A shotgun shell works by using a small explosive charge to launch projectiles down a smooth barrel at high speed. Unlike rifles and handguns, which fire a single bullet, most shotgun ammunition releases multiple pellets that spread out in a widening pattern after leaving the muzzle. This spreading pattern is what makes shotguns effective for hitting moving targets like birds, but it also means the physics involved are quite different from other firearms.
What’s Inside a Shotgun Shell
A shotgun shell (technically called a “shotshell”) has five main components stacked inside a plastic or paper hull. Starting from the bottom: the primer sits in the center of the brass base. Above that is the powder charge (propellant). Next comes the wad, a plastic cup-shaped piece that serves multiple purposes. And finally, the payload, which is either a cluster of small pellets or a single large projectile called a slug.
The wad is the component most people don’t know about, and it’s doing critical work. It creates a gas-tight seal between the burning powder and the pellets, preventing hot gases from blowing through the shot and deforming it. It also acts as a protective cup that keeps the pellets from scraping against the barrel walls. When the shot exits the muzzle, the wad peels away and falls to the ground within a few yards, releasing the pellets to fly on their own.
The Firing Sequence
Pulling the trigger releases a spring-loaded firing pin, which strikes the primer at the base of the shell. The primer is a small amount of impact-sensitive explosive that ignites on contact, sending a stream of flame (roughly 2,000°C) into the powder charge above it. The powder doesn’t explode in the conventional sense. It burns extremely fast, producing a rapidly expanding volume of gas.
That gas has nowhere to go except forward, so it pushes against the wad, which in turn pushes the pellets up the barrel. The entire process, from primer strike to the shot leaving the muzzle, takes just a few milliseconds. A typical 12-gauge buckshot load launches its pellets at around 1,200 feet per second, while a slug can leave the barrel at 1,850 feet per second.
Three Types of Shotgun Projectiles
Shotgun ammunition comes in three broad categories, each designed for very different situations.
Birdshot contains a large number of very small pellets. A one-ounce load of #8 birdshot, for example, holds roughly 399 tiny pellets, each just 0.09 inches in diameter. The idea is to create a wide cloud of projectiles that increases your chance of hitting a fast-moving bird or clay target. Each individual pellet carries relatively little energy, which limits damage at distance but makes birdshot effective for small game and sporting clays.
Buckshot uses far fewer but much larger pellets. A standard 00 (“double-ought”) buckshot shell holds about 8 to 9 pellets, each 0.34 inches in diameter. That’s roughly the size of a 9mm pistol bullet per pellet. A nine-pellet load of 00 buck at 1,200 fps delivers about 1,694 foot-pounds of energy at the muzzle, which is why it’s widely used for home defense and deer hunting.
Slugs are a single large projectile, making the shotgun perform more like a rifle. A one-ounce 12-gauge slug at 1,850 fps produces around 3,320 foot-pounds of muzzle energy, which is comparable to many rifle cartridges. Slugs are the go-to choice when you need accuracy and power at longer range from a shotgun.
How Slugs Stay Stable Without Rifling
Most shotgun barrels are smooth inside, with no rifling to spin the projectile. This is fine for pellets, which don’t need to be aerodynamically stable on their own, but it creates a challenge for slugs. A large, unstable chunk of lead tumbling through the air won’t fly straight.
Rifled slugs solve this with angled grooves molded into the slug’s outer surface. Despite the name, these grooves don’t actually spin the slug much. They mainly allow the slug to squeeze through various choke constrictions without damaging the barrel. Stability comes instead from the slug’s weight distribution: it’s heavy in the front and hollow in the back, so it flies nose-first like a badminton shuttlecock. Rifled slugs work in any standard smoothbore shotgun.
Sabot slugs (pronounced “say-bo”) take a different approach. They use a smaller-diameter projectile wrapped in a plastic sleeve (the sabot) that engages the rifling in a specially rifled shotgun barrel. The barrel spins the sabot, which spins the slug, and then the sabot falls away after the slug exits the muzzle. This produces significantly better accuracy at longer distances, but you need a rifled barrel or rifled choke tube to make it work.
What Controls the Spread
When pellets leave the barrel, they immediately begin spreading apart due to air resistance and the turbulence of exiting the muzzle. How quickly they spread depends largely on the choke, a tapered constriction at the very end of the barrel. Most modern shotguns accept interchangeable choke tubes that screw into the muzzle.
A cylinder bore (no constriction) lets the shot spread quickly, which is useful at close range. A modified choke tightens the pattern and works well out to about 35 yards. A full choke constricts the bore the most, keeping the pellet pattern concentrated at 40 yards and beyond. The pattern from a modified choke at 40 yards is noticeably more scattered than the same load through a full choke at the same distance. Choosing the right choke for the situation is one of the most important decisions a shotgun shooter makes.
How Energy Changes With Distance
This is where shotguns differ most dramatically from rifles. A rifle bullet is a single aerodynamic projectile that retains energy well over hundreds of yards. Shotgun pellets are small, round, and not aerodynamic at all. They lose velocity quickly to air drag, and the farther they travel, the more they spread apart, distributing their energy over a wider and wider area.
At very close range, the pellets haven’t had time to spread. The entire payload hits in a tight cluster, delivering virtually all of its kinetic energy into a small area. This creates devastating impact, with broad, deep tissue damage that exceeds what a single rifle bullet typically produces at the same distance. As range increases, the pellets separate, each one carrying less energy individually, and some miss the target entirely. This rapid energy falloff is why shotguns are considered short-to-medium range weapons, generally most effective inside 40 to 50 yards for buckshot and considerably less for birdshot.
Slugs behave more like conventional bullets in this regard, maintaining their energy better over distance since they’re a single heavy projectile. But even slugs lose velocity faster than most rifle bullets because their shape isn’t optimized for aerodynamic flight, particularly the blunt-nosed rifled slugs designed for smoothbore barrels.