How come when I see footage or war films from WW2 where flak cannons fill the air above them with shrapnel rounds that explode everywhere, said shrapnel doesn't rain down on the AA positions and harm the soldiers using the guns?
In short there was definitely a chance of it happening. I've linked to a previous post from u/Georgy_K_Zhukov talking about casualties during Pearl Harbor attributed to friendly flak rounds. Additionally there was damage caused by falling flak during the "battle of Los Angeles." It was just lucky that most people probably would have been inside or away from the scene, otherwise there would have been a chance of injury or death. During some of the large bombing campaigns in Europe it's possible that deaths from shrapnel would have been misattributed in all the chaos, but I'm not aware of friendly fire figures from say Germany. Perhaps someone else out there knows them.
Hi there I remove unexploded ordnance for my job in NL, and in short there is an unending stream of evidence that AA ammunition, whether explosive or not frequently and irregularly fell back to the surface following the firing procedure.
There must be made a distinction between faulty and functional ammunition in this regard. Faulty ammunition would not explode airborne, and could be completely dysfunctional. Functional ammunition would be either non-explosive, or might be explosive but limited in its explosiveness which possibly creates rather large and heavy chunks of metal in the process of falling down to the surface.
During WW2, German air defence in the Netherlands was extensive, and with almost dayly flights over the country from 1943 on always in action. The colossal amounts of expended ammunition - often of low quality itself - led to high percentages of faulty ammunition not hitting its mark or not exploding as intended. The nearly 1200 municipalities of the Netherlands kept detailed records of incidents with falling munitions in their local areas. In the majority of cases non-explosive munitions would damage roofs, windows and woodwork, all meticulously calculated in myriad damages. In the case of explosive munitions cattle, crops, sheds, and stores tend to be most often the casualties. The problem was so relatively common that a national fund helped out the affected, and meticulous background checks were done on a regular basis to ascertain the origin of the damage and the possible presence of more munitions or more debris.
The debris and munitions would then be collected by the German economic administration of the occupied Netherlands and often recycled or repurposed if there was any local use for it.
Regarding the effects on specific firing positions and the damage caused by fallback munitions I unfortunately do have any hard data, although it seems rare for it to cause casualties on a structural basis.