To add to what Hoser said. A TSA approved case. Also as said check the online rules for the carrier. My case was 3" too big for United and they tried to charge me an extra $175 one way. Check all the rules for firearms and oversize cases as the two charges can add up.

Basically when you go to the counter declare that you are traveling with a firearm. You will get a card to fill out. Make sure it is unloaded. none has ever asked to touch or handle my firearms. You then are directed to the TSA part and for me at least they ask to see the card and if it is unloaded. They ask to open it they look and have you place the card inside and lock it. Done. I have only got complements from the TSA gang that handles firearms.

Once coming back from Portland the guy said close it, no open it.... He looked and said "very nice" you can lock it.