Assault rifles are not illegal in MA.
No, but is it registered?
I was planning a hunting trip to Maine a few years back and was told by everyone who makes the trip to go around Mass because if you got caught with the weapons you would be in serious trouble. Apparently you could get some kind of waiver to carry them through the state but it was burdensome. I don't know if this was myth or reality, but everyone sure believed it and avoided Mass.