The ATF has reported for many many years that the VAST majority of firearms used in crimes come from two sources:
1) Straw purchases (where someone eligible to own a gun buys one for someone who would fail the background check).
2) Corrupt FFL gun dealers who are selling guns out the back of the store.
Both of these are already illegal, and both laws are poorly enforced. The problem with the straw purchase thing is that it’s often a girlfriend who is under implicit or explicit threat if they don’t comply and buy the boyfriend a gun. So the ATF and other locales tend not to do much about straw purchases.
There are honest gun dealers who will tell you that it’s EXTREMELY easy to spot a straw purchase in progress. And many of them have reported them to the cops as they’re happening. They have no legal right to refuse the purchase to the law abiding woman, but you can see her nervousness and uncertainty. But the cops generally don’t act on them. So the FFL dealers stop reporting them since they know nobody will do anything.
The corrupt firearms dealers are harder to spot. There have been some high profile stings, but not that many. It doesn’t take a genius to figure out what store a particular gun’s serial number came from, and when a whole bunch all came from the same store, it doesn’t take a genius to figure out what’s happening. And yet, big busts of corrupt FFL dealers are very rare. I’ve never heard a policy explanation for why the way I have for why they don’t enforce against straw purchases.
I will also add a third thing. The fact is that most of the gun violence in America is an offshoot of the gang war that exists in almost every city which itself is an offshoot of the drug war. Want to cause US gun violence to plummet almost overnight? Turn off the money spigot that motivates it and legalize recreational drugs, using the money for harm reduction policies instead. The gang wars will dry up and so will the violence. I’m not saying there will never be any violence or other crime, but it will go WAY down.