When you check in at the luggage check, look at the gate agent and say "I have an item in my baggage that I need to declare". There is ONLY one item that ever needs a declaration, and they know exactly what you mean.
This is sort of an aviation code-phrase so you do not need to use the terms "Firearm", "Gun", "Pistol", etc., in a location where use of those words might attact unwanted attention of bystanders. Funny... the last time I did this the gate agent (Southwest), said "Oh, you mean a GUN" and handed me the paper. Go figure. At least she whispered.
The gate agent will hand you a TSA form so that you can declare your "Item". Fill it in and hand it back and forget about it.
Make sure your bag is locked with a TSA accessable lock so that it can be opened without them needing to cut off the lock if they do decide to inspect it once it's checked. This lets them relock it... which is what you definately want. Pistol needs to be in a hard case inside another suitcase. Ammo "legally" needs to be in it's original packaging... but a loaded magazine (not installed in a pistol) seems to pass the test at least in my experience. They DO NOT want loose ammunition anywhere in the shipment.
Fly SouthWest in Texas... you will have no hassle.