My opinion is that the only thing that could bring down the rate of gun violence - either in mass murders or the regular kind - is to reduce the number of guns in the country from the current 300 million to ... something drastically lower. How would you do that? Ban the sale of new guns and confiscate existing ones. There really is no other way; guns are just inherently dangerous items, like cars, and after you do what you can to reduce unlawful and unsafe use, there is an irreducible lump of risk. Leaving aside the pesky Second Amendment roadblock that the current Supreme Court has created for us, let's think about the political and cultural issues involved in such an effort.
First, you have to line up enough political support to get Congress to pass the necessary legislation. The NRA aside, about 45% of Americans own at least one gun. The vast majority of them probably feel like they are not the problem, and don't want to give up their weapons. On the other hand, you have a fairly small group of people who would be full-tilt in favor of outlawing guns, with everyone else maybe vaguely supporting it. The gun owners and manufacturers would be highly mobilized and motivated, while the pro-ban forces would have to be self-funded and constantly reminding wavering allies why it was so important to do this.
Let's say you overcome that obstacle and get the votes necessary to pass a gun ban and confiscation law. What's the mechanism for identifying and confiscating the guns going to be? As many people don't know, the ATF doesn't have a database of gun owners. Individual gun shops are required to maintain records, but this law will put them out of business, so I doubt all of them will be able to "find" the records you would need to track down the original buyers of guns they have sold. You're going to need to hire a lot more cops and investigators.
What are the penalties for violating the law going to be? You have to think about how many people are going to become criminals solely by operation of your new law. If 120 million people own guns right now, how many would resist turning in their weapons? You'd probably have some measure of voluntary compliance. But largely I imagine you'd have to go find the guns and take them by force. What percentage of those incidents will end well? Probably, almost all of them, but the ones which don't ... you get the picture.
I did leave the Second Amendment to one side, but what about the Fifth Amendment? Confiscating guns is a Taking which requires compensation. How would you determine how much each individual gun seized is worth? What would be the procedure for paying the former owner? How much money are we talking about?
The picture I am trying to paint here is a parallel to the various kinds of prohibition America has tried in the past. Obviously, there are significant differences between guns on one hand and alcohol and drugs on the other - including the fact that it's harder to make your own guns than it is to distill spirits or make drugs. But the same problems come into play. Creating a black market for guns will have unforeseeable consequences. The value of an individual gun will skyrocket. Smuggling will be rampant. And while it is hard to make a gun, it is not impossible.