In watching a WWII movie recently, I was reminded by how many bullets were fired from aircraft, fighters and bonnets alike. Is there any evidence of those bullets that didn't hit their intended targets, falling to the ground any causing any damage?
Most certainly! Bullets and shrapnel fragments do not disappear of course, and this leaves the question of what does happen to them when they inevitably fall out of the sky sometime. Well, even relatively small modern conflicts see a significant amount of civilian casualties from stray bullets, hitting houses and streets, that were fired at far beyond the normal effective range of the firearms they came from. There are several known cases of people getting wounded or dying from celebratory gunfire alone in the past years, and military conflicts see a thousandfold more bullets being fired.
But first, let's take a look at what happens to bullets that miss their target:
physics part - skip if not interested
All objects fall at the same rate in vacuum, but our atmosphere slows them down to an equilibrium velocity -a 'maximum falling speed' if you will- which is also known as 'terminal velocity'. Terminal velocity is entirely dependent on the mass of the projectile and the drag, so a pointy shaped piece of lead can fall much faster than a styrofoam brick.
Normal bullets, especially supersonic rifle bullets, are designed very aerodynamically for obvious reasons. Hence when oriented normally, they have low drag, and as they are made primarily from either lead or steel, they have terminal velocities on the order of ~100 m/s, which is enough to seriously harm a human, and potentially kill them. Rifle bullets will be vastly more dangerous than pistol bullets, given their increased mass and lower drag. It's also very noteworthy that a rifle bullet shot at say 30 degrees elevation, will likely be travelling many times faster than terminal velocity on impact, because it does not slow down instantly. Stray rounds fired under a small elevation in general are likely to still carry a lot of ballistic energy, making them much more dangerous than rounds that are literally 'falling from the sky'.
Tumbling bullets have the same mass, but more drag, resulting in a lower terminal velocity. The specific velocity is again very much dependent on the type of bullet, as with the non-tumbling. Falling at speeds below 100 m/s, and travelling sideways instead of straight, their penetration will be much, much smaller. This means they could well bounce of without even braking skin, but it'd depend largely on the type of bullet and the drag increase from tumbling.
End of physics part
Now, as for your question you'll have noticed I've only mentioned pistol and rifle fire from the ground. Machine guns are statistically a much greater hazard though, both because of their ballistic properties and their larger volume of fire. Aircraft (both fighters and bombers) often employed several types of heavy machine guns, and any bullets travelling downwards would end up hitting the ground at more than their (already dangerous) terminal velocity.
I can't find any WW2 reports or estimates of people on the surface, killed or wounded by aerial combat, but personally I suspect the incident rate of stray aircraft rounds killing people on the ground isn't bound to be very high, for a different reason entirely, which is that it's a bad strategy to fight aerial battles above your own densely populated areas. Interception of both bombing missions and close air support was preferably done well ahead of the target area, because as soon as a bomber has reached the target area it's already too late. This holds true for close air support missions on military formations as well as strategic bombing of cities. In addition, even in WW2 military forces already factored in friendly fire and stray rounds when deciding on how to act. Plans to stop incoming V-2 rockets above London using a barrage of AA fire were rejected on the simple grounds that a small percentage of shells would fall back from the sky and explode on the surface, and the resulting casualties would exceed the deaths form the V-2 rocket. If surface casualties from stray machine gun fire had been a major factor, we'd likely seen reports on this, though again, fighting above your own populated area was already considered unfavorable.
I am assuming that you mean bullets fired at other aircraft in the sky - and not bullets fired at targets on the ground (since, obviously, misses hit something)
don't fire a bullet up into the air, but were you to, it would steadily lose speed, stop, and fall - but it doesn't return at the same speed (somewhat counter-intuitively) due to air resistance and tumbling
it's not that it can't do damage, but it's not the same as being fired "at" whatever it hits