Just the Tip: Understanding Ammunition

Understanding ammunition
September 17, 2019  
Categories: Learnin'

People gain firearm savvy in lots of different ways. Some are raised in a family culture that cultivates an intrinsic knowledge about guns and ammunition. Others learn through professional training and become active law enforcement or military personnel.  Many arrive at the doorstep of the firearm world through choice or necessity. No matter how you get there, you need to understand the ammunition that you feed your firearm. This is a very basic primer to beginning the learning process (see what we did there?) 


Ammunition is packaged in a cartridge, which has four parts: the primer, propellant, case, and projectile (the bullet). A cartridge is often referred to as a ‘round.’ Shotgun cartridges can also be called ‘rounds,’ or ‘shells.’ They are called ‘shotshells’ when they contain pellets instead of a slug or other projectile — but regardless of what you call it, there’s no such thing as a magic bullet. 

cartridge parts - understanding ammunition

There are four basic components to ammunition: Casing, Projectile, Propellant, and Primer. Gun laws may vary (Kansas Concealed Carry is different from Virginia, for instance), but bullet science does not.


The cartridge head has a copper or brass alloy cup with a brass anvil. The cup is filled with an impact-sensitive lead styphnate igniter, called the primer. When the firing pin strikes the cap, the primer ignites the powder, causing the propulsion of the projectile.

Ammunition- cartridge primer. Centerfire vs Rimfire

Rimfire vs. Centerfire

Regarding primer, there are two types of ammunition. Rimfire cartridges have the primer on the outer rim of the cartridge base, whereas Centerfire cartridges have the primer at—you guessed it: the center. There are two types of centerfire primers: Boxer and Berdan. Boxer is the most commonly used and is identified by having a single flash hole. Berdan has 2 flash holes.


Generally speaking, the propellant is the chemical that propels the projectile through the barrel and out toward the target.  There are a few different types of possible propellants, including gunpowder (also called black powder), nitrocellulose-based powders, cordite, ballistic, and smokeless powders.


How does ammunition work? Gunpowder - propellant

Once ignited, the nitrogen and carbon dioxide gases expand, which provides the propelling action.

Some of these are out-of-date; for instance, cordite has not been a commonly used propellant since the end of WWII. If you’d like to learn more history about gunpowder, consider reading Gunpowder facts and history. Or, read up on powder burn


The case (or casing) is the metal package that holds the cartridge components together, making it a complete round of ammunition. The inside is hollow and holds the propellant. It is thicker on the end so that when the striker hits the casing, the projectile moves away from the shooter. The other end is open so that it can hold the projectile.

parts of ammunition - cartridge casing

Ammunition casings – Image credit Hornady.com

Most often the casing is made of brass, but it can be made out of other metals as well. In fact, some manufacturers now offer lighter-weight polymer composite cased ammunition. Obviously, weight affects performance, so it’s important to know if the casing material matters for your purposes.


Commonly called a “Bullet,” this is a piece of metal (usually lead) that is seated at the open end of the casing. The bullet is the part of the cartridge that is expelled out of the end of the barrel and hopefully hits your target. Bullets come in several different shapes, for various intended purposes.

Bullet Types

Understanding ammunition - bullet shapes

Image source: aegisacademy.com

Bullet Shapes

  • Round Nose: The end of the bullet is blunted. These are multi-purpose and inexpensive.
  • Open Tip: Also called OTM (Open Tip Match), these bullets have a small opening at the top. They are often used for competitive shooting and do not expand on impact.
  • Jacketed: This is a bullet that is intended for high-velocity purposes. It is made of soft lead wrapped by another metal (usually copper) that helps the bullet penetrate more easily.
    • Hollow Point: This type of bullet has a hole at the end, which expands on impact, creating more damage.
    • Soft Point: A bullet with a soft metal core that is enclosed with a strong metal jacket with an opening at the front tip. They offer more expansion and greater expansion and are used for hunting large and small game.
    • Full Metal Jacket: Does not leave a deposit of lead in the barrel. Most often used for training.
  • Wadcutter: This bullet has a flattened front that is specifically designed for shooting paper targets, usually at close range.
  • Semi-wadcutter: This one is a cross between a round nose and a wadcutter. It is considered an all-purpose bullet for both revolver and pistol cartridges. Used for hunting, target shooting, and plinking
  • Ballistic Tip: A hollow point bullet that is covered by plastic which improves long-range accuracy.
  • Frangible: Instead of a soft lead core, these bullets are made with compressed copper granules that shatter on impact with a hard surface. Mainly used for training purposes in shoot-houses because they are less likely to over-penetrate.
  • Steel Core/ Armor Piercing: The core is a solid dart-shaped piece of steel. They can ricochet when used improperly.
  • Tracer: A projectile that is painted with a chemical that burns as it flies downrange. It lets the shooter see where it flies, but can cause fires after landing.

Shotshell Projectiles

Understanding ammunition - three types of shotshell projectiles

Image source: rem870.com

  • Birdshot: Smaller pellets. The cartridge holds more small pellets than large ones, so there are more pellets scattered over the target area.
  • Buckshot: larger pellets for more damage.
  • Slug: A single bullet weighing about 1 ounce of metal inside a shotgun cartridge. 410 slugs are much lighter at about 1/4 ounce.

Ammunition Measurements

Diameter and Length

Ammunition is described by measurements. In the United States, the commercial market uses the US standard measurement, but most of the world uses a metric rating. The diameter of the cartridge is especially important for firearm safety and performance. If the cartridge is too large it won’t fit into the chamber, and if it is too small, the bullet will rattle around through the barrel producing unpredictable accuracy. And of course, the wrong sized cartridge can cause the barrel to explode, which should obviously be avoided.

In metric, the diameter is listed before the length. So a 5.56mm x 45mm cartridge is 5.56 mm wide and 45 mm long. Bullet diameters can be listed as whole numbers (as in 9mm) or as decimal points (6.5 Creedmoor). 

understanding ammunition - measurement

Image source: Shooting Illustrated; and by the way, while size matters, “stopping power” is a bit of a misnomer.

Shotshells are measured by gauge, which indicates the diameter of the shell case, and length. A larger gauge indicates a smaller shell diameter. 

Shotshell cartridge measurements

Image source: hunter-ed.com


Bullet weight is measured by Grains. It takes 437.5 Grains to make an ounce, so the larger the number of Grains, the heavier the bullet. Some ammunition labels spell out the word Grains, and others abbreviate it with “Gr.”

This is not the same as counting grains of powder.

Bullet weight is measured in grains.

Image source: luckygunner.com

Bullet weight can impact on factors like ballistics, accuracy, and recoil. For instance, lighter bullets can perform better in long-range shooting.


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In order to know which size of ammunition to purchase, you have to know the caliber of your firearm. The caliber is the diameter of the inside of the barrel. It is represented by a decimal up to hundredths of an inch, as in .22, or in millimeters, as in 9mm. The caliber is identified on each firearm with a “rollmark” on the barrel.

Understanding ammunition - find your caliber by looking at the roll mark on the barrel of the gun.

The roll mark on this firearm indicates that it is chambered in 300 Winchester Magnum. Image source: gotogungirl.com

When purchasing ammunition, be sure that the ammunition caliber on the box is the same as the caliber on the firearm.

Other Ammunition Options to Consider


The velocity of the projectile is measured in feet-per-seconds at the moment it exits the barrel of the firearm. Velocity decreases downrange, and some ammunition boxes have charts that detail velocity differences at farther distances. For example, look at this Sniper Country chart that compares .270 and .308. 

Understanding ammunition - bullet velocity

Image Source: SniperCountry.com


Ammunition energy is measured as the kinetic energy of a bullet as it leaves the muzzle (or barrel) of a firearm. Commonly called Muzzle Energy, it is an estimate of the destructive (or stopping) power of the cartridge. It is measured by the Joule (J) or foot-pound (ft-lb) force.

From sciencing.com:

Calculate the energy the bullet will deliver to the target on impact using the formula Energy = Bullet Weight x Velocity^2 / 450,437. In words, the bullet energy (in foot-pounds) is equal to the bullet weight (in grains) times the square of the bullet velocity(in feet per minute) divided by 450,437.

Understanding ammunition - muzzle energy

Image source: Wikipedia

Heavier, faster bullets tend to cause more damage. Higher energy ratings generally mean greater force.

Overpressurized Ammunition

  • +P: this ammunition has a higher amount of pressure than standard rounds of the same caliber. It has a faster muzzle velocity with greater penetration but causes more stress on the barrel.
  • JHP+P+: Overpressurized Jacketed Hollow Point combines a jacketed hollow point with the increased velocity and penetration of a +P  round to increased damage to soft tissue.
Understanding ammunition - +P and +JHP

Image Source: concealednation.org

Read more about these options at Concealed Nation.

Anything Else about Ammunition?

These are the basics for understanding ammunition: the four components, how it works, what types there are, and additional options to choose from.

Remember: there’s no such thing as a magic bullet.

“If the bullets you’re considering are called anything that sounds like it’s from a SyFy movie (Annihilator, Devastator, Terminator, Double Secret Omega Discombobulator), stay away from it. If there’s a video about it and the narrator sounds like Don LaFontaine, steer clear.” No bullets from Billy Mays

Do you have anything to add?

More About Ammunition

How do you store your ammunition? Here are some tips from the Mad Duo.

Got problems with your ammo? Read Swingin’ Dick’s article about Ammunition, Chambering, Checking, and Problems.


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Stephanie Kimmell

Stephanie Kimmell

About the Author

Stephanie Kimmell is the firstborn daughter of Missouri's Pecan King, worthy scion of a Vietnam veteran sailor turned mad engineer-orchardist-inventor-genius. With a BA in technical writing, she freelances as a writer and editor. A Zymurgist greatly interested in the decoction of fermented barley and hops, she is in many ways a modern amalgam of Esther Hobart Morris, Rebecca Boone, and Nellie Bly. She hunts, fishes, butchers, and cooks most anything. When not editing or writing, she makes soaps and salves, spins wool, and occasionally makes cheese from cows she milked herself. Kimmell is a driven epistemophilic who loves live music and all sorts of beer. It will be interesting indeed to see what she writes in the coming days.


  1. Silence DoGood

    The roll mark illustrated denotes the cartridge fired by the rifle, not the caliber. Shooters tend to use the two terms interchangeably, to their detriment. A 300 Winchester Magnum is point of fact the same caliber as a .308 Winchester but even knowing that still does not tell you what caliber either it.

    Even introducing kinetic/muzzle energy into the conversation is pretty pointless because KE is relatively worthless information except for advertisers and mathematicians. For starters, KE (unlike momentum) is not conserved so there’s no guarantee that what has come out of the muzzle is what will impact the target. And more KE does not guarantee (or even imply) that the cartridge is more lethal or destructive. The fact that a .17 WSM has more ME than a .38 Spl+P shows the folly of putting any significance in the number. Momentum is easier to calculate and of more practical use in every respect.

    And you left out all mention of the ‘heeled’ bullet, which is essential to an understanding of why a .38 Special projectile is actually .357 caliber.

  2. Geoff

    This should be required reading for all Left Stream Media reporters.
    Then explain the difference between “clip” and “magazine”.


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