Check out mythbusters...they had a special about this very subject.
As long as you're firing "UP" not at an angle...it's technically perfectly safe.
They tested a lot of calibers. Basically once the bullet slows to a stop and begins to fall...it no longer is spinning, but tumbling down. Even with large calibers the terminal velocity of a tumbling down round is not enough to kill. It would hurt like a SOB if you got hit with one...but not fatal.
They set up a nail gun to shoot pennies at the terminal velocity of the rounds, and shot it at their hands from 2-3 feet. It hurt...but, didn't even break the skin.
I've shot 410's up in wooded areas...cool sound of buckshot falling through the trees