Lately, I've been watching the Libyan civil war in the media, and both sides in the conflict often celebrate success by firing their AK-47s into the air. Not to say that firing guns into the air is restricted to any particular group, I'm just using this as an example.
Will bullets fired into the air by small arms fire have the velocity to kill a person when they fall?
As for physics, it is really very simple, the kinetic energy is converted to a potential energy while climbing and then to kinetic again when falling, with some of it converted to a heat due to friction. The air friction is quite substantial, the landing velocity, which is reported to be in range 50-200 m/s, is significantly lower than muzzle velocity, which is usually 300-1000 m/s, but the velocity is still high enough to kill.
Celebratory gunfire - plenty of statistics and examples. My favourite: "every bullet that is fired up, must come down".
People are injured, sometimes fatally, when bullets discharged into the air fall back down. The mortality rate among those struck by falling bullets is about 32%, compared with about 2% to 6% normally associated with gunshot wounds. The higher mortality is related to the higher incidence of head wounds from falling bullets.
For example, in one study led by the U.S. Centers for Disease Control and Prevention (CDC), they found that 80% of celebratory gunfire-related injuries are to the head, feet, and shoulders. In the U.S. Commonwealth of Puerto Rico, about two people die and about 25 more are injured each year from celebratory gunfire on New Year's Eve, the CDC says. Between the years of 1985 and 1992, doctors at the King/Drew Medical Center in Los Angeles, California, treated some 118 people for random falling-bullet injuries. Thirty-eight Kuwaitis celebrating in 1991 at the end of the Gulf War by firing weapons into the air caused 20 deaths from falling bullets.
Firearms expert Julian Hatcher has studied falling bullets and found that on average .30 caliber rounds reach terminal velocities of 300 feet per second (90 m/s) and larger .50 caliber bullets have a terminal velocity of 500 feet per second (150 m/s). A bullet traveling at only 150 feet per second (46 m/s) to 170 feet per second (52 m/s) can easily penetrate human skin and at 200 feet per second (60 m/s), that same bullet can penetrate the skull. Even a bullet that does not penetrate the skull may still result in an intracranial injury.
Mythbusters did an episode on this and found that yes, a bullet fired into the air at a non-vertical angle (i.e. not equal to 90 degrees) can maintain its spin and high speed and can injure someone.
Source: Mythbusters results - Episode 50: Bullets Fired Up
In addition to the great scientific explanations given by others here, I'd like to share some empirical evidence. This is taken verbatim from the Wikipedia entry on celebrator gunfire:
- December 1859: An autopsy showed that a native servant in India, who suddenly fell dead for no apparent reason, was mortally wounded from a bullet fired from a distance too far for the shot to be heard. The falling bullet had sufficient energy to pass through the victim's shoulder, a rib, a lung, his heart and his diaphragm.
- January 1, 1996: A grandmother in Detroit named Sandra Latham was killed on New Year's Day from bullet from celebratory firing as she sat in her living room.
- December 31, 1994: A tourist from Boston was killed by a falling bullet from celebratory firing while walking on the Moonwalk in the French Quarter of New Orleans, Louisiana. The Police Department there has been striving to educate the public on the danger since then, frequently making arrests for firing into the air.
- July 22, 2003: More than 20 people were reported killed in Iraq from celebratory gunfire following the deaths of Saddam Hussein's sons Uday and Qusay in 2003.
- December 31, 2004: A 75-year-old man in Orlando, Florida, was mortally wounded in the heart from a falling bullet just before midnight. Police later traced the fatal bullet to a gun confiscated from a man firing into the air more than a mile away. The shooter was charged with manslaughter.
- January 1, 2005: A stray bullet hit a young girl during New Year celebrations in the central square of downtown Skopje, Macedonia. She died two days later. This incident led to the 2006 IANSA awareness campaign in that country.
- December 28, 2005: A 23-year-old U.S. Army private on leave after basic training fired a 9 mm pistol into the air in celebration with friends, according to police, and one of the bullets came through a fifth-floor apartment window in the New York City borough of Queens, striking a 28-year-old mother of two in the eye. Her husband found her lifeless body moments later. The shooter had been drinking the night before and turned himself in to police the next morning when he heard the news. He was charged with second-degree manslaughter and weapons-related crimes, and was later found guilty and sentenced to four to 12 years in prison.
- February 25, 2007: Five people were killed by stray bullets fired at a kite festival in Lahore, Pakistan, including a 6-year-old schoolboy who was struck in the head near his home in the city's Mazang area.
- July 29, 2007: At least four people were reported killed and 17 others wounded by celebratory gunfire in the capital city of Baghdad, Iraq, following the victory of the national football team in the AFC Asian Cup, Celebratory gunfire occurred despite warnings issued by Iraqi security forces and the country's leading Shiite cleric, Grand Ayatollah Ali al-Sistani, who forbade the gunfire with a religious fatwā.
- January 1, 2010: A 4-year-old boy was killed shortly after midnight while sitting in church during a midnight service in an Atlanta suburb. The cause of death was thought first to be due to falling ceiling debris until an autopsy confirmed an intracranial bullet injury to be the culprit.
The Wikipedia article includes citations to news stories and other evidence for many of the incidents listed above.
Dr Karl performed a calculation of the terminal velocity of a bullet and determined that it should be able to kill someone:
As the bullet falls, it's subject to two forces - the suck of gravity trying to pull it faster, and the wind resistance slowing it down. The suck of gravity is not as powerful as the explosive gases that push it out of the barrel. So it will accelerate to a maximum speed of not 3,000 kilometres per hour, but somewhere between 330 and 770 kilometres per hour - depending upon the weight and shape of the bullet.
The Straight Dope also calculates the terminal velocity as sufficient to kill someone, but disputes the reliability of the King/Drew Medical Center figures:
"There is some skepticism about the numbers reported by the King/Drew team," the article continued. "The Los Angeles County Sheriff's Department and the Los Angeles Police Department — which serve a vastly larger area — reported only about half a dozen deaths in the same period … Other hospitals contacted by The Times … reported few cases."
