You CANNOT legally purchase a fully automatic weapon in the US and haven't been able to do so for quite some time. The NRA and REPUBLICANS were the ones to ban that.
The true debate is on GUN REGISTRY which does absolutely nothing to stop these types of things and is just another tax on law abiding gun owners.
The VAST MAJORITY of mass shootings in the US are related to drug cartels and organized crime, many of whom cannot legally carry anyway, but get their supply from an open southern border smuggled from places like Brazil which the DEMOCRATS don't wish to do anything about.
We need to do a better job of educating why leftist ideology is a cancer to society and is creating hostility