Does Ammo Expire? Storage, Shelf Life, and Real-World Conditions

Properly stored ammunition lasts decades — often much longer. Military surplus ammo from World War II still fires reliably when stored in sealed cans. Commercial ammo in original packaging kept in a climate-controlled space will outlast you. But "properly stored" is doing a lot of heavy lifting in that sentence, and most people's actual storage conditions are far from ideal.

This guide covers the science of what actually degrades ammunition, addresses the specific real-world scenarios people actually worry about (car trunks, garages, humid basements), and gives you a practical storage plan.

The Short Answer

Ammunition does not have an expiration date. There is no chemical timer ticking down inside your cartridges. Ammo degrades when exposed to moisture, extreme heat, or chemical contaminants — and the degradation happens slowly over years, not days.

If your ammo has been stored indoors at normal room temperature (50–80°F) and humidity (below 60% relative humidity), it's almost certainly fine for 20+ years and likely much longer. If it's been stored in less ideal conditions, it depends on the specifics.

The Science of Ammo Degradation

A cartridge has four components that can degrade: the primer, the powder, the case, and the bullet. Here's what actually threatens each one.

Primer degradation

The primer is the most chemically sensitive component. Modern primers use lead styphnate compounds that are remarkably stable when dry but can degrade when exposed to moisture, petroleum-based solvents, or certain cleaning chemicals. A degraded primer means a misfire — the most common failure mode of old ammunition.

Powder degradation

Modern smokeless powder is chemically stable at normal temperatures. However, sustained high heat (above 150°F) accelerates decomposition, which can alter burn rates and pressure curves. In extreme cases, decomposing powder produces nitric acid, which attacks brass cases from the inside. You'll smell a strong acidic/ether-like odor if this is happening.

Case corrosion

Brass cases resist corrosion well but can develop green verdigris (copper oxide) in humid conditions. Surface tarnish is cosmetic and harmless. Deep pitting or corrosion around the case mouth or primer pocket is a problem — weakened brass can rupture on firing.

Bullet oxidation

Copper-jacketed bullets develop a dark patina over time. This is purely cosmetic and does not affect performance.

Real-World Storage Scenarios

Ammo in your car trunk

The concern: Car interiors can reach 140–170°F in direct sunlight during summer months, especially in southern states.

The reality: SAAMI testing confirms that modern ammunition is safe at temperatures up to 150°F. Short-term exposure (days to weeks) to car temperatures won't cause failures. However, repeated thermal cycling — hot days, cool nights, over months — accelerates primer degradation faster than constant moderate temperatures. Verdict: Fine for a few days. Don't store ammo in your car for months during summer.

Ammo in a hot garage

The concern: Garages in states like Texas, Arizona, and Florida can sustain 100–130°F for months.

The reality: Garage-stored ammo will last years, not decades. The sustained moderate heat won't cause catastrophic failure, but it will slowly degrade primers and powder faster than indoor storage. If this is your only option, store ammo in sealed ammo cans with desiccant packs, elevated off the concrete floor (concrete wicks moisture). Verdict: Acceptable for 3–5 years with proper containers. Not ideal for long-term storage.

Ammo in a humid basement

The concern: Basements in the eastern US commonly run 60–80% relative humidity without dehumidification.

The reality: Humidity is the number one enemy of ammunition longevity. Moisture attacks primers and cases simultaneously. Ammo stored loose in a cardboard box in a damp basement is the worst-case civilian storage scenario. Verdict: Only if you run a dehumidifier and use sealed ammo cans. Otherwise, move your ammo upstairs.

Ammo loaded in magazines for months

The concern: Does keeping rounds in a loaded magazine degrade them?

The reality: Magazine spring compression does not damage ammunition. The concern here is environmental — a loaded magazine in your nightstand is exposed to whatever temperature and humidity your bedroom is. If your home is climate-controlled, loaded magazine ammo is fine for years. Verdict: No issue from the magazine itself. Rotate carry ammo annually as a best practice.

Carry ammo exposed to body sweat

The concern: Does perspiration from inside-the-waistband (IWB) carry degrade ammo?

The reality: Sweat is salty, acidic moisture — basically the worst-case combination for brass cases and primers. Carry ammo in a pistol worn IWB daily will develop visible tarnishing within weeks. While modern sealed primers resist moisture penetration, this is the one situation where regular rotation genuinely matters. Verdict: Rotate IWB carry ammo every 3–6 months. Shoot the old stuff at the range.

How to Store Ammo Properly

The ideal storage conditions are simple: cool, dry, and consistent.

Temperature: Below 80°F is ideal. Room temperature in a climate-controlled house is perfect. Avoid locations with extreme temperature swings.

Humidity: Below 50% relative humidity is ideal. A standard indoor environment in most of the US is fine. If you're in a humid climate, use desiccant packs inside sealed containers.

Container: Military surplus .50 caliber ammo cans with rubber gasket seals are the gold standard — they're designed for exactly this purpose, they're airtight, they're stackable, and they're cheap ($10–20 each). MTM and Plano make polymer ammo cans that work similarly well.

Elevation: Store ammo off concrete floors. Concrete absorbs and releases moisture. A wooden shelf, pallet, or even a piece of cardboard underneath is sufficient.

When to Rotate Your Carry Ammo

A sensible rotation schedule:

"Rotating" doesn't mean throwing ammo away — it means shooting it at the range and loading fresh rounds. This also gives you regular practice with your actual carry gun and carry ammo, which is a training benefit.

Storage Products Worth Buying

Military surplus .50 cal ammo cans: The best value in ammo storage. Airtight rubber gasket seal, steel construction, stackable. Available at military surplus stores, gun shows, and online for $10–20 each. Inspect the gasket — it should be pliable, not cracked or dried out.

Silica gel desiccant packs: Throw one or two in each ammo can. They absorb moisture inside the sealed container. Rechargeable (bake in oven to refresh) versions last indefinitely. A few dollars for a multi-pack.

MTM Ammo Crates: Polymer alternative to metal cans. Lighter, won't rust, come in various sizes. Slightly less airtight than military cans but perfectly adequate for indoor storage.

Hygrometer: A small digital humidity/temperature monitor for your storage area. Tells you if your conditions are actually as dry as you think. Under $15 for a reliable one.

Browse our deals page for current ammo prices — buying in bulk now and storing properly is almost always smarter than buying at retail prices later.