I remember being taught that a bullet can have the same velocity as when it was fired if you shot it in the air during police training and could kill someone.
I don’t think that’s right. Once the apex of flight is achieved it just falls. It can fall fast and pick up speed, but wind resistance will eventually slow it to some max speed (e.g., a human body would be about 200mph IIRC). I doubt it will be anywhere near the same as the original muzzle velocity of over 1000 FPS.....
That said, it’s speed and energy falling could very well be enough to penetrate a human skull on the ground. Still very dangerous.
I would guess that a bullet, falling from a higher trajectory, might reach 300-400 fps or so. A lot slower than muzzle-velocity of most weapons, but a heavy slug would still be able to penetrate a skull.
The terminal velocity of a bullet falling is MUCH slower than that. Bullets that come from the sky and kill on New Years, and the 4th of July, etc. come in somewhat sideways, or less that 45*.