I've had old ammo that started to look funky and left it with my local police department.
I've also had old ammo that I dumped into an old spetic tank that was about to be crushed and backfilled.
At one old range I used to shoot at, the old timers would toss their duds into the swamp next to the range. Now that I live in a coastal town, I'd probably dispose of them offshore and let mother natures salty sea strip them down over time.
Warning though: sea water takes alot of time, more than you'd think. Local fisherman dragged up an old bomb of some kind in a net last year. He didn't know what it was, no markings, thought it was some kind of cylinder, dragged it on board and brought back to shore for scrap. Trailered it to the scrap yard who bought it and started cutting it up with a torch when the thing started burning and sparking and basically scared the cr@p out of everybody. They had to evacuate a few blocks and call the bomb squad!