A self-destructing bullet isn't the most intuitive thing: normally, you use projectiles to destroy targets, not the other way around. But people make mistakes, bullets can travel for kilometres, and civilian casualties (or friendly fire!) are sad realities. In an effort to counter this, the US Army's Armament Research, Development and Engineering Center (ARDEC) has patented self-destructing bullets. The projectile would be filled with a flammable material that would ignite when the bullet is fired. As it burns, it would react with another material inside the bullet, which would cause the bullet to lose shape and become "aerodynamically unstable" -- in other words, fall to the ground. By changing the composition of materials, it would be easy to tune the bullet to lose its shape at specific distances.
It's far from a perfect solution -- for one thing, the patent was filed in 2013, and the project has since lost funding. It would also be a hard sell for soldiers, who typically like their bullets to be accurate and not deform over long distances. But as a solution for police in built-up areas (or anyone worried about collateral damage), it's an innovative idea that goes a small way towards solving an age-old problem with guns.