The main problem is functional.
It would be easier to kill kids in a school with a pistol than a rifle. The school shooter idiots don’t realize that, but the gun owners do. So they know that if the magic gun evaporation fairy gets rid of all the rifles, the school shootings won’t stop, and then you are in your same predicament wondering what to ban.
So then what happens? Look into your crystal ball. You ban something else. And then something else. And eventually you have to be 18 and have a permit to buy a steak knife, like in England.