That UAV would have to get damn close for it to be any effect at all, much less the headshot that would be required to actually take out the target. The suppressor wouldn't do much good since the BG in question could both see and hear the UAV, and blast it with a shotgun or AK. The lowest flying UAV in use right now has a 2000 ft operating floor, and most UAVs operate in the 10,000 to 18,000ft altitude range. They would have to drop to about 200 feet to even have a chance with a .22. That's just not practical. A small Air to Surface rocket would be the only real option for a UAV armament.
There is a UAV that's still being designed called the WASP Block III which falls below the Tier I UAV requirements. This would be a hand launched UAV that has an operation time of 1 hour, and a 50-1000ft altitude range. As of now, it's only projected to carry cameras for squad recon, not weapons like the Reaper or Predator.
What about accuracy as well? Guided rockets work great because they have a large enough kill radius to make centimeter accuracy irrelevant. 1 unguided bullet would have to be aimed just like a gun. Good luck getting that accuracy looking through a small video camera flying through the air at a couple hundred feet away from the target. One light breeze or updraft and your one bullet would be a few feet off target.