Senate Majority Leader Harry Reid, D-Nev., was glum after background checks went down April 17. (AP Photo/Evan Vucci) A week after gun legislation suffered a stinging defeat in the Senate, an uncomfortable realization has settled over the Capitol that it will likely take another mass shooting or similar tragedy to reignite momentum for gun control. President Obama called last week’s vote “round one.” Senate Majority Leader Harry Reid, D-Nev., pledged that it was “just the beginning.” But gun-control advocates, both inside and outside of Congress, have identified no immediate path forward to alter a political landscape that left them five...