Ammunition 101: Understanding the Basics of a Firearm Cartridge [or Bullet]
If you’re a newbie to guns & ammo, then it’s quite likely that ammunition and cartridge definitions may seem somewhat confusing, while naming conventions and components may leave you a little baffled. The good news is that today we are here to help. This article will simplify the study of ammunition and projectiles into laymen’s terms, bringing you up to speed and further away from that rookie status. So to kick things off, let’s answer this: what is a firearm cartridge, and how does it work?
A cartridge – sometimes referred to as a round – is a fully-assembled firearm ammunition consisting of a primer, projectile, propellant, and a casing that holds it all together. These four items are known as the components of a cartridge; or cartridge components, and are used in a variety of firearms which include pistols, rifles, shotguns and automatic weapons or machine guns. They all work together to release the projectile under high velocity, through a process of expanding gasses which are the driving force behind the bullet.
Having a basic understanding of cartridges and their individual components is vital to any avid shooter. Not only does it make you look like less of a noob, but the correct knowledge plays an import part of ensuring that you’re achieving the best results from your shooting goals, whatever they may be. So let’s get to work, and bring you up to scratch.
The 4 Components of a Firearms Cartridge
- Primer – this is the explosive that sets off the propellant. The primer is struck by the firing pin once the trigger is squeezed and creates a spark that ignites the propellant
- Casing – typically made from brass or mild steel, the casing holds everything together
- Propellant – otherwise known as powder, the propellant is ignited by the primer and creates the gasses and energy necessary to drive the bullet
- Projectile – otherwise known as the bullet, the projectile leaves the firearm and travels through the air causing physical damage onto the intended target
The complete product – all four components combined – is know as the cartridge.
Cartridge Primers: What Are They, And How Do They Work?
As we have already mentioned, the primer is responsible for creating the spark which ignites the propellant. Upon being struck with sufficient force, the primer reacts chemically to produce heat which in turn ignites the main propellant charge and fires the projectile.
Without primer ignition, there would be no bang; the gun would misfire. It is therefore essential to always use high quality primers that have been stored correctly and away from moisture.
Rimfire vs. Centerfire Ammunition: What’s The Difference?
Rimfire and centerfire are two categories of primer ignition systems for ammunition cartridges. If you own a rifle or pistol, or have shot one before, you would have used a cartridge with one of these two systems. Let’s briefly explore some of the differences between the two.
- Rimfire ammo is limited to low velocity loads, and is found amongst smaller calibers (.17 and .22 are most common)
- The firing pin strikes the rim of the case, rather than a primer located in the center
- It is cheaper to manufacture and purchase than centerfire ammunition
- The brass is typically discarded after use
- The majority of calibers are centerfire
- The firing pin strikes the centre, external primer
- It is more reliable and safer to store than rimfire ammunition
- Centerfire brass is re-usable, provided they are boxer primed
When it comes to centerfire ammo, there are two common types of centerfire primers that are used, namely Berdan and Boxer primers. Should you concern yourself with the differences between the two? Well, if you intend on hand-loading your own ammunition, then yes!
Berdan Primers Explained
- These primers were invented in the US, but are more commonly used in other countries
- They are cheaper and easier to manufacture than Boxer primers
- The case houses two smaller flash holes which can be observed by looking into the empty casing
- The primer anvil is part of the case, rather than housed in a removable primer cap
- These cases cannot typically be reloaded, unless the primer hole is punched out and converted into a Boxer primer, which requires special tools
Boxer Primers Explained
- These primers were invented in Europe, but are more commonly manufactured and sold in the US
- They are typically found in ammunition purchased by civilians
- The primer anvil is part of a separate, independant primer cup
- The case houses one larger flash hole, which can be observed by looking into the empty casing
- The case can easily be reloaded by removing the spent primer – known as depriming or decapping – and replacing a fresh primer
Tip: once the cartridge has been fired, you can determine which primer was used by looking down into the empty casing and observing either two flash holes, or one (as seen in the image above). You may need to use a flashlight to see into deeper cartridge cases such as rifle calibers.
The Cartridge Case
The cartridge case, often referred to as brass, is the container which holds all of the cartridge components together. They are most often made from brass or mild steel, and in some cases even aluminium.
Brass Cartridge Cases
Brass is the alloy of choice when it comes to higher-end cartridge cases, particularly those used for precision shooting. It is an expanding material that provides a gas-tight seal within the chamber, which stretches and shrinks at a rapid rate allowing the casing to be extracted immediately after firing. It is also soft enough to allow for a longer barrel life; reducing throat erosion when compared to other casings such as mild steel.
Some manufacturers produce higher quality brass than others, which promotes accuracy due to the way in which it expands, causing a tighter and more consistent seal within the chamber and therefore resulting in more consistent muzzle velocities. Higher quality brass also results in a longer life-span when used during reloading. Lapua is a prime example.
Steel Cartridge Cases
Other manufacturers such as those who most commonly cater for the Ak market sell cheaper ammunition options with mild steel casings. These steel casings are sometimes coated with lacquer to prevent rust, and are often mistaken for brass.
The harder properties of steel casings – that being almost 50% harder than a brass casing – increases throat erosion, supposedly to a point where a rifle’s throat can become spent after only half of its expected lifespan.
Although appearing more cost effective in the short term, steel cased ammo should typically be avoided when fired from high quality weapon systems with tighter tolerances.
Cartridge Case Design
There are two types of general shapes that we may find in a cartridge case.
These are either straight-walled cases or bottleneck cases.
Straight-walled cases (image right) are most commonly found in pistols and revolvers, whereas bottleneck cases (image left) are most commonly found in rifles and machine guns, although there are some exceptions to this rule.
Types of Case Rim
At the base of the cartridge case we find the rim, which provides a lip for the extractor to engage, and in many cases also serves to headspace the cartridge. Upon close inspection of various cartridge cases, you will come to notice that there are various types of case rims – 3 types to be more specific – with each serving their own purpose.
Rimmed – while sometimes referred to as the flanged cartridge due to the prominently protruding rim, this is the oldest type of cartridge case which has a rim that is larger than the base. The rim is used to hold the cartridge in the chamber of the firearm, making the case length of less importance.
Some types of rimmed cartridges such as rimfire, use the rim to contain the priming compound, as we have already discussed above. Under the metric cartridge designation system, a capitalised “R” added at the end of the designation indicates a rimmed cartridge. An example of this can be seen in the 7.62x54mmR.
Rimless – on the rimless cartridge case, the rim is the same diameter as the base, and there is a gap formed between the rim and the body of the cartridge known as the extractor groove. Because there is no protruding rim, the cartridge must headspace on the case mouth for a straight-walled case, or on the case’s shoulder for a bottleneck case. The lack of rim makes feeding from a box magazine very smooth.
Semi-Rimmed – upon close examination of a semi-rimmed case, it is evident that the rim projects slightly beyond the base of the case, but not as much as a rimmed cartridge. This provides minimum interference when being fed from a box magazine, while still providing enough surface to headspace on. The semi-rimmed case is less common.
Modern Firearms Propellant
Modern propellant which is used in the majority of firearm cartridges today is known as smokeless powder. This propellant produces a negligible amount of smoke when fired in comparison to black powder, which is where it gets its name. Although not completely free from smoke, the combustion products of smokeless powder are mainly gaseous and produce very little smoke when fired from small arms, capable of producing six times more gas and pressure than black powder.
Smokeless powder is a highly combustable single based propellant – namely Nitrocellulose – which has a clean and equal burn rate, and is typically sold in one of three different shapes. The size and shape of the propellant grains can increase or decrease the relative surface area, and can significantly change the burn rate of the propellant. This allows the burn rate to be controlled. Additives and coatings can be added to the propellant to further modify the burn rate.
Smokeless Powder Shapes
Ball Powder – also known as spherical powder, this propellant shape is faster and easier to manufacture, reducing the overall cost. It is commonly rolled or flattened slightly to change the shape, enhancing loading density and ignitability. It feeds best through a powder dispenser, allowing for more accurate loads when hand-loading bulk ammunition.
Ball powder can have a greater shelf life than other powders, often burning at lower temperatures which may assist in extending barrel life. However, it is often considered the worst shape for precision loads, starting with a large pressure spike and weakening as the powder burns up.
Flake Powder – in the form of flat, round flakes or disks, this propellant is most commonly used in lower-velocity handgun and shotgun calibers. The powder produces inconsistent loads when fed through a powder dispenser, but thanks to the very high nitroglycerin level, it features superior cold temperature performance; vital in some shotgun loads.
Tubular Powder – otherwise known as extruded or stick powder, it comes shaped like small cylinders and is most popular in rifle cartridges. When fed through a powder dispenser, it is difficult to produce consistent measurements and is therefore best measured manually.
Most stick powder burns hot, which may increase barrel wear when compared to other powders, but it produces the highest level of shot consistency and accuracy which is vital for precision rifle loads.
Small Arms Projectiles
As we have already mentioned, the bullet – or projectile – is the part of the cartridge which leaves the firearm and travels through the air causing physical damage onto the intended target. Bullets come in many different shapes, sizes and weights, each specifically designed to achieve their own result during free-flight and upon impact of a target.
When referring to the mass or weight of a projectile (the bullet head itself), we use grain as a measurement. A grain is a very small unit of measurement whereby 437.5 grains is an ounce, or 15.43 grains is a gram.
- A 55gr .223/ 5.56mm bullet weighs 55-grains, 0.125 ounces, or 3.5 grams
- A 168gr .30cal bullet weights 168-grains, 0.38 ounces, or 10.9 grams
The heavier a projectile is in its caliber category, the more stopping power it will typically retain. Heavier bullets also travel further and are generally capable of flying truer along a predetermined trajectory, at least when crosswinds are present. The lighter bullet on the other hand will produce a higher velocity and a flatter trajectory, which can also be advantageous. So as the shooter, it is your job to determine which bullet weight is ideal for your intended use.
By now we should all understand that bullets are not the same. Even though some may look similar, or have similar specifications, they are all in fact very different from one another. There is a huge difference between different bullet brands, types, powders, and loads, even if the bullets you are using are both the same weight. Even bullets of the same weight, shape, size and load, but from different manufacturers will have completely different trajectories.
Two very distinctive characteristics that differentiate one bullet from the next – aside from weight – is the shape of the projectile’s base and tip, along with the presence or a cannelure, or lack thereof . The base is found in one of two shapes.
Flat Base Projectiles
Differentiated by the obvious flat base, these bullets have a number of advantages when engaging targets at closer ranges. The design of the flat base bullet is inherently more accurate and is often preferred amongst short range benchrest shooters. The shorter bullet profile of the flat base will also stabilise easier in rifles with a slower twist, and upon impact, the flat base bullet experiences less jacket and core separation. In other words, the jacket holds onto the core better, allowing for deeper penetration with maximum weight retention. When engaging targets at further distances, the flat base design is slower, producing greater drag and more wind drift. They are specially designed to cause greater damage at close range, and are commonly found in hunting and self-defence ammunition.
Boat Tail Projectiles
The outline of the boat tail bullet resembles the shape of a boat, with a thin nose and a tapered base. The design produces a far flatter trajectory, retains more energy, and overcomes wind better than the flat base bullet. The higher ballistic coefficient and superior aerodynamical shape improves wind resistance and reduces drag by cutting down the size of the bullet’s base. But, this design which is superior for long range engagements does have its disadvantages, the main one being the bullet’s terminal performance. The design often causes jacket and core separation upon impact. However, with modern advancements in bullet technology, there are various options available that solve this issue, such as Hornady’s InterLock boat tail bullets and other core bonded bullet brands. The boat tail is specifically designed for medium to long range engagements.
Very Low Drag Bullets have an ultra streamlined boat tail design, which typically sacrifices bullet retention upon impact for a superior ballistic coefficient, very flat trajectory, and great ability to overcome wind. The VLD bullet is generally characterised by its pronounced boat tail, short shank (the straight section of the bullet), and very long, highly tapered ogive (the curve of the bullet’s forward section) with a small hollow point.
The Bullet Cannelure
These are the tiny striations that can be seen (when present) on the bullet’s main body, as seen in the images above. The main purpose of the bullet’s cannelure is to allow the cartridge case to tightly crimp around the bullet in order to produce a more secure fit. In other words, the cannelure allows for an enhanced grip or biting point when seating and crimping the projectile.
So do all bullets have a cannelure? No. The cannelure is typically present in military grade ammo or ammunition that is hand-loaded for general use where the bullet may otherwise be pressed deeper into the cartridge case during rough handling. Precision ammunition generally lacks a cannelure and rather has a smooth surface, as this aids in accuracy. So it really depends on your intended use as to whether you require a bullet with a cannelure or not.
But be aware that crimping a case into a cannelure will increase pressure, and should be load developed & monitored accordingly.
Although bullets come in many different shapes and sizes that are all designed to meet their own specific needs, each bullet will generally fall into one of three categories.
- Ball Ammunition
- Expanding Ammunition
- Frangible Ammunition
Ball rounds, which despite their name are not spherical, have a hard outer surface most commonly made of copper with a lead core. This hard outer surface assists the bullet in retaining its shape and dimensions as much as possible after impact, while transferring the least amount of energy into the target in comparison to other bullets.
Ball rounds are designed for maximum accuracy and maximum penetration at varying distances, providing the deepest penetration into a target, which can be an advantage when engaging targets from behind cover. They also the most likely bullet to over- penetrate a target, which in some instances can be significant.
Expanding bullets include soft-point and hollow-point rounds, which are designed to expand upon impact, increasing their diameter and therefore delivering far more energy into the target. They are also sometimes referred to as hunting bullets or self-defence loads.
Once an expanding bullet enters a target, it can open up to twice its original diameter. This wider surface travelling through a living target creates a larger permanent wound cavity, and has more chance of passing through critical organs and nerve tissue, creating far greater damage when compared to ball ammunition.
Frangible ammo is far less common than ball and expanding ammo, and is often made from compressed copper powder. The bullets are designed to shatter or disintegrate upon impact of a hard surface, such as steel or bone. This eliminates any chance of over-penetration, but the bullets are still deadly when striking their intended target.
Frangible bullets are most commonly used amongst law enforcement personnel for training purposes.
Cartridge Dimensions Explained
To a newbie, cartridge dimensions and naming conventions may seem confusing. So by studying the following image, we can see how the cartridge and bullet dimensions are defined.
- If you examine the projectile in the image above, you will notice that bullets are measured by their diameter and not by their length, as bullet length will vary amongst different brands and types. This particular .30 caliber bullet measures 7.62mm in diameter, which is equivalent to 0.30″, hence the .30 caliber
- The case however is measured by its length, from end to end. This measurement from case rim to case mouth is 51mm. These combined measurement – of the bullet and the case – give us the 7.62x51mm cartridge
- The cartridge length as a whole – otherwise known as the cartridge overall length (COL) – will vary amongst different brands and bullet types, and may be intentionally sized to suit a specific rifle & bullet combination during hand-loading