I use a security cable to secure the firearm hard case to my luggage by running the cable with a combo lock through the handle extension tubes inside the suitcase. That way even an unlocked suitcase is pretty secure as far as having the gun case removed from your bag. I stopped putting TSA locks on my luggage. TSA cuts the lock off for reasons I can't understand. They are supposed to use the TSA key for that purpose.
But it doesn't need to make sense. It's TSA.