How dangerous are the bullets coming down ?

The determining factor is the angle that the gun was being held at when fired.
If the barrel was pointed straight up when the bullet was fired it will go up
until it reaches zero velocity and then fall back down in the general area
where it was fired from ( wind drift changes that). On the way back down the
bullet will reach what is known as terminal velocity. Gravity will accelerate
the bullet on its way back down at 32 feet per second squared IIRC. The
limiting factor is wind resistance so the bullet will reach terminal velocity
and go no faster. I have seen studies that estimate that speed at about 180 mph
for most bullets.

If the bullet leaves the barrel at anything less than a 45 degree angle it can
really be moving when it comes back to earth since it will not reach zero
velocity before it starts its downward trajectory. A round like that used in an
AK-47 might travel over 4 to 5 miles before coming back to earth if fired at a
shallow angle and it would still be moving fast enough to be very dangerous.
The bullet fired straight up would hurt like hell if it should hit you but more
than likely it would not be fatal. A good reloading manual with ballistic
co-efficients will give you an idea what kind of velocity can be expected under
different conditions.

Dennis