The Winchester Model 12 is a classic, the one that other pump guns are measured by and are found lacking. Those fine guns are still seen with regularity in the duck blind, the pheasant field and on the trap/skeet fields across the nation. Some of us have never owned one but have shot them, fine weaponry indeed. I still see them in gun shops occasionally along with its rack-mate the Winchester 97.
It always helps to measure the barrel on both those guns when looking them over. Quite a few of them sported polychokes and cutts compensators in earlier years, especially here in the midwest, and their subsequent removal has left more than one old classic with a barrel length that is chokeless and of an odd length.
Lots of "farmers shotguns" were modified over the years, even the aristocratic Model 12. A lot of them were stored in barns, which is how mine looks. But it's also an old soldier, and deserved a better retirement.
A well-made firearm is one of the few objects that can be used for centuries, as long a it gets minimal maintenance. Springs are perhaps the weakest link, but as long as someone knows how to make springs (and I think they'll be around for a long time), the gun can still function.
Ammunition is another matter. I have some original GI Krag ammo dated 1917. The National Guard still had Krags, so they were still making new ammo for it.
The ammo came in bandoliers, with ten loose rounds inside a pocket that was sewn closed with flimsy thread. I looked at some of the rounds, and each case neck was cracked, meaning air got inside the case, and oxidized the powder. If the powder ignited at all, the cracked case neck would never hold the bullet long enough for gas pressure to push it out the barrel.
I guess after 90 years that even annealed brass case necks re-harden, and then fail. Of course, drawn-brass cases were still pretty new metallurgical technology back then.