I had a dream, kind of Robocop-ish, in which a military robotic drone aircraft flew over a crowd and fired off many blank rounds, and the crowd was showered with hot, empty shell casings. Which, when I woke up, moved me to wonder if that had ever happened. Had there been, or were there now guns on aircraft that spewed the empties into the air?
The short answer is, Yes, there were. And in some cases, still are.
Not a lot of material on this–most links to machine guns in planes don't speak to where the empties go, but there were some that indicated that warcraft just ejected them into the air, and one story by a WWII bomber crewman was that one returning ship out of twenty showed damage from spent .50 cal shell casings, and that his personal experience was when shell casings from the plane in front of him broke through the plexiglas window and smashed up some gear, putting the nose gun out of commission, and bruising his shin through a flak suit.
So, yeah, though these days, the electric gatling guns in jet fighters drop them into a bin.
Check out the demo of how fast the M61A1 Vulcan 20mm machine gun fires–it takes a few seconds to warm up and slow down, but the average is 4000 rounds a minute. Look how fast it dumps those empties into the bin.
And the baby brother, 7.62mm minigun, out of a helicopter, firing tracers: