I travel with firearms on a regular basis. I've had no unusual experiences at O'Hare or Milwaukee. I sign the card, tape it to the small gun safe inside the suitcase, and hand it to the ticket agent to place on the conveyor.

I have been asked at some airports to wait for the TSA inspection and a not that it's good to go. At Seattle, I carry the bag to a special booth near the ticket counter. It's inspected and a porter carries it to the baggage checking area. I was asked to do the same in Ft. Meyers, FL, but had to carry it to the far end of the terminal. Fortunately, that was just once.

You should not hand the key or combination to TSA. They may ask you to open it if they see something unusual. If it's inside another suitcase, they may remove the gunsafe to better inspect the other contents, which might be hidden under all that metal. I put TSA locks on the outer suitcase, not on the "hard sided case" which holds the firearm.

They will not cut the locks or force the gunsafe unless they detect an immediate danger or unlawful condition. Checked baggage X-rays can easily see whether it is loaded or not. If they need to open the case, they will attempt to notify you and refuse to process the luggage until you respond.

Arrested for opening a suitcase in the baggage claim area? You cannot be arrested for violating a company rule, except on the aircraft itself. A law would apply to all airlines, not just Delta, and since it is an unusual requirement, it would be posted, citing the applicable law or ordinance. The shooter in the Ft. Lauderdale airport went into a restroom to retrieve his weapon. There is always someone looking over your shoulder in Florida restrooms, but not the TSA type

It is a violation of Federal law for anyone to mark the suitcase or otherwise indicate there is a firearm enclosed, including the airline or local authorities.