How long would current weapons last?

Question

In a post-apocalyptic world where humans stopped producing the weapons and ammunition as we know today, how long would one be able to rely on one of these weapons (guns, rifles, etc.) they posses? How long until it becomes very unreliable?

I think I'm asking how long the gunpowder inside ammunition would last because I think that's the first thing to fail over time, but I don't know of other factors that could cause malfunctioning (weapon rusting maybe?).

Storage and maintenance matters a lot, so let's say these weapons didn't receive any special maintenance or storage, they are just weapons stored as they would be in a store in the pre-apocalyptical days.

Answer 1

In a dry environment without a lot of temperature fluctuation (for example, Los Angeles) and properly stored, quality commercial ammo will last about a decade or two. Maybe more if they're stored in a well sealed container as one might store surplus ammo during the off-season.

The article How To Property Store Ammo in Guns & Ammo discusses what causes ammo to degrade and how long you can expect it to last.

According to Rick Patterson, Managing Director of SAAMI, “In fact as long as your ammunition is stored at normal room temperatures with low humidity, it can function reliably for decades.”

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“It’s easy to forget just how much conditions vary through the year, let alone over the course of many years,” said Tim Brandt, Media Relations Manager for Federal Premium Ammunition. “Here in Minnesota it can hit 100 degrees in the summer and negative 30 in the winter.”

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Ensuring a low-humidity environment is another storage priority. According to Patterson, “there’s no hard-and-fast level to strive for, but generally the lower the better.”

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Then there are the many types of Russian-made, surplus “Spam Cans.” The steel cans are sealed tightly against air and water, requiring a special tool or screwdriver to pry them open. Some shooters swear by these cans to keep ammo fresh for decades. The cans are commonly shipped with 7.62×39 or 7.62x54R loads—quantity varies, but 640 rounds is typical.

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Luckily, as long as you adhere to these basic recommended guidelines, you’re almost certain to get a decade of use out of every box of shells. Our grandfathers couldn’t expect that.