Couldn't one just get an 80% lower, complete it and make sure you have a fixed stock and a non-threaded barrel to make it technically not an "assault rifle" in the state of Mass? I'm probably wrong but I'm sure someone here will school me.
Under the word of law, yes.
Under Mad Mauras bullshit non-law edicts, no.
Currently the best option is a pinned 10rd magazine. Make sure it's in place before you install the upper.