However, a bullet dropped out of a plane or shot straight up in the air will not come down with enough force to kill someone under normal circumstances.
As with many topics along these lines the problem is that it's not as simple as most people believe.
A bullet dropped out of a plane duplicates one of the three potential scenarios that need to be considered. I agree that a bullet dropped out of a plane is fairly unlikely to have lethal potential, but even if we consider only the case where a bullet is fired perfectly straight upward, there are still two scenarios which differ significantly in lethal potential.
Scenario 1. The bullet is fired upwards but at some angle from vertical--it doesn't really take much of an angle off true vertical for this scenario to work. In this case the bullet will come down nose first with some retained muzzle velocity as well as velocity imparted by gravity. The total velocity is significant and bullets that fall into this scenario definitely have lethal potential.
Scenario 2. The bullet is fired straight, or nearly straight upwards and comes almost straight back down. In this case the bullet achieves terminal velocity but retains no significant muzzle velocity. There are two possibilities for this scenario.
Scenario 2A. The bullet remains spin stabilized and falls base first. This has been demonstrated by Hatcher's experimentation to happen with some rifle bullets. In this case, terminal velocity can be in the 300-400fps range because the spin-stabilized bullet falling base first is relatively aerodynamic. Impact energy is in the 30ftlb range which did not impress Hatcher much, but is well above the typical pellet gun energy levels. Sadly there are news stories that prove on a fairly regular basis that even the typically anemic pellet gun can be lethal to humans.
Scenario 2B. The bullet destabilizes and tumbles as it falls back to earth. This has been demonstrated by Hatcher to occur with some rifle bullets and by Mythbusters to occur with the pistol bullets they tested. Mythbusters was never able to recover any rifle bullets and did not test anything other than straight up shots so they provided absolutely no information on scenarios 1 or 2A. In this case, terminal velocity is around 150fps. I would not like to be hit by a tumbling bullet at 150fps, and I suppose it has some small potential to be lethal but it's not terribly likely to kill someone or even cause a significant injury.
It's not possible to tell without testing which scenario will be created with a "sky shot". It might be scenario 1 with guaranteed lethal potential, scenario 2A with a high likelihood of lethality or 2B which is fairly unlikely to be lethal. The only responsible answer is that one should never shoot a firearm upwards unless they can guarantee that the impact area is totally free of personnel.
Wasn't it VP Biden that told people to fire two shots in the air?
Shotguns using birdshot are very different from centerfire rifles and pistols. While a rifle or pistol bullet has the potential to travel for a mile or more and still retain some lethality, birdshot is generally considered to be safe after it has traveled only some 400 yards downrange. Similarly, birdshot is such an inefficient projectile it has a very low terminal velocity and can not cause injury as it falls--although I wouldn't want to catch a falling pellet in the eye.
Conventional pellet guns with conventional pellets have a range similar to shotguns using birdshot and are generally considered safe with only a few hundred yards of safety area downrange.
