I don’t recall if we had to turn in the spent brass or just attest that it was fired during training. (I was an 18-year-old officer candidate being told to fire them off, not in charge.)

Hearsay, but I think the done thing is to bury rounds somewhere still in their cans. I occasionally see people complaining about it online when one government department or another buys up a couple dozen million rounds.

We would just ask the supply sergeant to "lose" them for someone else to "find" later (ammo would be the exception, for obvious reasons). I always loved taking over another unit in a war zone. We'd open containers and "find" all kinds of neat things no longer on the books: routers, switches, km's of networking cable, office supplies, etc.