A definitive answer to whether ammunition can rust requires looking beyond the common term “bullet” to the complete cartridge. The cartridge is a complex assembly of the case, primer, propellant, and projectile, each made of different metals. Corrosion is a significant threat, but the specific process depends entirely on the material. Steel-cased ammunition is susceptible to true iron oxide rust, while brass-cased ammunition degrades through tarnishing. Any material degradation can compromise the cartridge’s performance and safety.
Anatomy of a Cartridge and Material Susceptibility
A modern cartridge consists of four main components, each made from a material with varying susceptibility to environmental degradation. The case, which holds the primer and propellant, is the most vulnerable part and is typically made of either brass or steel. Brass, an alloy of copper and zinc, is the most common material due to its ductility and natural resistance to general corrosion.
Brass does not rust in the traditional sense but is susceptible to tarnishing and stress corrosion cracking (SCC). SCC occurs when residual stresses in the metal combine with corrosive agents like ammonia or nitrogen compounds. This combination causes microscopic, brittle cracks that compromise case integrity even without visible surface rust.
Steel cases contain iron and are prone to true ferrous rust (iron oxide) when exposed to moisture. These cases are commonly coated with a lacquer or polymer layer for protection. The projectile, or “bullet,” is generally a lead core encased in a thin jacket of gilding metal. Gilding metal is a high-copper brass alloy, typically 95% copper and 5% zinc, which is highly resistant to corrosion.
Environmental Factors Driving Ammunition Corrosion
The primary trigger for all forms of ammunition corrosion is moisture, specifically high relative humidity and condensation. Corrosion can begin at a relative humidity level as low as 45% and accelerates quickly as humidity rises. Temperature fluctuations are particularly damaging because they cause air inside a storage container to cool and condense into liquid water on the cartridge surfaces. This condensation introduces moisture into the microscopic gaps between the projectile, case neck, and primer seal.
High temperatures accelerate the chemical reactions that degrade the propellant powder and primer compounds. Storing ammunition in uncontrolled environments like basements, attics, or garages exposes the rounds to daily temperature and humidity cycles that encourage destructive condensation. Chemical exposure from handling also contributes significantly to case degradation. The salt and oils from human sweat act as an electrolyte, accelerating corrosion on case surfaces.
Contact with certain reactive materials can also induce corrosion. Ammonia, found in some cleaning solvents or decaying organic materials, directly attacks the copper content in brass cases. Exposure to harsh chemicals or environments with high levels of airborne pollutants, such as those found near industrial activity, can accelerate metal degradation.
The Functional and Safety Risks of Corroded Ammunition
Corrosion presents two main categories of risk: a loss of function and a direct safety hazard. Functional failures often manifest as a failure to feed or extract a round in a firearm. Corroded steel cases develop a rough surface of rust, and brass cases can swell or develop pitting. This increases friction against the chamber walls, preventing the cartridge from smoothly entering or being reliably removed, leading to a jam or malfunction.
A failure to fire occurs when moisture degrades the primer compound or the propellant powder inside the case. If the primer ignites but the powder burn is weak, the projectile may not exit the barrel, resulting in a dangerous “squib load.” Firing a subsequent round into an obstructed barrel causes a catastrophic over-pressure event. This event can rupture the firearm and cause serious injury to the shooter.
Severe case corrosion, especially in steel, can also lead to a complete case rupture upon firing. While this failure is rare, it releases extremely hot, high-pressure gas backward into the firearm’s action. This risks injury to the operator and immediate, irreversible damage to the weapon. Corroded ammunition often exhibits inconsistent performance, with varying muzzle velocities and reduced accuracy.
Storage Techniques to Ensure Longevity
Protecting ammunition for long-term reliability focuses on controlling the immediate microenvironment surrounding the cartridges. The most effective method is to use airtight, sealed containers, such as military-style metal ammo cans. These cans feature rubber gaskets that prevent moisture and air exchange, creating a stable, isolated environment for the ammunition.
For additional moisture control, include a desiccant material, such as silica gel packets, inside the sealed container. Desiccants absorb residual moisture, helping to maintain relative humidity below the critical 45% threshold. The storage location itself should be cool and dry, ideally maintaining a stable temperature between 50°F and 70°F and a relative humidity between 30% and 50%. Avoiding areas like attics and garages prevents the rapid temperature cycling that leads to condensation.
Ammunition should also be kept away from direct contact with materials that can harbor moisture or corrosive chemicals. This includes avoiding storage on bare concrete floors or in contact with certain types of foam or wood. These materials can trap moisture or emit corrosive vapors that accelerate degradation. Proper storage in a sealed, stable environment ensures the longevity of both brass and steel ammunition, allowing it to remain reliable for decades.