When a live cartridge is exposed to an external heat source, such as a house fire or a bonfire, the event is a predictable sequence of physical and chemical reactions driven by thermal stress. A standard metallic cartridge is a miniature chemical system composed of four distinct components, each with unique material properties and thermal tolerances. Understanding the composition and heat sensitivity of these parts—the casing, projectile, propellant, and primer—provides the framework for predicting the final, energetic outcome.
The Cartridge Components and Their Heat Thresholds
A typical modern cartridge contains four main parts: the projectile, the casing, the propellant, and the primer. The casing is the metal container, usually made from cartridge brass or aluminum, which holds the other elements together. The projectile is typically a lead core jacketed in a copper alloy, seated firmly at the open end. Inside, the propellant is a granular chemical mixture, often a nitrocellulose-based substance called smokeless powder. The final component is the primer cup, a small metal cap at the base containing a sensitive chemical compound.
The thermal tolerance of the casing material is significantly higher than the chemicals inside, but it is the first component to show structural stress. Cartridge brass melts around \(916^{circ} text{C}\) (\(1680^{circ} text{F}\)), while aluminum alloys melt around \(660^{circ} text{C}\) (\(1218^{circ} text{F}\)). In contrast, smokeless powder begins to chemically break down and can autoignite at temperatures as low as \(160^{circ} text{C}\) to \(200^{circ} text{C}\). This difference explains why structural changes occur long before the casing itself melts.
Physical Changes to the Casing and Projectile
As a cartridge is heated externally, the metal casing experiences thermal expansion. The brass or aluminum casing expands outward, while the smokeless powder and air inside heat up, increasing the internal pressure. This rising pressure, combined with the softening of the brass, causes the casing to weaken structurally. Cartridge brass begins to anneal significantly in the temperature range of \(427^{circ} text{C}\) to \(760^{circ} text{C}\) (\(800^{circ} text{F}\) to \(1400^{circ} text{F}\)), well below its melting point.
This thermal softening compromises the friction fit that holds the projectile, known as the case neck tension. The projectile seal can fail, allowing the bullet to loosen or even fall out before ignition occurs. If the heat is intense enough, the structural integrity of the casing wall is compromised, potentially leading to a rupture or split.
The Trigger Point: Primer Detonation Versus Propellant Ignition
The sequence of ignition is governed by the relative thermal sensitivity of the propellant and the primer. Smokeless powder is a chemical compound, typically nitrocellulose, that requires high heat and high pressure to burn efficiently and produce a rapid release of gas. When lit in the open air, smokeless powder burns slowly, similar to a flare, because it lacks the necessary confinement to build pressure. The powder’s autoignition point, the temperature at which it spontaneously combusts, is relatively low, around \(160^{circ} text{C}\) to \(200^{circ} text{C}\).
The primer compound is designed to be highly sensitive to heat and mechanical shock. Modern primers often contain lead styphnate, classified as a primary explosive, meaning they detonate with minimal energy input. In a heating scenario, external heat transfers through the casing, concentrating on the thin-walled primer cup at the cartridge base. Because the primer is engineered for sensitivity, it will almost always reach its thermal detonation threshold and explode before the main propellant reaches its sustained autoignition temperature. This primer detonation acts as a small internal explosion that provides the concentrated heat and pressure needed to ignite the bulk of the propellant.
The Result of External Ignition and Pressure Dynamics
When the primer detonates and the propellant ignites, the resulting event is energetic but fundamentally different from a bullet fired from a firearm. For a projectile to achieve high velocities, the enormous pressure generated by the burning propellant must be contained and directed down a rigid, sealed barrel. This containment allows the gas pressure to act only on the base of the projectile, propelling it forward. The cartridge casing is designed to be supported by the steel chamber walls of the firearm to withstand this pressure, which can exceed 60,000 pounds per square inch.
When a cartridge is heated outside a chamber, the casing is exposed and unsupported. The sudden, massive pressure spike from the burning powder instantly exceeds the structural strength of the softened metal casing. Instead of directing the force forward, the casing ruptures and fragments, releasing the high-pressure gas in all directions. Because the pressure is not focused behind the projectile, the bullet moves at a low velocity and erratically, often traveling only a short distance. The primary danger comes from the shrapnel created by the fragmentation of the metal casing, which is violently thrown outward by the escaping gas.