How long can you store ammo before it's no longer safe to shoot? The short answer is decades (maybe even centuries).
Many companies are in the business of selling surplus military ammo, that dates back to WWII, or older. This ammo is still considered acceptable to shoot my many, and it's sold at near-premium prices.
Let's look at the main components of a cartridge, how long each will last, and what it's biggest enemies are.
- Casing: Most commonly made of brass, which is used in other fields specifically for its corrosion-resistant properties, such as marine/boat fittings. While the brass will take some abuse, if it has begun to become noticeably corroded, dispose of that cartridge safely (other uncorroded cartridges are likely still ok). Steel-cased ammunition will degrade much more quickly, because steel obviously rusts much easier. Stored correctly, in a cool, dry place, the ammo's case should last centuries.
- Powder: You might be thinking, "Won't the powder degrade over, become more volatile, and cause an explosion?" This is unlikely. The powder was designed to be as explosively efficient as possible (e.g. powerful), so they generally only lower in capabilities over time. This can have the effect of causing a "squib load", or a bullet that doesn't make it out of the barrel when fired - if you unknowingly fire a second bullet into the obstructed barrel, you're likely to catastrophically destroy the gun, and injure yourself or others. Powder is very susceptible to moisture, and most military surplus ammo has a sealant applied to the case mouth and primer pocket, so old commercial cartridges may degrade more quickly. Modern powders should last for centuries if handled appropriately.
- Primer: Like powder, the primer will last for decades, maybe centuries. They are vulnerable to moisture, but in assembled ammo will outlast their owners.
- Bullet: As long as the bullet isn't obviously corroded, it essentially lasts indefinitely.