There really is no simple answer, besides being illegal many places.
If fired perfectly straight up then the bullet on its way down should only reach terminal velocity maximum on its way down, which may or may not be deadly depending on its mass and drag, but will injure and could possible seriously injure or kill. Firing a shotgun slug for example into the air, which is quite heavy, would be more than lethal on its return to earth without even reaching terminal velocity as it would still impact with a high amount of foot pounds due to its mass. However small birdshot would be more along the lines of being hit by a bb from a low powered bb gun, still unpleasant, and dangerous, but not likely lethal or to cause serious injury unless very unlucky.
However that is when fired perfectly up, a feat not likely to be achieved from a hand held arm, and in which case it will still have a trajectory. Then it turns into a question of the power of the round and the trajectory it is fired as to how dangerous it is on its return. Basicly though it is still very lethal quite often.
Shot however is round, and has high drag, and small shot high drag and low mass. So it is not as dangerous, however a bullet has more mass and less drag and can remain dangerous.
So lethal or not, nobody wants themselves or thier property to have rocks thrown in thier direction, or a kid shooting bbs from his daisy bb gun at them, so why you would think they would want the same from bird shot is beyond me. Return velocity is still going to be about the speed of a weak bb gun for bird shot.
Terminal velocity is likely to be around 200-250mph for the height of most powerful small arm rounds, and less powerful rounds will be somewhere in between. At 250mph that is 366.6 feet per second. So how lethal something is at 366 FPS really depends on how much mass it has. However I don't really want to get hit by something at 350+ fps whether it weighs over an ounce like a shotgun slug, or whether it weighs very little. Very small birdshot however has a lot of drag which limits the height it can reach.
Crosswind of course plays a role as well, drag and mass another.
So it really depends on who you ask, but mythbusters used very poor math, and assumed something in air would travel a certain speed based on the distance it traveled through a gel denser, which is quite false. He multiplied the distance traveled in air by how how much less dense air was to the gel. That is entirely incorrect data.
Here is a source
http://www.recguns.com/Sources/XD1.html
They mention the return velocity of a returning round at 300 fps, keep in mind though the return velocity is going to be dependent on how high the round travels which is going to be dependent on many factors.
"It was concluded from these tests that the return velocity was about 300 feet per second. With the 150 grain bullet, this corresponds to an energy of 30 foot pounds."
I sure do not want 30 foot pounds hitting me in the head, even if the army judges that to not be a disabling wound (likely in the torso).
However keep in mind all of this data is depending on firing the round perfectly straight into the air, any arc changes this and imparts more power to the returning bullet.
The real difference in danger posed is the distanced the round travels based on drag. Very small birdshot has a ton of drag. Non lead Birdshot (unless it is as dense or desner like hevishot) even greater drag due to less mass. So the danger posed is limited to a much smaller area, and the drag imposed on the round on its flight up limits the height it reachs which also limits the speed it reachs against drag on its return to earth. So birdshot is safest, bullets generaly are not. Of course even the term birdshot is relative and the smaller the better.
