I’ve had some .38spcl reloads in a plastic bag since the early 1980s. These were left laying on a shelf out in a hot, non-AC, shop too. Shot about 30 of them the other day. Every single of em fired is if reloaded yesterday. A cool, dry, storage place in a sealed ammo can will let them last a long, long time. I had some old Chinese SKS/AK rounds that were stil in their sealed tins in stripper clips from the early 60s. Opened the tins and they were like brand new....every one of em fired without a hitch.
Also, a properly cared-for firearm will last generations.
S&W 686 revolver, stored clean and empty, will work fine 300 years from now.
Not sure of the shelf life of properly stored modern ammo.
I’ve fired 50 yr. old 32-20 ammo....in a 1934 Colt revolver.
(cost Grandpa $11 mail order)