Most of the guns used in these mass shootings were legally obtained. This particular guy who murdered the firefighters did not steal his, but purchased it from an individual who did purchase it legally. Only one that I know of (the Portland mall shooter) stole or borrowed his from an acquaintance. If these weapons were still banned, they would not have been able to so easily obtain their weapons, stolen or otherwise.

The are tactical weapons of war that only belong in the hands of the military or law enforcement. Nobody will ever convince me otherwise.