Also, who knows what happens if a conservative justice passes in the next4 years.
It's possible that they could hear a case in which "common use" gets a narrower definition, and that "assault rifles" are somehow found to be "dangerous." However, that's a real stretch, and given that the Chief Justice was on our side in Heller
, that seems highly unlikely.
In a way, Scalia and Alito did a good job of boxing them in for the foreseeable future. The court can't go reversing its own decisions within a short time-frame without damaging their credibility.