So three wrongs make a right, in the end.
1) TSA shouldn't have prohibited the item to begin with, since spent ammunition casings are explicitly allowed as carry-on baggage.
2) TSA cannot confiscate items, they are "voluntarily surrendered". So he should have been able to get it back and mail or check it (though possibly at the risk of missing the flight).
3. TSA normally doesn't return "voluntarily surrendered" items but did in this case, so the guy got his necklace back.