Sniping is HARD...
all the comments about using a projectile are apparently all from people that only see sniping in movies or in computer games. In real-life, it is HARD. And annoyinigly range limited. A Barrett 50caliber sniper rifle has accuracy out to about 1400 yards in combat. The Hellfire II can hit from 600 yards to 8000 yards. The difference in ranges is the difference in the drone staying covert or not staying covert - and remember that the range has to include both a horizontal component and the vertical component of the drone's height.
Also that 1400 yard range is from a stable platform, off a bi-pod. From a moving aircraft, your reliability of actually hitting the target will be much lower, especially as the windage issues are multiplied with different air currents at different heights - plus the effect of the drone's own airstream must be compensated for.
It would be cool if they could put a 50cal in a drone, but I suspect that it is just too hard and exposes the drone to counter attack due to the range.
Now, a 50cal in a DIRIGIBLE, floating noiselessly and slowly at night, with a night vision TV scope...much cheaper, you could have a cloud of them hovering over a target area that could decend to 500 meters at night, they could even be autonomous and engage anything that they detected (like say in some remote areas known for hiding caves and Al Queda members...). Note to anyone building these - ensure the rifle is permanantly disabled in the event of a dirigible crash or failure...I suggest a small charge in the breech that can be remotely detonated, or even that detonates itself upon loss of control signal.