Modern ammunition is made from four main components: a brass casing, a lead or copper projectile (the bullet), a smokeless powder propellant, and a small primer that ignites everything. Each part is built from specific materials chosen for how they behave under extreme heat and pressure. Here’s what goes into each one.
The Four Parts of a Cartridge
Every round of ammunition, whether it’s a tiny .22 or a large rifle cartridge, follows the same basic design. The case is the outer shell that holds everything together. It attaches to the bullet at the front and the primer at the back, with propellant powder packed inside. When the firing pin strikes the primer, a small flame shoots through a tiny hole called a flash hole, igniting the powder. That powder combusts rapidly, producing high-pressure gas that pushes the bullet out of the casing and down the barrel.
What the Casing Is Made Of
Most cartridge cases are made from brass, specifically an alloy of copper and zinc. Brass is the industry standard because it’s soft enough to form into precise shapes during manufacturing but strong enough to handle the pressure of firing. It also has a critical physical property: when the round fires, the brass case expands to seal the chamber and prevent hot gas from blowing backward. Once pressure drops, brass contracts back to roughly its original shape, making it easy to extract from the chamber. This expansion-and-contraction cycle is so important that the dimensions of most cartridges were designed around brass specifically.
Steel and aluminum are cheaper alternatives. Steel cases show up in budget training and target ammunition, but steel doesn’t expand and contract the way brass does. Brass expands about 1.5 times as much as steel, so steel-cased rounds can be harder to extract and may cause more wear on a firearm over time. Aluminum cases are the lightest option and cost less to produce than either brass or steel, but they’re not reloadable and are generally found in lower-cost handgun ammo.
What Bullets Are Made Of
The bullet, the part that actually leaves the barrel, is most commonly a lead core wrapped in a copper jacket. Lead is dense and heavy for its size, which gives the bullet the mass it needs to carry energy downrange. Copper jacketing surrounds all or part of the lead core, protecting the barrel from lead fouling and controlling how the bullet behaves on impact. A full metal jacket covers the entire lead core and tends to stay intact. A hollow point leaves the tip of the lead exposed, allowing the bullet to expand when it hits a target.
Solid copper bullets have become increasingly popular, particularly for hunting. These are machined from a single piece of copper with no lead at all. Copper is less dense than lead (8.96 g/cm³ versus 11.3 g/cm³), so solid copper bullets are often slightly longer to make up for the weight difference. Solid brass bullets exist as well, though they’re less common.
Military ammunition sometimes uses steel components inside the bullet. The U.S. military’s M855 round, for example, contains a steel penetrator tip in front of a lead core, all wrapped in a copper jacket. That steel insert makes it significantly better at defeating body armor and fiber composite materials compared to conventional lead-core designs. At the more extreme end, armor-piercing rounds use cores made from tungsten carbide, a material roughly twice as dense and more than twice as strong in compression as hardened steel. These cores are bound together with small amounts of cobalt, nickel, or iron.
Propellant: Smokeless Powder
The propellant inside a cartridge is smokeless powder, which replaced black powder in the late 1800s. Despite the name “powder,” it typically comes in small flakes, cylinders, or ball shapes. The base ingredient is nitrocellulose, essentially cotton or wood cellulose treated with nitric acid. This is called single-base powder. Double-base powders add nitroglycerin to the mix, which increases the energy output and burns at higher temperatures.
Smokeless powder doesn’t explode in the traditional sense. It burns extremely fast in a controlled, progressive way. The confined space inside the cartridge case causes pressure to build rapidly until it overcomes the friction holding the bullet in place, launching it forward. Different powder formulations burn at different rates, and manufacturers match specific powders to specific cartridges to control pressure and velocity precisely.
For historical context, the original black powder used in firearms for centuries was a simple mixture: 75% potassium nitrate (saltpeter), 15% charcoal, and 10% sulfur by weight. Smokeless powder produces far more energy per grain, generates less residue, and creates dramatically less smoke.
What’s Inside the Primer
The primer is a small metal cup seated at the base of the cartridge, filled with a tiny amount of impact-sensitive chemical mixture. When the firing pin strikes it, that mixture detonates and sends a jet of flame into the main powder charge. Modern non-corrosive primers contain a mixture of lead styphnate (the primary explosive sensitive to impact), antimony sulfide (a fuel), and barium nitrate (an oxidizer that supplies oxygen to sustain the reaction). The amounts are minuscule, just enough to produce a reliable flash.
Shotgun Shells Are Built Differently
Shotgun ammunition follows a different structure than rifle or pistol cartridges. The hull has two parts: a brass base that holds the primer and powder, and a plastic upper section (usually polyethylene) that contains the payload. Between the powder and the pellets sits a plastic cup called a wad, which seals gas behind the shot and cushions the pellets during firing. Older shotgun shells used paper hulls and cardboard wads, but polyethylene took over because it holds up under the extreme temperatures and pressures of firing.
The pellets themselves are traditionally lead, ranging from tiny birdshot to large buckshot. Steel shot is required for waterfowl hunting in many areas due to lead toxicity concerns in wetlands. For the same reason, lead-free alternatives have expanded to include bismuth, tin, and tungsten-based shot. Tungsten is particularly effective because it’s extremely dense, and manufacturers mix it with polymer binders (roughly 95% tungsten, 5% nylon or other polymer) to form pellets that perform close to lead without the environmental harm. Shotguns also fire single-piece slugs, which are usually solid lead projectiles of various shapes designed for hunting larger game.
Lead-Free and Non-Toxic Alternatives
Growing concerns about lead contamination in soil, water, and wildlife have driven a push toward non-toxic ammunition materials. The metals used as lead replacements are chosen based on density, ballistic performance, ease of manufacturing, cost, and proven safety for wildlife. The primary alternatives are iron (steel), tungsten, bismuth, tin, and copper.
Each comes with trade-offs. Steel is cheap and widely available but much less dense than lead, which reduces energy at longer distances. Bismuth is closer to lead in density and softer than steel, making it gentler on older shotgun barrels, but it costs more. Tungsten is the premium option, nearly matching lead’s density, but it’s expensive to produce. Copper works well for rifle and shotgun slugs and has become the standard material for lead-free hunting bullets in states and regions where lead ammunition is restricted or banned.