From Cato Institute:

Massachusetts law currently prohibits ownership of “assault weapons,” the statutory definition of which includes the most popular semi-automatic rifles in the country, as well as “copies or duplicates” of any such weapons. As for what that means, your guess is as good as ours. A group of plaintiffs, including two firearm dealers and the Gun Owners’ Action League, challenged the law as an unconstitutional violation of their Second Amendment rights. Unfortunately, both a federal trial judge and appellate court upheld the ban—though they could not agree on why.