Mythbusters:
Terminal velocity of a falling bullet
Adam built an acrylic wind tunnel (much like the one in the Penny Drop myth). Air was shot up through the bottom and a bullet was dropped into the chamber. The terminal velocity was calculated based on the speed of the air needed to make the bullet stop falling. They figured that the terminal velocity was 100mph (150 ft/s). The wind tunnel also showed that the most stable falling position for the bullets was on their side.
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.
It was time for them to figure out what would happen with real bullets fired into the air. They went out to the Mohave Desert, where they setup a rig to fire straight up into the air. They planned to fire a bunch of bullets into the air and hopefully find at least one of the bullets where it landed. To maximize their odds, they stationed their crew in bulletproof listening posts.
They first fired bullets straight into the ground as a control:
BB: 3"
9mm: 6"
.30-06: 12"
Jamie fired a clip of 9mm bullets up into the air. 39 seconds later they heard the bullets hit the ground.
Adam: "I'm searching across the desert for a pencil-sized hole"
The first bullet that Adam found went only 2" into the ground and appeared to have hit the ground on it's side. The bullet had traveled 330ft horizontally. Jamie found another bullet hole almost identical to the first.
Jamie then fired the .30-06 rounds. Big problem: after 40 rounds fired into the air, they weren't able to hear any bullets land. The .30-06 rounds travel over twice as high, so they were simply traveling too far for them to find.
Adam brought out plan B: a balloon attached to an instrumented platform that could drop bullets remotely. The platform had a wireless video camera that fed an image of the platform, including the altitude gauge, down to Adam.
The bullets were dropped in a bundle from a height of 400 ft. The .03-06 made a 2" hole. The 9mm made a 2" hole as well, matching up perfectly with the actual 9mm bullet firing.
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.
That being posted - I would say that anyone firing at an angle is playing russian roulette with innocent people and a gun's main function being deadly force, the shooter had the intention to harm innocents - therefore criminal,
Firing straight up can not kill and though stupid would expose the shooter to severe civil damages but not criminal penalties.
But any gun owner knows it is a lethal weapon whereas a car is not.