Just ban the production of ammunition altogether. Pull whatever stock is out there and destroy it. Gradually what is left out there will deplete to the point its value is prohibitive for most people, apart from real criminal enterprises that make substantial money or the rich. Sure you can make ammo in your basement, but it will run the same premium, as well as the risk of lengthy imprisonment - not to mention the materials used to produce it can also be outlawed, forcing further illegal production of powder etc. Alternately, simply raise the price of ammunition 10,000 fold. A gun is useless without something to shoot out of it.
Sure you can use any old thing to kill another person, but a knife, baseball bat, bare hands etc. all are nowhere near as efficient or safe to use as a gun is. Any other method involves getting in close to your victim or takes more time than a split second to pull a trigger.
As well, ban the sale of handguns and automatic/semi-automatic weapons. They serve no purpose but killing human beings.
Of course none of this does anything to change the dysfunctional mentality that appears systemic to the United States with how often this lunacy happens there compared to other countries.