I’ve wondered about that. I have some empty 30mm from an A-10 Warthog and those things are heavy enough to sting, plus they come out in big quantities.
Nothing like a little depleted uranium to rattle the tank hull...
The F-16 holds the spent casings so nothing but bulllet falls from the sky.
The worst casings dropping out of the sky are those from parachute flares. IIRC, they are about 4” OD and 3 feet long. We did an Op sealing off the Batangan Peninsula and pushing out to the end. C-130’s were dropping parachute flares all night, every night.
We’d wake up in the morning to find the outside canisters of the flares buried a foot deep into the paddy mud. My foxhole for perimeter watch was set in front of a huge boulder. We’d rotate watch every 2 hours. About 2am when another Marine was on watch, one of those canisters hit the boulder right behind him. As you can imagine, he nearly filled his britches! If it had hit him, it would have killed him!