Personal experience:
I was gifted a Colt AR15 SP1 a few years ago that hadn’t been out of its box since it was bought new in 1973 after the owner fired five rounds through it and put it away. When I received it, it was dry as a bone and had zero wear, rust, or discoloration. Bore was like a mirror after I ran a patch down it. This rifle was stored up in a closet in its original box in a desert ranch home since Nixon was President. It function tested perfectly, needing no maintenance or new springs or anything.
The oldest ammo I’ve ever shot was US military .30-06 that was packaged in a sealed can in 1934. Every round fired flawlessly.