Sounds like we have a real pickle here. We NEED to have background checks before ANYONE can buy an ASSAULT rifle (the scary looking ones the epa uses) to make sure MENTALLY ILL people can't get them, BUT the "background" check has to be prospective because you only become violent crazy spontaneously, and in any event the constitution, as interpreted, allows ANYONE to buy a gun whenever they want no matter their background, which is no good, because of the spontaneous nature of violent mental illness.
Fantasy land is a complex place.