The FPS Russia people made this video showing a "prototype" quadrotor equipped with a machine gun. Yes, a remote-controlled flying machine gun. They take it for a spin and shoot some targets, as they usually do.
In reality, this weaponised quadrotor or quadrotorised weapon is just a fun 3D animation, lifting the concept from Metal Gear Solid. But you can bet the US military is working on stuff like this right now. It's only a matter of time before the weight/power ratio is good enough to make it happen. The Army and the Marines would love this thing.