“But if all the weapons the kid had were knives then the law worked as intended.”
Wouldn’t a “red flag” law have provisions to prohibit someone who has just had their weapons confiscated due to threats of suicide and/or murder from buying new weapons, of any sort? Why wouldn’t the names of such persons be placed in the federal database that is supposedly checked before firearms sales?
Legally I don't see how you can. If a person is forced to surrender their weapons under Red Flag laws that isn't a permanent surrender. In Illinois they take them away for six months and unless the court issues another order, called a firearm restraining order, then the weapons get returned to the owner. People are forbidden from purchasing any kind of weapon while the retraining order is in place but once it expires then constitutionally they have the same rights as anyone else. The restraining order itself is sealed three years after it expires so there would be nothing to search for.