Depends on how it was disposed of. … and how well it’d maintained.

There’s plenty of 1-200 year old firearms that still work; and even as far back as the old west, firearms were meant to be kept up with replacement parts.

Now, that said there’s plenty of programs where you can turn in a firearm, no questions asked, and those are supposed to get destroyed. In the US, they’re frequently run by city cops to get guns off the street.

If you do find a firearm… call the cops. Nobody just “looses” a firearm. And, ah, don’t touch it. You don’t want your finger prints anywhere near it when they do get there. (Cops are dicks… and that gun probably has a body on it.)

Most guns don't really wear out in a reasonable timeframe. Properly maintained they can last quite a while. My first gun was from the 80s.

For gun owners in the U.S. if we no longer want a gun, don't want to go through the hassle of selling it, or the gun is unsafe (due to wear and tear or defects), or wherever reason really if we just want to get rid of it we have many options.

We can surrender a gun to our local police, though they may run its serial which might lead to awkward situations if you aren't certain of its history. There are also gun buybacks which are essentially events where you can discard a gun for cash incentive, and are typically no questions asked. You could also donate it to a local gunsmith for practice. And finally, you could render it inoperable (the ATF has guidelines that basically boil down to "weld the important stuff") and simply discard it like trash, use it as decoration, or whatever really.

Ultimately they either end up melted down, welded inoperable, or simply discard / forgotten.

They go into your bedroom drawer completely unsecured so your kid can steal and missuse it, as per the American way.

The modern notion of "interchangeable parts" was developed for guns; they're made to be maintained and repaired. Parts that wear out can be replaced. You can still get new replacement parts for guns made over a hundred years ago.

Gun heaven, where they can run around and frolic in the fields and enjoy the rest of their well deserved life.

I toss mine in the river when I'm done with them and have wiped the prints and filed off the numbers.

what happens to them at the end of their useful life.

When the gun buy back happened in Aus after the Port Arthur massacre they were just melted down and the steel was sold for scrap.

in germany you can trade old guns in at a dealer, and they decide if it's still usable or not. then it gets recycled i believe.

As others have said for the most part with the advent of interchangeable parts guns don't get beyond a state of repair often. Some probably do go to landfills but they're metal and metals tend to be one of the few materials worth recycling. Other might be parted out if their damage is isolated to a few parts.

They do wear out eventually, but the end of their useful life is a whole crapload of rounds if they're well-made, and most good guns can have parts replaced either by the user or by a smith, so you can keep them running a long time if you oil them and keep them relatively rust-free. If a gun is so messed up it can't fire or if it fails catastrophically and blows up, if it's truly and permanently at the end of its life, I think most folks do indeed throw them in the trash, assuming they have no historical value. Maybe disassemble them or cut them up with a hacksaw first. Once it stops being able to be a gun anymore, it's just junk.

Technically I believe it is illegal to throw away a gun in the US because of it's weapon status. That said, I'm sure it still happens. Guns, though, go on for a long time if properly maintained, and are often still in use even if they are not. Typically they'll get repaired or turned into pieces of "art" and hung on the wall. In some cases they may get donated/sold to someone who works on guns for a living or a hobby. Mostly, they hang around and rust. All of this information is from my own experience growing up in the rural south, and YMMV depending on the area you are in and the moneyed status of the owner.