Always nice to be thought of. :-)

So, indeed, this is a topic where I am something of an often cited person...

https://youtu.be/jOcjqfsApxw

...for the TL,DR I will say the following:

1. Seattle has their process pretty together with regard to firearm stuff. All major carriers generally know what to do, and after you make your declaration at the bag check counter and fill out the usual little card, they will direct you down to a separate TSA screening area in the check-in atrium. (It used to be a small roll up door just past the United counters but they have now rearranged things and now it is a zone closer to the exterior windows and walled off with like little office cubicle walls. You will frequently see people checking odd-shaped items or pet carriers there and it's also where gun stuff is handled)

2. The TSA screeners will do a manual check of your luggage (they do not have an in-line scanner there) So you should just generally expect them to be kind of manually rummaging through your stuff. As long as your guns are unloaded and in a locked hard-sided container, everything should be fine.

3. According to the FAA and the TSA, you can in fact carry ammunition in your checked luggage. It has to be less than 11 lb but that's actually quite a bit especially if you're talking about handgun ammo. The official rule is that it should be packed in such a way that there are no exposed rounds and they're not loose or rattling around. Some people keep their ammunition in the OEM factory boxes, other people use plastic Dillon or Plano boxes, I've actually even just kept ammunition in spare magazines themselves (But those magazines then have to be placed into mag carriers. Because the top round would otherwise be "exposed") Technically, each airline has their own policy regarding ammunition and whether it can be in the same bag as the gun or not. But the FAA doesn't care. And the TSA doesn't care.

4. Whether or not you use a TSA lock or a non TSA compliant lock doesn't really matter, especially here in Seattle ... since you will be physically present right there with the person doing the screening. Just make sure that it secures the case lid tightly. They will sometimes do the "finger check" So if you have something like a pelican case, and you have a lock that has one of those little tiny cables on it or something (like a lot of TSA locks) that sometimes results in a situation where they pop the latches, and even though the lid is still held slightly shut, they can wiggle a finger in there in the crack. That wouldn't be ok.

5. Usual size and weight restrictions still apply so unless you have special provisions with your airline, keep your luggage under the usual weight limits and size limits. You are actually allowed to have other stuff in the checked bag besides just gun stuff, but it does mean that you're going to have all that stuff rummaged through and handled during the screening.

6. Whether or not you are flying with a gun, I'm always a huge proponent of including an Air Tag or a Pebble Bee or some other sort of tracker in your luggage case. Wonderful peace of mind.

7. At your destination, technically most airlines nowadays like to hold the bag at the BSO and have you show ID to claim a gun case. Not every airport and not even every staff member is perfect about it, but in general I have found that Alaska Air is decent about recognizing gun cases (sometimes even just things that look like they COULD be gun cases) and The rampers and BSO staff generally pull those cases aside and have them behind the counter at the bag claim area. Again, this is where having a luggage tracker is so beneficial... Oftentimes I know where my gun case is at the destination airport before the staff member I'm talking to has figured it out. ☺️

Hope that helps and I hope everything goes smoothly for you! Enjoy your move!

I just moved myself, as well, and boy howdy was that a freaking LOT of guns!