A couple of years ago i was serving my conscript term as a flak gunner/airport security personnel.
In gun handling and safety training they told us that whenever we want to clear a potential gun jam or make sure a gun is empty before placing it in storage during peace time (which entails taking off the clip, pulling the arming handle and pressing the trigger x2), we should first make sure that if a bullet is inadvertently fired it should be in the direction less likely to cause damage.
First and most used method was to aim in an obliquely placed barrel containing sand, the idea being that the bullet will be slowed down or fragmented before exiting the barrel and even if it does exit, it will go into the soft ground placed underneath the barrel for this purpose.
If a barrel was not present, we were instructed to "empty-fire" at the ground but only if it was soft ground and the angle was shallow, in order to prevent possible ricochets.
If none of these was applicable, we were told to "empty-fire" into the air but sideways (about a 30 degree angle) and not directly up, in a direction with a visibility about equal to the gun's effective range, so that we could first determine there were no materials or people in the way of the bullet within it's effective killing range.
The one thing they stressed most of it all however was "whatever you do, don't aim the gun straight up when clearing jams or practicing safety measures".
A bit of high-school Newtonian physics is enough to calculate that even though friction with the air and the effects of tumbling result in a bullet that comes back down in a probably non-supersonic speed and maybe not with the pointy end facing the ground, it' still a bad idea to fire directly upwards into the air because the downwards speed on the return trip is still big enough to injure or kill.
Sure, it might not be fully effective after it goes through its reversal of trajectory, but getting hit on the head by a chunk of metal that's doing 150-200 meters per second is still a bad idea no matter how one looks at it.
After all, it if was safe people in the age of muskets wouldn't have had so much success killing each other with non-rifled weapons that were shooting round bullets.
