I keep thinking about the question of "the last gun I'd ever give up" and I finally came to a decision.
I would choose my flintlock pistol.
It wouldn't be my first choice, but if things got bad enough that I could only keep one, it is the most sustainable long-term. If I were a nomad without a home base, I might choose differently.
Ammunition is the main reason for my choice.
Black powder can be made at home, so there's less risk from the supply chain or the murderous BATFE's rules. Of course, in such a situation, the murderous BATFE wouldn't exist, so their empty opinions could be ignored.
Powder manufacturing is also why I would choose a different gun if I were nomadic with no place to set up to make black powder.
Lead can be found as trash. I find lead wheel weights on the street all the time, although the zinc ones are becoming more common. You can get a bullet mold the size you need and use it practically forever. In a pinch, you could probably find a way to make do even without a bullet mold.
Patches for sealing the bullet in the barrel can be made from old shirts or handkerchiefs, or can be skipped altogether if you don't mind a loss in accuracy.
Flint or chert can be found just about anywhere, and would be easy to pick up and carry to use later. No, I don't have any locally, but I pick it up whenever I do find it in my travels.
The gun is also simple to disassemble and work on. I think I could keep it running to be passed on to the next generation. If all else failed, someone could probably turn it into a matchlock without too much trouble.
Yes, for defense I would rather have a revolver or a modern semi-auto with a magazine. But unless you can get brass and primers, those won't keep being useful under post-apocalyptic conditions. And, I doubt anyone has enough ammo.
Feel free to explain why I'm wrong and what you'd keep.
Also, until the apocalypse happens, if you want to sign up for the ammo subscription service, AmmoSquared, you can use my link https://ammo2.me/dullhawk and help us both. I've also added the link to the sidebar.
If you sign up from my link, using the "join today" button, once you've spent $20 we each get an additional $25 in our ammo accounts. I put in very little every month, but it's adding up. I hope you'll try it. We both win!