Bullets fired up into the air can be lethal: busted, plausible, and confirmed.
The "all of the above" ruling on the bullets fired into the air myth was a new one for MythBusters. All of their tests showed that if you fire a bullet perfectly straight up into the air, it will not kill you as it will fall down on its side and have too low of a terminal velocity to kill, much like the Penny Drop myth. However, it is very difficult to fire perfectly straight into the air and they even found an international expert in falling bullets who was able to confirm for them that people have died from bullets fired up into the air.
Myth: A bullet fired up can come down and kill you
...Firing bullets at terminal velocity...
The rigged up an air hose to an aluminum pipe to launch the bullets at terminal velocity (150 ft/s). Their first shot put a good dent in the metal door. Their next target would be a pig's head, just as soon as they got the amount of air pressure tuned correctly. A chronograph was used to measure the speed of the bullet and a solenoid valve was attached to the tube to control the air flow.
They fired bullets from the pipe into the pig's head and recorded it all on the high-speed camera. At 166 ft/s, the 9mm bullet bounced right off of the pig's head. The .30-06 bullet did only slightly better, piercing the skin and then bouncing off.
It was looking like this was going to be busted, but, as it turns out, there is an international expert on falling bullets working in nearby Stanford. The expert, Dr. David G Mohler, told them about a case in Menlo Park where a woman sitting in a lawn chair was struck in the leg by a bullet that was fired into the air 1 1/2 miles away during a 4th of July celebration. Mohler recovered the bullet from her leg and the police were able to match the ballistics to a shooter.
Mohler also told them about a case of an elderly man in Alameda who was talking to his wife underneath a plastic corrugated roof in his carport. His eyes rolled up and his wife thought he was having a stroke. When they got to the hospital they found out there was a bullet in his brain and, unfortunately, he died.
"I know for a fact that bullets fired at a distance, returning to Earth, with terminal velocity, have the ability to kill people." - Dr. Mohler
This contradicted their findings so far, so it was back to the drawing board.
Mohave Desert testing
They figured out what was different from their original assumptions: the bullets in Dr. Mohler's cases weren't fired straight up into the air. They were fired at an angle, which meant that they remained spin-stabilized and on a ballistics trajectory.
For the first time ever, they deemed this one busted, plausible, and confirmed. All of their tests, from the pig's head to the 9mm firing to the balloon, showed that a bullet fired perfectly straight up into the air is not lethal. However, it is also very difficult to shoot perfectly straight up into the air and, with the cases cited by Dr. Mohler, they have confirmed that people have died from bullets falling from the sky.