Shotgun shells are rather loose and open at the crimp compared to a rifle or pistol cartridge. If you keep your ammunition secured in a air/watertight ammo can in a cool dark place they will easily remain viable for decades . Cartridges and their components can degrade over time ( such as old cordite rounds) to the point of failure. On the other hand, people have been killed by old blackpowder antiques that were still charged.