We had a long discussion on this. There are so many out there that you cannot control them leagal or not. Hate to say this but control the ammo, say serialize it and show Govt ID to buy it?
This dude apparently has only a speeding ticket on his criminal record. No amount of background checks will show that and would have stopped him. Or 99 percent of the other assholes that pull of these terrorist attacks.
No, what needs to happen is mental health issues like being admitted to a mental hospital AND making threats to shoot up a NG base needs to be input into the existing NCIS system AND a temporary seizure of his/her firearms until a court with advice from medical officials to determine his mental state. That would have prevented this. Nothing else