Physics state that anything that goes up must come down. I'm surprised that I never considered this, but what happened to all that flak that was shot up into the air over Germany in World War 2? Surely the vast majority didn't hit anything with enough traction to lodge and stick, so it must have been raining shrapnel during and just after air raids, no? Do we have any primary sources that touch on this? (And this is to say nothing about all the bullets that were shot during the intercept battles above.)
As far as I know there has been no comprehensive study done on that in Germany so far. What we know is that it did happen, both shrapnells as well as unexploded FlaK grenades came back down. While many of those grenades exploded upon contact with the ground or structures, the damages were not as high as a bomb, obviously. The most common AA-grenades were 8.8 and 10.5 cm, and they could still do significant damage or kill at close range. Unexploded AA-grenades are still found as frequently as dud bombs (which is a common occurence here in Germany).
So what we know is mostly anecdotal, there's lots of archival material that hasn't yet been comprehensively analyzed. For Berlin, e.g., there's the reports of the 'Hauptluftschutzstelle', the main air protection bureau, for Berlin. These are detailed reports for the damage done by each air raid, usually compiled quickly after the raid. For example, for the Air Raid on Berlin from 7th September 1941, the damage report lists, after incendiary and explosive bombs, damage done by 57 Flak-grenades, among them 10 duds. And for the early raids on Berlin, until about Autumn 1940 (after when this would quickly change), they list more damage due to Flak-grenades than allied bombers.
The first air raid on Berlin on August 26, 1940, did only light damage due to allied bombs. But in the middle of Dragonerstraße street, a Flak-grenade exploded, wounding four persons and breaking all glass in the vicinity; the district hall of Tiergarten lost most of its windows to another German Flak-grenade, and Flak-shrapnel wounded seven Berliners in the Wedding district, one of them heavily.
For the raid of September 7, 1940, the only damage in the district of Zehlendorf was done in Herwarthstraße 9, where a house was damaged by a Flak-grenade, and in Prenzlauer Berg district, an unexploded grenade struck the roof of a flat in Dunckerstraße 84, and the family had to be evacuated to friends.
Similar reports can be found for many other German regions and cities.
There are also many anecdotal and biographical accounts of kids playing with what they called 'bomb' shrapnels, but which were actually the remains of exploded flak-grenades, they traded them with each other and hoped for big and impressive specimens.
I couldn't find much more but I'll have a look in our library tomorrow.
Paging u/Astrogator & u/TerribleTauTG - any info on this?