Installing an integrated thermal imaging camera, drones can easily be suited for search and rescue missions and possibly play an integral role in law enforcement agencies. Micro UAV's might as well be an effective way for environmental activists and biologists to easily track down wild life under suspicion of imminent danger by human sources. It'd be cheaper than using helicopters.
And I'm quite sure that on a Law enforcement (civilian model/ micro UAV) quadrotor drone, it is possible to mount a very light pistol. But I doubt such feature will increase the effectiveness of the drone unless considerable amount of automation programs is implemented into the technology to automatically calculate and predict trajectory instantaneously in flight. But besides that, the legality of that matter would be in question.
On that train of thought, I am seriously considering the effectiveness of a paint ball gun mounted on a sizeable quadrotor drones with a camera. This can definitely add another layer of tactical role at a sporting level.