Even though there are written rules by both TSA and each airline, you never know what you will encounter traveling with firearms and ammo. So you have to be prepared for any eventuality.
I always put my ammo in a separate bag the factory boxes in a locked plastic box. Why? Because different folks will give you different instructions and that’s always going to be fine no matter who you run into.
I also have both kinds of locks because I have received completely opposite instructions by different workers (airline and TSA) on locks - once within 5 minutes at the same airport.
I still have the same Cabela’s aluminum double rifle case I have used for almost 20 years now, but they are not made as well today. If I were buying new I would get a Pelican.