It depends on how straight you fire the gun.
If you fire a bullet straight in the air, it will travel upward until it expends all of the energy it gained from the powder blast in the gun. At this point, it's pulled back to earth by gravity, but slowed by friction from the air. At some point the aerodynamic properties keep the object from falling any faster. This is called terminal velocity. Bullets are small, aerodynamic, and dense, but they only manage a terminal velocity of around 150 feet per second. Tests on a variety of bullets have placed a minimum velocity to penetrate flesh at around 200 miles per hour. In other words, it would hurt, but it would barely be able to break through skin, let alone kill you.
If you don't fire quite straight, and you probably won't, the bullet will arc like an artillery shell. Gravity will eventually pull the bullet back to earth, but not before the momentum from the initial blast has been expended. This means the bullet could be moving fast enough that it would still be lethal.
Mythbusters covered this in their "Bullets Fired Up" episode. When testing their vertical firing rig, they showed just how unpredictable these bullets can be:
While researching this myth, they met Dr. David G. Mohler, an expert on falling bullets. He identified one case where a man was killed by a falling bullet reaching his brain, while in another case a woman was hit in the leg by a bullet matched to a gun that had been fired 1 1/4 miles away. In both cases, there was some distance between the gun and the victim, which means the bullets were shot in an arc, not straight up.
Posted 4169 day ago