A more serious complaint is that Loughner was able to legally buy a weapon even though he was weird enough to induce fear at his community college. School authorities finally told him not to come back until he got a bill of health from a mental health professional. But none of this showed up in the background check when he went to buy a gun.
Right now, federal law excludes a purchaser only if he "has been adjudicated as a mental defective" or involuntarily committed to a psychiatric facility. But some states bar sales to those who have been voluntarily committed, which makes sense. No one would argue against better use of available records.
Stopping a troubled person whose behavior is not alarming enough to trigger action by his family or friends, though, borders on the impossible. We don't want to give every gun buyer the burden of proving mental stability -- any more than we would require each taxpayer to take a polygraph when filing a 1040. The only real hope for keeping a lunatic away from guns is diligence by those who know him.
It's hard to imagine that stricter gun control laws would have any discernible value in averting tragedy. Homicides have actually declined since the demise of the assault weapons ban.
Utah has the nation's most permissive gun laws, according to the Brady Campaign to Prevent Gun Violence, but it has one of the lowest murder rates in the country. California, with the strictest laws, has a homicide rate higher than the national average.
There are plenty of lessons to be drawn from the ineffectuality of firearms regulations. But gun control supporters are in no mood to learn.