Here's an idea: Why don't we levy a healthy excise tax on bullets?
Something like an ammunition tax obviously wouldn't do much to deter deranged spree killers, but emotionally gripping as such things are, mass murder incidents are a vanishingly small share of America's overall violent crime problem. And while it's relatively easy for firearms skeptics to whip themselves into a frenzy about scary-looking "assault weapons," again these guns account for very little killing in the United States. The much larger issue is the routine use of small, easily concealable weapons as an instrument of robbery or gang violence. To make a big dent in this problem by targeting guns would require extremely draconian curbs on a large class of weapons, a huge number of which are already in circulation—in other words, an incredibly heavy lift.
But while I'm not sure it's a true story, I can at least tell a plausible story about how an ammunition tax could lead to a much lower level of violence.
The key thing here is that the use of handguns in gang conflicts is at least in part an equilibrium problem. If two rival organizations are conducting disputes with guns rather than knives and fists, that's worse for both gangs and for the city at large. The legal risks are higher, the risk of death is higher, and up-arming yourself gives you no systematic advantage over rivals. But whole cities get stuck in the bad high-fatality equilibrium because nobody wants to be the guy who brings the knife to a gunfight. Yet at the same time, these gangs are at least in part economic institutions that should be sensitive to the price of production inputs. If bullets get more expensive, you need to start conserving them. And if your rivals have the same problem, then perhaps the citywide basis of competition can ratchet down to a less-deadly dynamic of melee rather than drive-by.
Maybe it wouldn't work. But given that we are going to have to raise some taxes of some sort and this would raise revenue, it seems worth a try. As an added bonus, a higher price of ammunition shouldn't be a serious obstacle to anyone hoping to use a firearm for home defense. The costs would be borne by habitual gun users rather than people trying to prepare for rare contingencies.