You may have forgotten how it works.Handguns must be in a locked case (non-TSA lock). TSA lock goes on suitcase. Easiest to put a small locked case into your checked bag than in a case on their own that gets checked. I did it once about 6-7 years ago.
From Transporting Firearms and Ammunition | Transportation Security Administration:
"Firearms must be unloaded and locked in a hard-sided container and transported as checked baggage only. As defined by 49 CFR 1540.5 a loaded firearm has a live round of ammunition, or any component thereof, in the chamber or cylinder or in a magazine inserted in the firearm. Only the passenger should retain the key or combination to the lock unless TSA personnel request the key to open the firearm container to ensure compliance with TSA regulations."
How this applies to transporting scuba gear is that you can declare, for example, a starter pistol, put it in the hard-sided bag with the scuba gear, hard-lock the bag, and off you go. It also all weighs more. But the big danger is not TSA, it is baggage handlers. they can easily open a zipper bag; a locked hard-sided bag is more difficult; one with a non-TSA lock is really hard.