Given the number of people who celebrate with gunfire, it's indeed not terribly dangerous overall. Nevertheless, "it's still your fault if it kills someone."
"They found that while bullets traveling on a perfectly vertical trajectory tumble on the way down, creating turbulence that reduces terminal velocity below that which would kill, it was very difficult to fire a bullet in this near-ideal vertical trajectory. In practice, bullets were likely to remain spin-stabilized on a ballistic trajectory and fall at a potentially lethal terminal velocity."
So I'm not completely wrong. Of course, if you fire a bullet non-vertically, it's much more likely to have a lethal velocity when it returns.
My comment wasn't really one of correct or incorrect.
It was meant to highlight russellbeattie's propositional logic ("if it kills someone" then "it's still your fault"). I agree with you that it's "not terribly dangerous"[1]. Your comment seemed to have omitted the premise, so I wanted highlight how it was a non-empty set. [2]
[1] It's also not terribly dangerous to drive without a seat belt on either. I don't advise people do so.
[2] "Firing a bullet into the air" does not mean it must have time to slow to terminal velocity, nor is a bullet hitting a body the only way to kill. (See "Twenty-three people were electrocuted after celebratory gunfire brought down a power cable".)
Look, I agree with you - celebratory gunfire is pretty safe, and if you want to be safer, shoot straight up. Not only will it slow to zero, but you will be the one most likely to be hurt by it.
That said, just because you shoot straight up doesn't mean it will slow to zero.
russellbeattie is still right - the person who shot that plane is partially at blame for the damage to the plane and the injuries of the two people onboard.
(I say partially because from what I gather, neither person was a trained pilot and were joyriding.)
Mythbusters found pretty much the same thing - IF (big IF there) you can fire it EXACTLY vertically so that the returning bullet is just like it was dropped from a balloon or something, the terminal velocity is not lethal
However, with any significant angle off-vertical, the bullet still travels in a stable trajectory and orientation, so will impact with a speed much higher than random-orientation terminal velocity, and be potentially lethal, even if passing through some roofing material on the way into your body.
So yeah, don't shoot into the air, and don't be around where anyone does...
