Gun control is a joke... there is no one way to stop the wrong people from getting them, heres why.
1) The gun show loophole makes it easy for anyone to get a firearm because it's private sales and depending on your states laws you can just walk in and out in less than 30 minutes with your gun. No background checks, or wait periods. I live in the state of Virginia for example and here I can do exactly that; I can go to a gun store and walk out asap with an AR-15. There's no wait periods, no permit is required, no registrations need to be done (unless you purchase a machine gun but you would have to fill out ATF FORM 4 and pay a $200 NFA tax stamp to possess the gun.)
2) There are some states who are not reporting criminal convictions, or persons who are aquitted as mentally insane. If that information doesn't get passed on to the federal NCIC database then FFL dealers have no record to tell them to refuse the sale to unlawful persons.
3) There is a huge demand for firearms and it extends way beyond just going to Cabelas, Dicks, or Walmart for a gun. The black market is a huge source for illegal firearms and illegal transactions. So even if the US Congress decided to: increase wait periods, regulate accessories such as bump stocks, extended mags, etc, and raising the age limit to 21 will not remedy the situation. It only makes it harder for regular, honest - law abiding citizens from obtaining self protection.
4) There's been talks about raising the purchase age for shotguns and long guns to 21 which I think is absurd because: There is no research that shows that someone who is 21 years old or older is more responsible and less likely to go on a killing spree. A person of any age can build a grudge towards someone , any person can get bullied, any person can just all of a sudden [snap] and go out and kill innocents. So whether you're 10, 18, or 21 or 25 it doesn't matter because we can't control the circumstances that lead someone to do what they do.
There is only so much that can be done to help "better the system" but you can only add so much rules and regulations before it gets declared unconstitutional because of our 2nd amendment rights protect our rights to keep and bear arms.