A crowd of Libyans fired guns in the air and chanted slogans in support of Muammar Qaddafi at a rally in the city of Sirte on Monday. Isn't it kind of dangerous to shoot bullets into the sky?
Yes ... well, probably ... maybe ... it kind of depends. The Explainer is far from being the first to ask this question. Everyone from the U.S. military to The Straight Dope's Cecil Adams has probed the lethality of falling bullets. That includes forensic scientists, cardio-thoracic surgeons, and the hosts of the Discovery Channel's Mythbusters—which devoted nearly a whole episode to the matter. And yet, no one has been able to come up with a straightforward answer. The general consensus is that a bullet fired straight up—at precisely 90 degrees to the horizontal—is unlikely to kill a healthy adult when it returns to Earth. That's because, on the way down, air resistance prevents the bullet from returning to its initial velocity. The bullet would deliver a painful wallop but could only have a chance of killing you with a direct hit to the eye, ear, or mouth.
Things aren't likely to be much worse at angles just off the vertical. That said, bullets fired at an upward angle of 45 degrees or less can be far more lethal, since they're likely to hit someone on the ground while traveling at a much greater speed. In this case, gravity isn't directly opposing the bullet's motion, so the projectile stays at a higher velocity throughout its flight path. It's also more likely to maintain its initial, aerodynamically favorable orientation. Bullets fired vertically tend to fall nose-up or sideways, which creates a lot of drag.
Why has this question confounded so many experimenters over the years? (British and German soldiers were firing vertical test shots way back in 1909, and American servicemen did it in World War I.) In part, because it's impossible to calculate the exact minimum velocity required for a bullet to perforate the skin. Based on hundreds of years of shooting at pigs, oxen, and human cadavers—not to mention ballistics gel and other objects—munitions experts estimate that a bullet must be traveling at least 200 feet per second (or 136 miles per hour) in order to break the skin, although one traveling as fast as 330 feet per second (225 mph) might bounce off your body under certain circumstances. The broad range depends on several factors, like how pointy the bullet is and which part of the body it strikes. Skin thickness varies significantly from person to person, and in different places on the same individual. Upper-lip skin is 50 percent thicker than cheek skin, for example. Babies have thin skin, and elderly skin has poor elasticity, which makes it easier to puncture.
Even if you have a good sense of a bullet's minimum lethal velocity, it's still pretty difficult to clock the speed of a round fired in the air. Gunshots can travel as high as 10,000 feet, and the wind takes them in unpredictable directions. Julian Sommerville Hatcher, the U.S. military ordnance expert whose work on this topic is often cited, managed to land only four of his 500 vertically fired bullets in the target range. The Mythbusters crew lost all of its rifle shots.
As a result of these obstacles, the experimental results are mixed. Hatcher calculated that his .30-caliber rifle bullets reached terminal velocity—the speed at which air resistance balances the accelerating force of gravity—at 300 feet per second. You might die from a bullet moving at that speed, but it's unlikely. Lighter bullets, like those fired from a 9mm handgun, max out at even lower speeds, between 150 and 250 feet per second, according to computer models.
All this depends on the bullet's orientation during the fall. In the rare case where a bullet descends nose-first, it picks up more speed. In a 1923 experiment in which soldiers loaded bullets into their shells upside down, the total trip time dropped by as much as 80 percent. Air resistance also decreases at altitude, so falling bullets are more lethal in La Paz than in Amsterdam.
Here's the good news: Bullets fired vertically spend at least 15 seconds in the air, and many hang for well over a minute. So, the next time you're at a party and some drunken lout goes Yosemite Sam, you've got time to grab your drink and take cover.
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Explainer thanks L.P. Brezny of Gun Digest and Michael G. Haag of Forensic Science Consultants.