It's pretty safe to fore a gun in the air. When the bullet comes back down it will only be traveling at its terminal Velocity, not at the extremely high Velocity it has exiting the gun.

Terminal Velocity for a bullet would be around 45fps, feet per second, its muzzle Velocity would be between 900 and 1500fps for most hand guns.

If a 9mm were fired straight up and came down on someone's head, it would hit with the force of a very small rock being rather gently tossed. A good baseball pitch is roughly three times the Velocity.

In order for the bullet to pose a significant threat it needs to travel a ballistic trajectory and stay within an effective range, given enough time it will always decelerate to its local terminal Velocity.