The firing of a projectile from a firearm relies on a chemical process that rapidly converts a solid material into high-pressure gas. This transformation of potential chemical energy into kinetic energy is the fundamental principle behind how a gun functions. The sequence, from the pull of the trigger to the bullet exiting the barrel, is a controlled chain reaction designed to create immense pressure in a confined space. This process involves the propellant, the ignition system, and the physical interaction within the barrel.
The Chemical Composition of Propellants
Propellants used in modern firearms are classified into two main types: traditional black powder and smokeless powder. Black powder, the earliest known explosive, is a mechanical mixture of three components: potassium nitrate (saltpeter), charcoal, and sulfur. Potassium nitrate acts as an oxidizer, supplying the oxygen needed for the rapid combustion of the fuel sources.
Modern smokeless powder replaced black powder and is primarily based on nitrocellulose, or a combination of nitrocellulose and nitroglycerin in a double-base composition. Unlike black powder, smokeless powder is a chemical compound rather than a simple mixture, making its combustion cleaner and more efficient. The formulation of these propellants is controlled to determine the burn rate and the overall energy release for consistent performance in a firearm.
The Ignition Sequence
The firing process begins with the mechanical action of the firing pin striking the cartridge primer. The primer is a small cap at the base of the cartridge containing a shock-sensitive chemical mixture. When crushed by the firing pin, this compound undergoes an instantaneous, small-scale explosion.
This initial explosion generates a jet of hot gas and burning particles. This heat and flame travels through a flash hole in the cartridge case, making contact with the main propellant charge. The primer provides the energy spike necessary to initiate the burning of the larger, less sensitive main propellant.
Deflagration and Gas Generation
Once ignited, the main propellant charge rapidly burns in a process known as deflagration, which is a subsonic combustion reaction. This is distinct from a detonation, which is a supersonic shockwave explosion that would destroy the firearm. The flame front travels quickly through the propellant, but slower than the speed of sound.
The chemical reaction converts the solid propellant grains into a vast amount of hot, high-pressure gas, including carbon dioxide, nitrogen, and water vapor. This gaseous conversion is important because the products occupy a volume many times greater than the original solid powder. The confinement of this rapidly expanding gas within the cartridge case and the gun chamber causes a massive buildup of pressure and temperature.
Propelling the Projectile
The intense pressure generated by the deflagrating propellant gas exerts force uniformly in all directions inside the sealed chamber. Since the projectile is the only movable component, the pressure forces it out of the cartridge case and into the barrel. This interaction is studied under the science of internal ballistics, which tracks the projectile’s movement from ignition until it exits the muzzle.
As the bullet travels down the barrel, the high-pressure gas continues to push on its base, accelerating it to its final muzzle velocity. The bullet is forced into the rifling—spiral grooves cut into the barrel’s inner surface—which imparts a stabilizing spin. This rotational movement ensures the projectile maintains its point-forward orientation, converting the chemical energy of the powder into the kinetic energy of the speeding bullet.