We have background checks.
Comprehensiveness appears to be key. I remember Obama saying only 1 week before Orlando that there was someone that the FBI knew to be an ISIS sympathizer, and yet the law wouldn't let them put his name on a list to prevent guns from being sold to him. What's the use of a background check that doesn't turn up the necessary information?
If background checks exist, they do not appear to be consistent, nor do they appear to be given the teeth necessary to be effective.
Is it a crime to be an ISIS sympathizer? Has he been convicted of a crime? Did the FBI have enough information to charge him and throw him in jail? Should they be able to say anybody is a potential ISIS sympathizer and now nobody can fly on a plane or own a gun without any sort of remediation process?
The background check network can only be used by professional dealers (to "make money" selling firearms you need to be a licensed dealer and any firearms you sell need to include a background check) -- this includes professional dealers at gun shows. What doesn't involve a background check in many states is buying from a friend, or another third party person -- think more of an impromptu swap meet.
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On a side note, rather than editting my post I'll say here -- I realized what I said at the end was a little disengenuous to make it seem more righteous.
Buying a gun from a private person is more like buying a lawn mower from either your neighbor, girlfriend, or some random guy you meet on craigslist.
Will requiring background checks on all gun sales actually reduce the number of guns sold to bad people? Most of these states that don't have background-check requirements for private sales do have laws that make it illegal to sell a gun to anyone who is not legally able to possess one. So these sales are already illegal. Is making them "extra special illegal" going to make a difference?
I'm not actually opposed to universal background checks. I'm just not sure it will have a big impact on overall violent crime. I'm definitely not convinced it will have any impact on mass shootings. Mass shootings are just an incredibly different beast than most other violent crime. I'd say it has more in common with rape than with murder or assault, because they both come from deeply narcissistic world-views. What are we doing about the growing narcissism problem in the world?