I've seen this on CSI. He attached a gun to a large helium filled ballon and shot himself. The gun floated off never to be found.
Either that or some muppet fired some rounds into the air a few miles away for a laugh and this man was unlucky enough to be underneath one when it came down.
its common for a bullet to travel up to a mile+ hence why theres restrictions on land licenced for shooting with rifles (not shotguns), many accidents have happened in the winter when the ground is frozen a bullet can ricochet for up to 2 miles !!