Cartridges, Deconstructed And Explained

Bullet. Round. Shell. There are many words for ammunition, but cartridge is the most correct term in many circumstances. This article will break down the parts of a cartridge and answer a lot of those questions you may have had but were too embarrassed to ask.

Education @ TFB:

• Concealed Carry Corner: Types of Non-Lethal Items
• TFB Armorer's Bench: Techniques and Types - Punches
• Different Kinds Of Handguns, Explained
• TFB Armorer's Bench: Techniques and Types – Boresighting
• The 5 Best AR-15 Tools for Your Gun Bench in 2025

Cartridge Overview

A cartridge is a single, self-contained piece of ammunition. Back in the day, firearm ammunition was very different. The first guns did not fire fixed ammunition. Instead, it was loaded in parts. Shooters would pour gunpowder down the barrel and pack it in place with a rod. A bullet, usually a lead ball, would sit on top of the powder along with a cloth patch to hold things in place. The earliest guns used a lit match placed at an opening in the rear of the barrel, sometimes with a fuse, to set off the powder charge and fire the shot (think of how cannons fire in Pirates of the Caribbean or Master and Commander).

Some big guns like military artillery still use shells separate from the powder charge, but most firearms today use cartridges. This simplifies loading, is more reliable, and reduces the risk of accidentally blowing up the gun by adding too much powder. Cartridges come in many shapes and sizes, but they all share common features. This article will cover the basics of what makes up a cartridge.

One last thing we need to cover before we continue is measurement units. In America, most ammunition weights are provided in grains. Those are an anachronistic unit equal to 1/7,000th of a pound. As silly as that is, it is still the unit in common use. Also, when a box of ammunition lists a grain weight, it is referring to the weight of the bullet, not the gunpowder charge.

Cartridge Anatomy

There are several components of a cartridge, and a whole language for the different types and styles of components. There are four main parts of a cartridge: the case, the primer, the powder inside, and the projectile or bullet. Here are the four main parts laid out:

Let’s now take each of those components in turn.

The Case

The most visible portion of a cartridge is the case. This is most frequently made of brass and is the part that holds all of the other parts together. This is the shiny metal piece that's ejected from guns dramatically in slow motion in an action movie.

Each part of the case has a unique name and function. We'll start at the rear and work toward the front. The rear portion of the case is called the “head,” and the rim is the very back end of the head. The rim is used by the gun to extract the case from the chamber of the firearm. Older guns usually have a “rimmed” case where the rim extends beyond the sides of the case. More modern cartridges use a “rimless” design, which still incorporates a rim, but it is the same size as the case body. Rebated rims are smaller than the case body, but are comparatively rarer.

The case head also includes a “primer pocket” to hold the primer, and a “flash hole” that allows the flame of the primer to enter the case body. Certain older magnum cartridges have an external belt that surrounds the case body in front of the rim. The main cylindrical body is the case wall, and it tapers from the base toward the front of the cartridge to help both feeding and extraction once it is fired.

Some cartridges have a “bottleneck” while others have straight walls from base to the mouth, or front opening of the case. Bottleneck cartridges have a “shoulder” that angles inward, stepping down from the body diameter to the bullet diameter. The portion in front of the shoulder is the case neck. Bottleneck cartridges are more common in rifles, but there are bottleneck pistol cartridges, too.

While brass is far and away the most common case material, there are other options as well. Steel cases are the next most prevalent. They are generally cheaper, but can’t be easily reused and reloaded like brass cases can. Aluminum cases are similar to steel in most respects. Some newer ammunition types use a combination of steel and brass, like the US military’s new 6.8x51 cartridge.

The Primer

This is where the party starts. The primer is a small cylindrical device that holds a very volatile chemical called priming compound. Pulling the trigger of a gun releases a chain of events that results in a firing pin or striker hitting the primer forcefully. That sets off the primer, causing a jet of flame and sparks that hits the powder charge and ignites it. If you've ever played with a cap gun as a child, it functions somewhat the same, but primers fire with much more force. Primers are a one-time use item and cannot be reconditioned or brought back into service once used.

Primers come in all sorts of varieties. The two main sizes for pistols and rifles are called (very creatively) small or large. Those break down into more options like small rifle match, large pistol magnum, or small pistol. Shotguns use a #209 primer, which is much larger.

There is another entirely different type of primer that deserves a mention here, the venerable rimfire. Rather than having a separate primer inserted in the case, a rimfire cartridge has a priming compound inside a raised rim that runs around the base of the case. The firing mechanism hits the edge of the rim wherever it happens to line up when the round is in the chamber. Just like a centerfire cartridge, this ignites the powder.

The Powder

When a primer works properly, it ignites the powder charge. That creates a large volume of expanding gas and flame. If everything is working properly, the path of least resistance for those gases is to push the bullet out of the cartridge case and down the barrel. Almost all ammunition today uses smokeless gunpowder. It is not truly smoke free, but compared to the older black powder it replaced, it is dramatically reduced. Smokeless powder is highly flammable but not a high explosive.

Powder comes in many varieties, but manufacturers usually do not list types or amounts of powder used in loaded ammunition. The two main types are ball/flake powder and extruded/stick powder. As a generalization, ball powders are easier to measure consistently to get the same exact charge weight in each cartridge, but they tend to have more velocity variation with temperature change.

The Projectile

All of the other parts of a cartridge exist so that the projectile can be fired. It is the real star of the show, and it is the only part that actually interacts with the target. Modern bullets are precision manufactured, typically with a copper jacket and lead inner core. Older bullets were bare lead, which worked fine in lower velocity blackpowder cartridges but is less than ideal with faster smokeless propellants. Lead projectiles can leave substantial lead residue inside the barrel if they are pushed too fast, so their use is limited to lower velocities.

The copper jackets that enclose the lead on most bullets are made in two general ways. If the lead is pushed into the back of the jacket, it creates a full metal jacket (FMJ) projectile, which usually has an exposed lead base. If the lead is forced in from the front, it leaves an opening at the front of the bullet, which can be crafted into a jacketed hollow point (JHP), boat tail hollow point (BTHP,) or soft point (SP) design. The opening at the front of these projectiles causes them to expand or fragment on impact, depending on design. There are other specialized projectile types, like armor-piercing bullets with steel cores or solid copper machined projectiles used for extreme long range (ELR) shooting competitions.

A cartridge without a projectile is known as a blank. These are often used in movies, TV, and reenactments because they look and sound like a real gunshot but are less dangerous - no projectiles leave the weapon. Note that they are still dangerous in their own way, and all gun safety rules still apply! Other cartridges that deserve special handling care are incendiaries, which set targets on fire, and tracers, which burn in flight to show the flight of a projectile. Those are uncommon outside of military use though.

Shotgun Shells

Shotgun cartridges differ in many ways from rifle and pistol ammunition. The “hull” of a shotgun shell is equivalent to a cartridge case, but typically uses a plastic body and brass or steel case head and rim. The hull contains a primer and powder charge, but adds another component called a wad. The wad is a plastic cup that holds the load of pellets as it travels down the barrel. Once it exits the barrel, the sides of the wad open and it slows, allowing the pellets to fly down range and spread. The hull has a crimp over the front of the shell, holding the wad and shot inside the cartridge. Shotguns also differ from pistol and rifle ammunition by measuring the weight of the projectiles in ounces rather than grains.

Most shotgun loads contain many small pellets. The sizes of pellets are numbered, with higher numbers indicating smaller pellets. #8 or #7.5 pellets are common in cheap ammo for shooting clays or piles of trash. Larger pellets like #4 or #5 are more common in small game loads. The largest pellets are called buckshot, with 00 or “double aught” being the most common. There are also shotgun cartridges loaded with a single projectile, called a slug.

Safety Concerns

There are a few safety issues inherent to ammunition that we should discuss. Gunpowder is obviously flammable, so keep cartridges away from fire or heat sources. Primers can detonate if struck, so rough handling should be avoided. These are both very clear dangers, but others are longer-term. Most projectiles contain lead, as do primers, and lead exposure can be very dangerous over time. Always be sure to wash your hands with cold water and use a special lead-removing soap whenever possible.

Cases can sometimes fail, damaging the gun and possibly the shooter. The case itself should help the gun’s action contain the pressure of the shot. But if the case is compromised or does not fit in the chamber fully, it can rupture and release that pressure in an uncontrolled manner. This is one reason why firearm manufacturers often advise against using remanufactured ammunition, and a reason why you should always wear safety glasses. Be on the lookout for cracks in the shoulder or case neck, as these are clear signs of case failure.

Two types of ammunition malfunctions also deserve a mention. First is a squib round, which is when a gun fires but the bullet does not leave the barrel. This is very dangerous because firing another round when a bullet is stuck in the barrel can lead to a catastrophic failure of the gun. You could be injured or killed.

The other serious ammo failure is a hangfire. Every once in a while there is some kind of obstruction between the primer and the gunpowder. The primer functions properly but the flame cannot reach the powder immediately. But when it does reach the powder the gun will still fire. The danger here is obvious; if a shooter is attempting to diagnose the issue when the gun goes off it could cause serious injury. This is why it is critical to always observe the first rule of gun safety and keep the muzzle pointed in a safe direction.

Conclusion

In summary, there are four parts of a cartridge: the case, the projectile, the powder, and the primer. All four are critical, and a more thorough understanding of each can help you operate a gun more safely. This article was only an overview, and there is much more to learn about the vagaries of ammunition, but hopefully you now have a baseline understanding of how cartridges work.