This is a much better example of people finding a way to hurt others than the other person using the Oklahoma City Bombing. Rent a large truck, drive it into people. Just as easy, if not easier, than buying a gun and a bunch of ammo.

These types of attacks are definitely going to be hard to solve for, if they even can be solved, but I would argue that it's easier to defend against them. Cities have already begun to place heavy duty bollards around areas that large crowds gather, limiting such an attack to areas with weaker security or less people. Obviously, that will take some time to implement, but it's still at least something that can be done to prevent it/minimize the damage, unlike trying to prevent a person from going anywhere with a bunch of concealed weapons.