Curiosity about what happens when a cartridge is exposed to extreme heat is often fueled by dramatic portrayals in popular media. The reality is far less explosive than often depicted, but it is scientifically complex. Understanding the outcome requires examining the components of ammunition and the physics of how it functions within a firearm. The resulting event is a low-energy thermal reaction, not a high-velocity shot, though it still poses a significant hazard.
Anatomy of a Cartridge
A modern cartridge, often incorrectly called a “bullet,” is an assembly of four distinct components. The casing, typically made of brass, holds the other parts and is designed to expand and seal the chamber upon firing. At the base is the primer, a small metal cup containing a shock-sensitive explosive compound that serves as the initial ignition source. The casing holds the propellant, a measured charge of smokeless powder that burns rapidly to produce a large volume of gas. The projectile, the actual bullet, is seated at the open end, and the rapid expansion of gas pushes it down the barrel at high speed.
Heat Versus Impact
A cartridge is designed to ignite through a rapid, concentrated mechanical force, not a slow, uniform increase in temperature. When a firing pin strikes the primer, it crushes the sensitive compound against an anvil, creating a spark that ignites the propellant in a fraction of a second. Exposure to extreme heat, a phenomenon known as a “cook-off,” is a different chemical reaction entirely. As the temperature of the casing rises, it eventually reaches the auto-ignition point of the primer and the propellant. This thermal ignition, which can occur at temperatures achievable in house fires or bonfires, is slower and less controlled than the mechanical impact that occurs in a gun.
The Actual Danger and Trajectory
When a cartridge ignites outside of a firearm, the lack of a chamber to contain the pressure fundamentally changes the outcome. In a gun, the solid steel of the chamber and barrel forces all the rapidly expanding gas to push the projectile forward. Without this containment, the pressure seeks the path of least resistance, which is typically the brass casing itself. The pressure from the burning propellant causes the thin brass casing to rupture and fragment, releasing the energy in all directions.
The projectile, or bullet, does move, but it achieves only a fraction of its intended velocity. The velocity of the projectile is so low because the pressure is not sustained or channeled by a barrel. Testing has shown that a bullet from a cook-off event will often travel with an initial velocity that is too low to penetrate human skin or standard firefighting gear at a distance of a few yards. The greater danger comes from the fragments of the brass casing, which can fly out like shrapnel. The resulting event is a loud, uncontrolled pressure release that scatters hot debris, including the casing fragments and the low-velocity projectile.
Common Misconceptions and Safety
The most pervasive misconception is that a cartridge in a fire will propel the bullet with the same lethal force as if it were fired from a gun. This “Hollywood myth” ignores the physics of pressure containment, which is the entire purpose of a firearm’s chamber and barrel. Although the projectile is not launched at rifle speed, the event is still hazardous and should never be attempted. The rupture of the casing generates noise and flying metal shrapnel, which can cause serious eye injury or penetrate soft tissue. Safe storage involves keeping ammunition in cool, dry conditions, well away from heat sources like stoves, heaters, or open flames. This practice mitigates the risk of a thermal reaction and the resulting uncontrolled release of pressure and debris.