How about, you're born on the "can't purchase" list and stay on the list until you've reached the age of 21, passed extensive background checks including having every single thing you've posted online carefully scrutinized by the BATFE, taken and passed a gun safety course and having completed the above, applied for and received a federal license to own a firearm or ammunition.

If this sounds expensive and cumbersome, that's the point. It should be expensive and cumbersome to get a gun. The fact that it's so easy to get a gun in the USA is the problem.

If I were Biden, I'd get Congress to pass a comprehensive gun ban. No guns or ammo of any type without a federal license. Assault weapons completely banned. Mags over seven rounds completely banned. Trafficking in ghost guns or information on the manufacture of ghost guns including CAD files for 3D printed guns -- banned. I would get the IRS and the BATFE to conduct enhanced audits and investigations of businesses who sold guns to ordinary schlubs including gun shops and gun and ammunition manufacturers. Yes, this will raise constitutional issues, but the lawsuits would take years to work their way through the courts, during which time I would binge-nominate Supreme Court justices who aligned with my views on the matter, bringing the total number of justices to 12 or 14 or so, and ensuring that effective defanging of the Second Amendment becomes national precedent when the cases do finally reach the Supremes.

There is no excuse for so many guns being on the streets of a civilized nation.