Its altitude depends upon the type of weapon and type of bullet. On New Year's Eve 2017, a member of the Texas House of Representatives, Armando Martinez, stepped outside and was slammed in the head and rushed to the hospital where it was found the fragment of a .22 caliber bullet had penetrated his skull and lodged itself in his brain, requiring surgery to remove it. The bullet had been fired as part of New Year’s celebration. After the accident, Martinez unsuccessfully introduced legislation raising the penalties for discharging a firearm without an intended target.
Such cases are rare, but a 2015 article described two cases of children killed by falling bullets fired during Fourth of July celebrations in 2011 and 2012. A 2004 study by the Centers for Disease Control and Prevention found that on New Year's Eve and New Year's Day of that year, bullets from gunfire caused 19 injuries and one death. Thirty-six percent of the victims were struck on the top of the head.
Ballistics researchers have spent a lot of time studying the performance of bullets fired horizontally, but when it comes to firing straight up there's not nearly as much data.
In the mid-1940s, U.S. Army Maj. Gen. Julian Hatcher did experiments in Florida in which he fired various weapons, ranging from rifles to machine guns, straight up and tried to measure how long it took for the bullets to come down.
In his 1947 volume Hatcher's Notebook, he calculated that a standard .30 caliber bullet fired from a rifle pointed straight up would rise to an altitude of 9,000 feet in 18 seconds, and would hit the ground in another 31 seconds. He also calculated that during the last few thousand feet it would attain a nearly constant speed of about 205 miles per hour.
Ballistics researcher James Walker, department director of engineering dynamics at the Southwest Research Institute in San Antonio, says that the altitude attained by a bullet fired straight up would depend upon the type of weapon and projectile.
For example, a .22 caliber cartridge is the same diameter as the bullet, doesn’t have much powder and has a relatively low velocity. Bullets fired from rifles with a much larger cartridge will go faster because there is more powder.
When fired horizontally, bullets tend to slow down rapidly due to air drag, so that a rifle bullet may be down to half of its initial speed by the time it gets to 1,640 feet. When a gun is fired when pointed up, gravity will slow the bullet down even faster. Refer to THIS chart on the website of Close Focus Research, a ballistics testing company. Note: To convert feet per second to miles per hour multiply by 0.681818.
Of course, eventually the bullet’s velocity will be spent and it will begin to fall. The elevation from which it begins to fall back to Earth is a non-issue as to how fast it falls. As it is no longer spin stabilized, it will hit a terminal velocity based on its shape, orientation, and whether or not it is tumbling. Also, a bullet is unlikely to fall straight down, as wind can alter its path, so it’s difficult to predict where it will land.