angeles. hi, william. reporter: a cornerstone of that system to stop the bad guys from getting guns starts with this form that you fill out when you want to buy a gun. are you mentally ill, do you use marijuana or any depressant? if you lie, there s a huge fbi database that s supposed to stop that sale, but even the federal government admits it s full of holes and few lie years are ever punished. it is the first line of defense against gun crime, the background check. this is to make sure that you don t have anything prohibiting in your past that would keep you from owning a firearm. reporter: felons, fugitives, domestic abusives, drug users, illegal aliens, among others, cannot buy a gun. they re going to take ten days and put you through a background check. reporter: currently, all states require fbi background checks on weapons sold at gun stores against a criminal database. some states include gun show sales for most weapons, but a majority do not, a loophole some