It's all about ammo.

Once the SHTF and people are running around in MadMax World, it's all about the ammo.

A long gun is nothing more than a blunt object without ammo. Gotta find a long gun where you'll be able to scrounge up ammo along your journey in just about any location or country.

And to that, I'd pick a 12. ga double barel shotgun, suited to shoot 3.5" shells on down. You can shoot any 12 ga shell through that thing - slug, shot, steel, lead, whatever. Make and model would need to be researched a bit, as I'd want a scatter gun that was virtually indestructible and the fewest parts possible. Or, if parts (ejector springs) fail, it really doesn't matter a whole lot to the functioning of the gun. Doesn't have to be pretty...just reliable and damn near indestructible.

Shotgun ammo is probably going to be found in just about EVERY house in America and throughout the world. People have been using the 12 ga scatter gun across the world for ever!

Most, if not all, of your encounters will be at close range. Shotguns are well suited for close range destruction and mayhem.

Now, if I could take a 2nd gun, I'd probably grab an AK. No optics and just open sights. I hate AK's....But....they are used world wide and everyone has ammo for them. That is another rifle that you'd never have to clean, but easy to take apart and clean if had too, and will "just work" no matter what conditions.