When you pull a shotgun’s trigger, a chain of mechanical, chemical, and physical events unfolds in roughly 3 to 5 milliseconds. It starts with a small metal lever releasing a spring-loaded hammer and ends with a column of shot exiting the muzzle at over 1,000 feet per second. Each step in that sequence depends on the one before it, and understanding how they connect gives you a much clearer picture of how a shotgun actually works.
The Trigger Pull and Firing Pin Release
The sequence begins with the trigger. When you pull it rearward, you’re moving a small internal component called the sear out of a notch in the hammer. The sear is essentially a sharp metal bar that holds the hammer back under the tension of a powerful spring (called the mainspring). The moment the sear clears that notch, the mainspring snaps the hammer forward with considerable force.
The hammer strikes the rear of the firing pin, driving it forward through the bolt face and into the back of the shotshell. This entire mechanical chain, from your finger’s pressure to the firing pin hitting the shell, happens in a fraction of a second.
Primer Ignition and Powder Combustion
The firing pin strikes a small metal cup centered in the base of the shotshell called the primer. Inside that cup is a tiny amount of impact-sensitive explosive compound. The sharp blow crushes this compound, producing a burst of hot sparks and flame that jets through a small flash hole into the main powder charge inside the shell.
That flame ignites the smokeless powder, which burns extremely fast but does not technically detonate. As the powder burns, it converts from a solid into a rapidly expanding volume of hot gas. In a standard 12-gauge shell, this gas buildup creates peak pressures up to about 11,500 PSI inside the chamber. That’s roughly the same pressure you’d find 25,000 feet below the ocean’s surface, all contained in a space smaller than your thumb.
The Wad and Shot Column Move Forward
The expanding gas has nowhere to go except forward. It pushes against the wad, a plastic component sitting between the powder and the shot pellets. The wad serves two important roles: it seals the gas behind it so no pressure leaks past the shot, and it cushions the pellets from the violent acceleration so they don’t deform on launch. Without the wad, gas would blow through the gaps between pellets, dramatically reducing velocity and creating erratic patterns.
As pressure builds, the plastic hull of the shell expands outward and presses tightly against the chamber walls, creating a gas-tight seal. The only direction for energy to move is forward, so the wad and its payload of shot accelerate down the bore from a dead stop to full velocity in the span of roughly 18 to 30 inches of barrel. The crimp at the mouth of the shell opens under pressure, and the entire shot column exits the shell casing and enters the barrel.
Travel Down the Barrel
As the wad and shot column race through the barrel, the gas continues expanding behind them, maintaining pressure and acceleration. The smooth bore of a shotgun (unlike the rifled bore of a rifle) allows the wad and shot to travel without spinning. The wad keeps the pellets organized in a relatively tight column during this brief journey.
Near the muzzle end of most shotgun barrels sits the choke, a slight narrowing of the bore’s diameter. This constriction squeezes the shot column just before it exits, which influences how tightly or widely the pellets spread after leaving the barrel. A tighter choke holds pellets closer together for longer range. A more open choke lets them spread quickly for close targets. The choke is the last thing the shot interacts with before it’s airborne.
Muzzle Exit and Recoil
The instant the wad and shot clear the muzzle, the high-pressure gas behind them vents into the atmosphere. This creates the muzzle blast you see and hear. The wad typically separates from the shot column within a few feet of the muzzle, tumbling away while the pellets continue downrange in an expanding pattern.
Recoil starts the moment the shot column begins accelerating forward, but you feel it as the gun pushes rearward into your shoulder. Newton’s third law is at work: for every bit of momentum the shot and gas carry forward, the gun absorbs an equal amount of momentum backward. The heavier the payload and the faster it moves, the harder the kick. A 12-gauge firing a standard load produces noticeably more recoil than a 20-gauge for exactly this reason. The sudden release of gas pressure at the muzzle adds a secondary push that compounds the effect.
How Semi-Automatics Cycle the Next Round
In a pump-action shotgun, the sequence ends here. You manually rack the forend to eject the spent shell and load a fresh one. But in a semi-automatic, the gun uses energy from that same fired shell to do this work automatically. Two main systems handle this differently.
Gas-Operated Systems
A small port drilled into the barrel, partway between the chamber and the muzzle, bleeds off a portion of the expanding propellant gas as the shot passes by. This diverted gas enters a cylinder and pushes a piston rearward. That piston drives the bolt back, which extracts and ejects the spent shell. A return spring then pushes the bolt forward again, stripping a fresh shell from the magazine and locking it into the chamber. The entire cycle happens before the gun has finished recoiling into your shoulder.
Inertia-Driven Systems
These work on a different principle. When the gun recoils backward from firing, a heavy bolt body inside the receiver resists that movement due to its own inertia. The result is that the gun moves backward around the bolt, compressing a spring between the bolt body and bolt carrier. Once the recoil force fades, that compressed spring releases its stored energy, driving the bolt carrier forward. This unlocks the bolt, which then moves rearward to extract and eject the spent shell. The return spring then pushes everything forward again, feeding a new shell from the magazine into the chamber and locking the bolt closed.
In both systems, a component called the disconnector prevents the gun from firing again the instant the bolt closes. It forces the trigger to be fully released and pulled again before the sear can disengage from the hammer, ensuring each trigger pull fires only one round.
The Full Sequence at a Glance
- Trigger pull: Sear releases the hammer, which drives the firing pin forward.
- Primer ignition: Firing pin crushes the primer compound, producing sparks and flame.
- Powder combustion: Flame ignites the powder charge, generating rapidly expanding gas up to 11,500 PSI.
- Wad acceleration: Gas pushes the wad and shot column forward, with the hull sealing the chamber.
- Barrel travel: Shot column rides the wad through the smooth bore, picking up speed the entire way.
- Choke constriction: The narrowed muzzle end shapes the shot column just before exit.
- Muzzle exit: Shot, wad, and gas leave the barrel. The wad separates within feet. Pellets continue in an expanding pattern.
- Recoil: The gun pushes rearward into the shooter’s shoulder from the equal and opposite reaction force.
- Cycling (semi-auto only): Diverted gas or stored recoil energy drives the bolt back, ejects the spent shell, and chambers a fresh round.
From trigger pull to shot leaving the barrel, the entire process takes only a few thousandths of a second. The cycling of a semi-automatic action adds perhaps another tenth of a second. What feels like a single, instantaneous event is actually a precise cascade of mechanical and chemical steps, each one setting up the next.