How long does ammo last?

Manufacturers commonly print a ten-year figure. That number is a conservative warranty position, not a physical expiry date. Military stockpiles decades old are routinely fired without issue, and surplus ammunition from the middle of the last century still functions when it was stored well. Meanwhile a case left on a garage floor through a few humid summers can develop real problems.

Storage conditions dominate. Age is almost a side issue.

What's actually inside, and what ages

A modern centrefire cartridge has four components, and they age at very different rates.

• The case. Brass is corrosion-resistant but not immune. In humid conditions it develops surface oxidation, and under sustained stress with certain contaminants it can suffer season cracking. Steel cases are lacquered or polymer-coated precisely because bare steel rusts.
• The primer. Modern non-corrosive primers are chemically stable and sealed. Moisture reaching the priming compound is the failure mode, and it produces duds or hangfires rather than gradual weakening.
• The propellant. This is the component that genuinely ages, and it's worth understanding why.
• The bullet. Essentially inert. Lead and copper oxidise cosmetically and it changes nothing.

Why propellant ages

Smokeless powder is based on nitrocellulose, which slowly decomposes on its own, releasing nitrogen oxides. Those oxides are acidic and they catalyse further decomposition — so the process feeds itself if nothing intervenes.

Manufacturers therefore add stabilisers, typically diphenylamine or similar compounds, whose job is to chemically neutralise those oxides as they form. The powder is stable for as long as the stabiliser holds out.

Two things consume stabiliser: time, and heat. Elevated storage temperature accelerates decomposition substantially, which is why the same lot of ammunition stored in a conditioned closet and in an attic will be in very different condition after twenty years. Once the stabiliser is exhausted, decomposition can accelerate and the powder degrades in earnest.

This is a slow process measured in decades under reasonable conditions. It is not something to worry about with ammunition you bought this year, but it is the reason "cool" matters as much as "dry."

Warning signs

Deteriorating propellant announces itself. Open a container and look and smell for:

• A sharp, acidic or acrid smell — distinct from the faint solvent-like odour of healthy powder. This is the clearest indicator.
• Reddish-brown dust or staining inside the container or around case mouths.
• Green corrosion (verdigris) at the case mouth or around the primer pocket.
• Case cracks, particularly at the neck or shoulder.
• Bullets that move under light thumb pressure, indicating lost neck tension.

Ammunition showing acidic smell or heavy corrosion should be disposed of, not fired. Contact your local range, law enforcement agency, or hazardous waste facility — most have a process for it. Do not put it in household refuse.

What actually kills ammunition

Humidity

The primary threat. Moisture reaching the primer or propellant causes duds, hangfires and squibs. It also drives case corrosion. Aim to keep storage below roughly 50% relative humidity.

Temperature cycling

More damaging than steady warmth, and widely underestimated. As a sealed container heats and cools, the air inside expands and contracts, drawing humid outside air past any imperfect seal on each cycle. Over seasons, this pumps moisture in and deposits it. An uninsulated garage or attic cycles hard every single day.

Sustained heat

Accelerates stabiliser depletion as described above. Cool and stable beats cold and variable.

Direct sunlight

UV degrades polymer components and packaging, and sunlit storage runs hot. Keep it dark.

Storing it properly

• Use sealed containers with intact gaskets. A surplus steel can with a hardened, cracked gasket seals nothing. Inspect the rubber; replace it if it has taken a permanent set. A good seal resists the lid closing on the trapped air.
• Add rechargeable desiccant. Silica gel packs are inexpensive and rechargeable in an oven. Put one in every sealed container.
• Put a hygrometer in at least one can. This converts hope into knowledge for a few dollars.
• Store inside the conditioned envelope of the house. An interior closet beats a garage by a wide margin, mostly because of cycling rather than peak temperature.
• Keep it off concrete. Slabs wick moisture and stay cold, encouraging condensation on containers sitting directly on them. Use a shelf or a pallet.
• Label with purchase date and lot number. Rotate oldest first. If you ever chase a pressure or ignition problem, lot tracking is the difference between diagnosing it and guessing.
• Leave factory packaging in place inside the can. Boxes and trays prevent case-to-case abrasion and bullet setback in transit.

A note on corrosive surplus

Some older military surplus ammunition uses corrosive primers based on potassium chlorate. Firing it deposits hygroscopic salts in the bore that draw moisture and cause rust — sometimes within hours. This is not a shelf-life issue; corrosive ammunition can be in perfect condition and still require immediate cleaning with a water-based solvent to dissolve the salts, since petroleum solvents alone will not remove them. If you shoot surplus, find out which primers it uses before you fire it.

The short version

Sealed, cool, dry, dark, and inside the house. Do that and shelf life stops being a question you need to think about. Skip it and the ammunition you're relying on may not be there when you need it.

FAQ

Shopping for this?

Our Ammo storage guide covers the specs to compare and how to read them. Price classes, not live figures.