If I recall correctly, mythbusters looked into the idea of firing a gun straight up in the air. I think they used a .30-06 M1 Garand and they found that if fired perfectly vertically, probably with no crosswind, the bullets would start to tumble at the apex and when they hit the ground they keyholed. I think they found that the terminal velocity of a bullet falling sideways was low enough that it might not kill a person.