The Modern Cartridge Case | The Ballistic Assistant (2024)

It is commonly referred to as brass, cases or shells, and it is the result of over a thousand years of firearm innovation, without it, self loading rifles like the M1 Garand, AR-15, and the AK-47 would not be possible. There have been attempts to replace it, with “caseless ammunition” but they have largely been unsuccessful. More recently there have been innovative gains in improving the cartridge case.

Examples of case innovation can be found in the two piece cartridge case by Shell Shock Technology, which uses a nickel alloy instead of brass offering a stronger yet lightweight case. Sig Sauer’s recently announced 277 Fury, which uses a stainless steel head and a brass body, to achieve a maximum average pressure (MAP) of 80,000psi. Pressures that are unsafe in regular cartridge brass. There have also been numerous attempts to replace the case body with materials such as plastic, this substantially reduces the weight of the ammunition.

A Brief History

Prior to the modern cartridge case rifles, pistols and cannons were all some variation of muzzle or breech loading. This relied on the user to measure out the powder, bullet, and to rely on an external primer. Reloading was painstakingly slow by today’s standards. Early attempts of offering paper cartridges where the powder, bullet and wad were contained in a paper cartridge were used as early as the 1500’s and while an improvement, still limited the firearms to only a few shots per minute.

In 1845 Louis-Nicolas Flobert invented the first rimfire cartridge. While the first center fire cartridge was invented in 1829 by Clement Pottet , it wasn’t commercially viable until 1855. From there it went through a series of improvements, until in 1867 Britian adopted the .577 Enfield Snider cartridge for use in converted Pattern 1853 Enfields.

During the development of the modern cartridge case, two different priming systems became popular. The Berdan Primer, invented by an American Hiram Berdan in 1866, and the Boxer Primer invented by ColonelEdward Mounier Boxer also in 1866. The boxer primer gained popularity in the US, while the Berdan primer gain popularity in Europe.

While both methods of priming achieve the same results, the Berdan primer is cheaper, easier to make, and is every bit as functional and reliable as the Boxer primer. The Boxer primer, is more expensive to manufacture, but is far easier to reload and reuse the brass case. While Berdan primed cases can be reloaded, it is far more work.

The Functions of a Modern Cartridge Case

At the end of the day the cartridge case provides 4 main functions.

• Contains the bullet, primer, and powder in a package that is protected from the elements, and feeds/ejects smoothly in the firearm.
• Aligns the bullet in the chamber
• Expands to seal the chamber during firing.
• Acts as a heat sink, absorbing the heat from the combustion, removing it from the chamber when the cartridge is ejected.

It is important to note that SAAMI and CIP specify the external cartridge dimensions for a loaded cartridge which includes the external case dimensions. They do no specify the internal case dimensions, which is why internal case volumes can vary so greatly from manufacturer to manufacturer.

While the case is made as one piece, there are several individual features that each serve a critical function. Defects in any one area can result in a case that is unsafe to fire.

The Case Neck

The case neck is responsible for holding the bullet. The thickness of the brass is typically 0.010 to 0.015, and ideally the neck diameter is .001 to .0015 below that of the bullet diameter. This creates an interference fit with the bullet that is responsible for creating neck tension.

Neck Tension plays a critical role in holding the bullet in place while it sits in the magazine, is being chambered, or even when extracting a loaded cartridge from the case. The amount of neck tension required varies drastically as it is depended on bullet weight, caliber, recoil and firearms.

For example, a 44 magnum requires high neck tension due not only to the high recoil of a 44 Mag revolver, but also due to spring tension and recoil when loaded in a lever action rifle. A substantial crimp is usually applied to increase the neck tension, but it typically only prevents the bullet from being pushed back into the case, a crimp alone may not be sufficient to prevent bullet creep from occurring under recoil.

Fortunately 44 Magnum brass typically has high neck tension, on the order of several hundred pounds or more. More than what a crimp alone can achieve.

The case neck is also the softest part of the brass case. This is softness helps the brass to expand and contract without cracking. Since brass works hardens, repeated firing and sizing operations will harden the neck until it cracks. A handloader can anneal the case neck to extend the life of the brass.

The Shoulder

The shoulder in most cases is responsible for headspacing the cartridge in the chamber. This headspace determines how deep the cartridge sits in the chamber in references to the bolt face. The brass on the shoulder also tends to be thin and soft, which can cause issues.

As the case is seated in the chamber, the forward movement of the cartridge is halted by the contact of the shoulder with the shoulder cut in the chamber. Sometimes the cartridge is chambered with such force that it can actually crush the shoulder resulting in a case that is sitting deeper in the chamber then what is ideal. The shoulder also has to support the impact of the firing pin on the primer, if the shoulder is crushed on chambering, the energy of the firing pin can be absorbed by the shoulder flexing. This results in misfires.

300 Blackout is one example of where a thin, small and soft shoulder can result in misfires in a firearm. There are other cartridges out there with similar small shoulders and they all are prone to the same issues with crush up.

The Body

The body of the case is sometimes called the Powder Chamber, performs two primary functions. The first is to contain the powder charge, and the second is to swell under pressure to form a gas tight seal between the chamber and the case. This seal prevents hot gasses under pressure from being ejected into the action, and potentially injuring the shooter.

This seal also holds the cartridge in place. The gripping power is such that it substantially reduces the amount of force that is transmitted to the bolt face. I first came across the accounting of this “gripping” force in Hatcher’s Notebook, written by Julian S. Hatcher.

He wrote of experiments done with a 30-30 lever action rifle where the locking lugs were ground away. The only force that held the cartridge in the chamber was the brass expanding and gripping the inside of the chamber. In the case of the 30-30 this force was sufficient. However when he applied any oil to the outside of the case, the bolt flew open under firing.

This account helped when troubleshooting blown primers in a 300 RUM during a cold test. We would freeze the rounds for 24 hours at -20 Degrees F and then check them for function. The rounds would sit on the bench and accumulate a thin layer of frost. When firing, the primer pockets would blow out despite the pressures being within normal range. Remembering General Hatcher’s experiment, I decided to wipe off the frost and chamber the round. The issue disappeared.

Let this be a lesson that cartridges should be kept free from oil or water, and you should probably refrain from oiling the chamber. Also if you don’t have a copy of Hatcher’s Notebook and are a serious student of Ballistics, I suggest you pick up a copy.

The Case Web

The case web forms the junction between the case body and the case head. This crucial junction can weaken after repeated firings as the brass is stretched under firing and then compressed when sized in a sizing die. Known as a case head separation, this can render the firearm inoperable until the body of the case is removed from the chamber. Some actions are “springier” than others allowing the case to stretched more, and when reloading for these actions it is important to be aware of that problem and check the cases for premature stretching (Lee Enfield rifles have this problem).

The Case Head

The case head is the thickest and hardest part of the case, consequently it is also the strongest part of the case. The case head contains, the flash hole, the primer pocket, the head stamp, and the extractor groove. Some cases also headspace off the head of the case via a rim or a belt.

When the the powder is ignited the pressure builds rapidly and the case expands to fill the chamber. It pushes the primer back to make contact with the bolt unseating it momentarily until the case head expands rearward making contact with the bolt face and reseating the primer. With cartridges that have a low pressure, it is not uncommon to see the primer sit slightly proud of the case head after firing, due to the case head not stretching far enough to reseat the primer. The primer should not be loose in the primer pocket, this is a different issue, and can be indicative of high pressure, or a tired case.

The case head is compressed between the high pressure gases inside the chamber and the locked bolt face. These compressive forces are resisted by the strength of the brass. When the pressure is too great the brass will begin to flow, similar to how compressed lead will deform. The case head expands outwards, the primer pocket is expanded and usually results in a dropped primer. If there are extractor plungers the brass will flow into those, the primer will be forced first against the bolt face, flattening it, and then down into the firing pin hole creating a visible “crater” . All of these signs are known as “Pressure Signs” and are signs that the brass is beginning to fail.

In order for the brass to resist deformation under the high pressures of firing it has to be hard. Brass naturally work hardens as it is formed. The drawing, stamping and machining of the brass case all add to its hardness and it’s strength, but if it is subjected to a substantial amount of heat it can be annealed. Anneal case heads are very dangerous as they will fail, and they can destroy a firearm and or injure the shooter. While you can anneal the case neck, special precaution must be taken to ensure the case head is kept cool.

The Flash Hole

The flash hole is typically .060 in diameter and is responsible for ensuring that the sparks from the primer ignite the powder charge. The flash hole also allows gases to flow back into the primer during combustion. The size and uniformity of the flash hole does have some impact on pressure and velocity. With that said, most of the flash holes I’ve seen have been pretty uniform in diameter all falling between 0.060 to 0.080in.

The Primer Pocket

The primer pocket provides two important functions, the first being it retains the primer through handling and firing of the cartridge, sealing the gas within the cartridge. The second function is to seat the anvil in a Boxer Primer in order for the firing pin to crush the primer pellet between the anvil and the primer cup.

If the primer pocket fails to retain the primer during firing two things can happen, the first is the primer drops down in the firearm action causing malfunctions and issues in the firearm, the second is gas cutting of the bolt.

Failure to seat the primer so the anvil is able to make contact with the primer pellet may result in misfires.

The Extractor Groove

The extractor groove is responsible for providing a surface for an extractor to hook onto and remove the case from the chamber. In the case of rimmed cases, there may be a slight undercut on the rim to aid in extractor gaining a more positive purchase on the case.

The extractor groove is typically shaped a little differently on each cartridge depending on it’s intended firearm. However all exist to perform the same function. Provide a means to remove the cartridge from the chamber. Due to the forces involved in some semi automatic rifles, the groove is subject to a substantial amount of force.

It is not uncommon to see grooves dented or malformed after only one or two firings in firearms that have especially high bolt/slide velocities.

The Cartridge Case Shortcomings

For almost a century the material of choice for cartridge cases has been brass. Specifically a 70% Copper 30% Zinc alloy aptly named Cartridge Brass. This is why cases are commonly referred to as just brass, despite the fact that there are vast quantities of Aluminum and Steel cartridge cases made each year. The brass alloy is ductile, and springy making it a near perfect material for cartridge cases. However it has its shortcomings.

Brass is expensive and it is heavy. Typically the case is the most expensive part of a load, and it is more often than not, discarded after a single firing. Aluminum and Steel cases are cheaper to produce, and are lighter than brass, but they are not reusable. In countries where reloading or handloading isn’t as popular, steel and aluminum cases are more commonplace. In the United States, the majority of ammunition is made with a brass case.

Due to the expense of brass, steel and aluminum are used in budget ammunition, and we can expect to see more and more two piece cases where plastic is used for the body and neck, while the case head is made of brass, steel or aluminum. Plastic cases reduce the weight of the ammunition by up to 60% percent. This might not seem important to the hunter, but to the soldier on the battlefield, it’s a big deal.

In many ways the brass cartridge case has also limited the pressures that modern firearms can operate at. These pressures are capped around 65,000psi. While some people may be tempted to load hotter, and they get away with it, it is generally accepted that a working pressure of 65,000 psi is the limit for a brass case. This is due to needing to maintain a factor of safety. Average proof pressures for a 65,000psi load are permitted to be up to 93,000psi, with individual shots permitted to be up to 168,000psi. At these pressures the brass case would have to be quite thick in order to pass.

This is where we begin to see the innovation. Caseless ammunition has been under development for some time but has yet to be commercially viable. This reduces the cost and weight of the ammunition by removing the case all together in favor of a bullet embedded into a solid chunk of propellent. One of the obstacles of cases ammunition is keeping the chambers cool enough. When a spent conventional case is ejected it carries a substantial amount of heat with it. Caseless ammunition has no case to eject and thus no means to dump that heat.

Sig Sauers 277 Fury cartridge uses a stainless steel head, and a brass body in order to achieve an maximum average pressure of 80,000psi. This two piece design is quite interesting, and if I can ever get ahold of one I’d like to get it sectioned to take a look and see how it is done. This is a very new cartridge that has yet to be accepted by SAAMI. So we will wait to do a full review until SAAMI has accepted it.

Wrapping it up

We are not likely to see the brass case fade out anytime soon. However with the rising price of copper, and with innovations in the industry we can expect to see more and more alternative case materials and designs being used. Fundamentally this doesn’t change the functions of the cartridge case, but it will impact how we load, shoot and test ammunition.

Hopefully this has been a good introduction into the basics of the modern cartridge case, and will help to establish a baseline understanding as we take a look at new cartridges and new case technology that comes out in the coming years.