A voluntary gun buyback program would be effective at preventing gun violence. Plenty of people own a gun they don't want or use anymore, but hang onto because it's valuable. They could be given an attractive price to sell it back to the program, which could then sell working guns to police departments to partly offset the cost.
One criticism of earlier buyback programs is that they create an implied gun subsidy (since a gun can be turned in for cash). This can be avoided with intelligent program design, such as focusing on guns that wouldn't likely be sold otherwise, paying in gift cards, never guaranteeing a price, etc.
Not only does this idea have potential, it stands a decent chance of gaining broad support.