A gun fires through a rapid chain reaction: pulling the trigger releases a spring-loaded striker or hammer, which slams into a tiny impact-sensitive explosive in the back of the cartridge, igniting propellant powder that generates thousands of pounds of pressure per square inch, and that pressure pushes the bullet down the barrel and out the muzzle. The entire sequence, from trigger pull to bullet exit, takes only milliseconds.
The Cartridge: Four Parts Working Together
A modern round of ammunition has four components, each with a specific job. The brass (or steel) casing holds everything together and seals the chamber so expanding gases can only go one direction: forward. Seated in a small pocket at the base of the casing is the primer, a tiny cup filled with a shock-sensitive chemical compound. Above the primer sits the propellant, a carefully measured charge of powder. And pressed into the open end of the casing is the projectile itself, the bullet.
The primer is the critical first domino. Its chemical compound is so sensitive to impact that even bumping a container of it against a hard surface can set it off. When struck by the firing pin, the primer produces a jet of hot sparks that shoots into the casing and ignites the propellant. The propellant doesn’t explode in the way most people imagine. It deflagrates, meaning it burns extremely fast rather than detonating all at once. That rapid combustion fills the casing with high-pressure gas. In a .30-06 Springfield rifle cartridge, for example, that pressure reaches roughly two and a half tons per square inch.
Once the pressure overcomes the friction holding the bullet in place at the neck of the casing, the bullet detaches and accelerates down the barrel, pushed by the wall of expanding gas behind it.
What Happens When You Pull the Trigger
The trigger isn’t directly connected to the firing pin. Instead, pulling the trigger trips a small latch called the sear, which is the only thing holding back a spring-loaded hammer or striker. Once the sear releases, the spring drives the hammer or striker forward with enough force to crush the primer compound and start ignition.
There are two common designs for this mechanism. In a hammer-fired gun, an external or internal hammer pivots forward and strikes a separate firing pin, pushing it into the primer. The hammer can be cocked manually (by thumbing it back) or automatically by the slide’s rearward movement after each shot. In a striker-fired gun, there’s no hammer at all. A simple spring-loaded metal rod sits inline behind the cartridge. Pulling the slide to the rear cocks the striker automatically, and the only way to release it is to pull the trigger. Most modern polymer-framed handguns use this striker system.
Quality firearms include an interrupter mechanism that disconnects the trigger from the firing pin until the action is fully locked. This relationship, called timing, prevents the gun from firing before the cartridge is properly seated in the chamber. Some designs add a firing pin block, a physical barrier that sits in front of the firing pin and only moves out of the way when the trigger is deliberately pulled.
How the Barrel Shapes the Shot
The barrel does more than point the bullet in the right direction. Inside most barrels are helical grooves cut into the metal, called rifling. As the bullet is forced down the bore under enormous pressure, it’s engraved by these grooves, taking on their mirror image. This engraving forces the bullet to spin as it travels, much like a football thrown in a tight spiral.
That spin is what keeps a bullet stable in flight. Without it, a bullet would quickly begin to wobble, tilt sideways, and tumble end over end. Shooters can sometimes see evidence of this problem at the range: elongated or sideways holes in the target (called keyholing) indicate the bullet was tumbling rather than flying point-first. Once a bullet starts to yaw, accuracy disappears entirely because it veers in unpredictable directions. The rate of spin depends on both the spacing of the rifling grooves and how fast the bullet is moving when it leaves the muzzle.
How Fast the Bullet Travels
Muzzle velocity varies enormously depending on the type of firearm and cartridge. Handgun rounds are relatively slow. A 9mm round leaves the barrel at roughly 1,155 feet per second, and a .45 ACP travels even slower at about 850 feet per second. A .22 Long Rifle, the most common small-caliber round, exits at around 1,750 feet per second.
Rifle cartridges generate far higher velocities because their casings hold more propellant and their longer barrels give the gas more time to accelerate the bullet. A .223 Remington can reach 4,000 feet per second, while a .30-06 Springfield hits about 3,080 feet per second. These differences matter because velocity is the primary driver of a bullet’s energy and range.
The Full Cycle in a Repeating Firearm
In semi-automatic and automatic firearms, the firing doesn’t just end when the bullet leaves the barrel. The same gas pressure that pushes the bullet forward also drives the action through a complete mechanical cycle, preparing the gun to fire again. That cycle has a specific sequence: feeding a new cartridge from the magazine, chambering it into the barrel, locking the breech closed, firing, unlocking the breech, extracting the spent casing, ejecting it out of the gun, and re-cocking the hammer or striker.
In blowback-operated designs (common in smaller handguns and some submachine guns), the bolt isn’t mechanically locked at all. It’s held in place only by the tension of a heavy spring and the bolt’s own inertia. The gas pressure pushes the bolt rearward against that spring, and the spring pushes it back forward, stripping a fresh round from the magazine on the way. In locked-breech designs, which handle higher-pressure cartridges, a mechanical locking mechanism holds the bolt shut until the bullet has left the barrel and pressure drops to a safe level.
In a revolver or bolt-action rifle, several of these steps are manual. You rotate the cylinder or work the bolt by hand to chamber the next round. But the core firing sequence, the sear release, the primer strike, the powder ignition, and the bullet’s trip down a rifled barrel, is identical across virtually every modern firearm.