless grandfathered as I noted above. Most dealers will not sell ARs, but there may be a few that will work around this BS. I'm in the camp that says that no politician can create a law without legislative action. Nobody's been prosecuted for her alleged ban. Court challenges have been filed but she has outwitted us on procedural grounds each time.