Almost 300 murders in Baltimore this year, nearly all by bullet. In the face of years of devastating loss, curtailing the city’s homicide rate feels like an unsolvable puzzle. Harvard senior fellow Thomas Abt sees a way forward. His new book, “Bleeding Out: The Devastating Consequences of Urban Violence - and a Bold New Plan for Peace in the Streets,” offers evidence-backed tactics to reduce homicides, such as interventions with potential shooters, led by social workers, police, and leaders in the community. The message they are to deliver is clear: Stop the shootings.