I see this situation as almost exactly how various governments treat art that was stolen during WW II. A lot of GIs brought home art from Europe, or sold them to private collectors after the war, or the Nazis raided homes and museums. Once the original owners/museums/governments found out where their property was, they took action to get it back. The US government cooperated with a lot of European governments and private individuals to return their property to them. I have read several books about this.
Through no fault of his own, the most recent owner of the rifle bought stolen property. Can we fault the original owner of the property for getting it back? If someone broke into your home, stole your favorite 1911 and resold it, would you just write it off and not try to recover it from the person who bought your stolen property?