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.