I'm sure the ATF is going to drop everything and get right on that! His only chance of getting it back, rests with your willingness to return it. Unless somehow, your wife failed to return it in accordance with their divorce decree, or he previously reported it stolen, he's SOL. It's no different than any other property. No, I'm not a lawyer but I've had to deal with similar issues.
Bingo.
Substitute the word "guitar" for "firearm" in your musings.
Property is property. Let him call the ATF... your wife has committed NO CRIME!
This^^
Let him call the ATF, they'll laugh at him if they don't just hang up. Same thing with the local law enforcement. It doesn't matter that it's an AR, it's property and he abandoned it 10 years ago. It now belongs to your wife. Don't try and send it to NY, don't do anything. Just ignore him.
Doing anything other than nothing is just feeding the troll, it's like responding to a post by walt.
