Right, a gun fired straight upward or close to straight upward probably isn’t going to kill anyone. But when it’s angled sideways that’s when things get dangerous. The horizontal component of the bullet’s velocity doesn’t have time to get reduced by air resistance to a point where it’s non-lethal.
I have heard that a bullet shot horizontally to the earth will hit the dirt at the same time as if dropped to the ground from point of fire.
True or false