This video of a "homemade multirotor with a semiautomatic handgun mounted on it." is both impressive and scary. It's also possibly illegal - or not.
The drone is obviously hand-built but the firing seems very real as the craft itself recoils with every shot.
The FAA has rules that "would prohibit weapons from being installed on a civil aircraft."
But legal scholar Dan Terzian has suggested in a 2012 paper "The Right to Bear (Robotic) Arms" that there is a "very real possibility of robots being [defined as] arms under current Second Amendment doctrine." Presumably the same interpretation might also cover drones.
Here's the video