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AMMO 101: Part 1 Ammunition for Beginners

AMMO 101: Part 1 Ammunition for Beginners

Ammunition is what determines the purpose and performance of a firearm. This is the case for every projectile weapon, regardless of whether it’s a firearm, bow, cannon, artillery piece, or rocket launcher. It’s the ammo that ultimately achieves results.

This is true whether you’re disabling or destroying enemy personnel and materiel on the battlefield, repelling or incapacitating criminals, slaying game animals for sport or food, or blowing apart clay disks in flight. The weapon system itself is simply the mechanism that carries and projects the force.

Whatever the intended use, small arms ammunition is a complex topic with a rich background and has been evolving for more than a thousand years.

Ammunition Components and Terminology

If you’re new to firearms and ammunition, a good start is to know the standard terminology. This is not a complete glossary but should familiarize you with some of the most commonly used terms regarding ammunition.


Caliber may refer to either the bore’s diameter — the inside of the gun barrel — as measured from land to land or the bullet’s approximate diameter.

The caliber is typically expressed in hundredths of an inch (e.g., .30 caliber, .45 caliber); thousandths of an inch (e.g., .380 ACP, .270 Winchester); and millimeters (e.g., 9×19mm, 5.56×45mm NATO).


In the context of shotgun ammunition, a concept related to caliber is gauge. Gauge refers to the number of lead balls of a given size it takes to equal one pound. For example, in a 12-gauge shotgun, it takes 12 lead balls of .729 caliber — the nominal bore diameter of a 12-gauge shotgun in inches — to equal one pound.

Whereas an increase in caliber in handguns and rifles corresponds to an increase in the barrel’s diameter and projectile, the gauge of a shotgun is the opposite. As the gauge increases, the bore diameter decreases.

As a result, a 10-gauge shotgun has a larger bore diameter (.775/19.7mm) than a 28 gauge (.550/14mm) because it would take fewer lead balls of that size to equal 1 pound.

What is a Cartridge?

A cartridge or round is a single unit of ammunition. Sometimes referred to erroneously as a bullet, a cartridge typically comprises 4 components. These are the projectile, the propellant charge, the primer, and the cartridge case.


If it’s designed to be propelled to damage a target over space, it’s a projectile. While rockets are also projectiles, the critical difference is the propulsive force.

In a gun — firearm, howitzer, or recoilless rifle — the propulsive force ends once the projectile leaves the weapon system.

Shotgun shells are often loaded with multiple spherical projectiles referred to collectively as shot and individually as pellets made from several different metal alloys, from lead to stainless steel and tungsten.


A propellant is a low explosive or deflagrant that burns, producing high-pressure expanding gases when ignited in a confined space. From the 9th century to the 19th, this was known as gunpowder or black powder. As more than half of black powder’s combustion products are solid, it was known to create a thick white smoke that would blanket the battlefield and obscure the target.

In 1884, however, smokeless powder was invented. Smokeless powders generate higher chamber pressures and higher muzzle velocities; therefore, they place a greater demand on firearm action’s material strength. Most combustion products are gaseous, so the shot won’t obscure the target or disclose one’s position as readily.


This is the metal cup containing an explosive priming compound. When struck by the firing pin, the primer detonates, igniting the propellant charge through one or more flash holes in the cartridge case’s primer pocket.


This is the metal tube, closed at one end, containing the powder and holding the bullet and primer in place.

In shotgun ammunition, the cartridge case is also referred to as the shell. There may be a fifth component called the wadding, which acts as a spacer between the propellant charge and the shot.

Typical Ammo Calibers

.22 LR:

Introduced in 1887 by the Steven Arms Co., the .22 Long Rifle rimfire is a development of the earlier .22 Long cartridge. Prized for its low cost, negligible recoil, and accuracy, the .22 LR is commonly used for small-game hunting, competition, and plinking.

.380 ACP:

One of John Browning’s designs, .380 ACP (also called .380 Auto, 9mm Short or 9×17mm), was introduced in 1908 as a pistol cartridge intended mainly for compact semi-automatic pistols. Today, it is considered to be the minimum caliber for effective self-defense.

9×19mm (9mm):

Designed in 1902 to fulfill a U.S. Army requirement, the 9mm Luger cartridge, also known as 9×19mm Parabellum, is one of the world’s most popular centerfire semi-automatic pistol cartridges. Modern 9mm pistols offer a high capacity and mild recoil, making it an excellent self-defense choice.

10×25mm (10mm Auto):

A joint effort between Jeff Cooper and ammo manufacturer Norma, this cartridge was introduced in 1983 to serve as a more powerful alternative to the 9mm and .45 ACP. Once selected as the FBI’s standard service cartridge and made famous by shows like Miami Vice, it was considered too powerful for law enforcement and was superseded in 1990 by .40 S&W. In the late 2010s, 10mm experienced a resurgence, as a powerful hunting and self-defense cartridge for semi-automatic pistols.

.40 S&W:

Introduced in 1990, .40 S&W was introduced to address the common complaint that the 10mm Auto had too much penetration power for use by police or other law enforcement entities. With a shorter case (22mm instead of 25mm) and a reduced powder charge, .40 S&W is more controllable yet still effective. Glock was the first company to introduce a .40-caliber pistol, the Glock 22, which several law enforcement agencies adopted across the United States.

.45 ACP:

Following the unsatisfactory performance of the .38 Long Colt in the Philippines during the Moro Rebellion, the U.S. Army decided that nothing less than .45 caliber would suffice. First adopted in the Browning-designed M1911, the .45 ACP is frequently used as a self-defense and competitive shooting cartridge due to its large bore size and high stopping power.

.45 GAP:

Introduced in 2003 by Glock, .45 GAP was intended to be a low-recoiling, easily-controllable alternative to .45 ACP, but with a shorter case length, allowing standard-sized pistol frames (generally intended for 9mm or .40) to be designed around this cartridge. Although initially well-received, firearm technology advancements rendered it largely obsolete, as better-designed compact .45 ACP pistols were released.

5.7×28mm FN (5.7 FN):

First chambered in the P90 PDW (Personal Defense Weapon), the 5.7mm cartridge combines high-velocity performance with low recoil and compact dimensions. In its military configuration, it delivered armor-piercing capability in a weapon the size of a submachine gun. In the aptly named Five-seveN pistol, the 5.7mm can be considered a more powerful .22 Magnum, affording a flat trajectory and high magazine capacity.

.38 Special:

Introduced at the turn of the 20th century, .38 Special is one of the oldest centerfire cartridges you can still find today. Initially designed by Smith & Wesson for use in the S&W .38 Hand Ejector revolver, which was later designated Model 10. This handgun became the most popular revolver of the 20th century.

.357 Magnum:

The first widely-used Magnum handgun cartridge, it was commercially introduced in 1935. It is dimensionally identical to .38 Special but featuring a longer case and a higher powder charge, achieving a higher muzzle velocity and energy. .357 Magnum is a versatile cartridge, suitable for self-defense, handgun hunting, and target shooting.

.357 SIG:

A cartridge based on the .40 S&W, but necked down to accept a 0.355” (9mm) bullet. When fired out of a typical semi-automatic pistol barrel, .357 SIG is intended to replicate the performance of a .357 Magnum revolver of similar barrel length. Some police agencies have adopted .357 SIG to replace their .357 Magnum revolvers, praising its accuracy and performance.

.44 Magnum:

Introduced in 1954, .44 Magnum is a powerful handgun cartridge primarily intended for revolvers, made famous by Dirty Harry movies. Today, .44 Magnum is used mainly for handgun hunting and target shooting.

Projectile Types

The projectile is the only cartridge component, other than propellant combustion products, to leave the barrel. It’s what inflicts ballistic injury or damage. Some common projectile types include:

Full Metal Jacket (FMJ)

Referred to as ball ammunition in a military context, FMJ typically consists of a soft lead-alloy core enclosed in cupro-nickel, mild steel, or brass envelope called a jacket. If it fully encloses the core, it’s referred to as a full metal jacket, although the bullet base may still be exposed. FMJ aims to protect the barrel from lead fouling and the lead core from melting, ensuring reliable cycling and penetrating more effectively.

Jacketed Hollow Point (JHP)

Designed to expand in soft tissue, hollow-point bullets consist of a lead-alloy core with a concave nose or tip. As the bullet penetrates soft tissue or another target medium, it expands or mushrooms due to the rise in hydraulic pressure in the pit of the nose cavity. As with FMJ, the core in the jacketed variety is enclosed, either fully or partially. This has the effect of controlling expansion.

Jacketed Soft Point (JSP)

Also known as soft-nosed bullets, the JSP is partially enclosed in a jacket with an exposed lead tip to promote controlled expansion. More penetrative than JHP but more likely to deform than FMJ, this type of bullet can be considered a hybrid between the two.

Open Tip Match (OTM)

The open tip may cause some confusion with hollow points. The hollow point is designed to expand, but the OTM is precisely manufactured. The lead core is poured through the nose of the deep-drawn jacket, allowing for increased uniformity.

Wadcutter (WC)

Also called a full wadcutter, this is a sharp-shouldered, flat-nosed cylindrical bullet, typically made from exposed soft or hard-cast lead. Wadcutter bullets are designed to punch neat round holes in paper targets for more accurate scoring during competitive matches.

WC is more commonly used in revolvers than semi-automatic pistols. While designed for target shooting, the wadcutter has seen increasing use as a self-defense bullet in snub-nosed revolvers due to the reduced velocity, which may not be sufficient to cause a hollow point to expand reliably.

Semi-Wadcutter (SWC)

Available with or without a jacket, the semi-wadcutter is a specialized flat-nosed bullet. There are multiple types of semi-wadcutter designs, from Keith bullets with lubricating grooves to those with a tapered, cone-shaped nose and a wide, sharp shoulder. SWC is useful for target shooting, hunting, and in some self-defense applications. This design variation is the lead semi-wadcutter hollow point (LSWHP), which the FBI used in its .38 Special service revolvers for many years.

Cartridge Case Types

The purpose of the cartridge case is to hold the other components together, seal the propellant charge against the ingress of moisture, and expand on firing to seal the breech. In addition, the case absorbs and removes heat from the chamber. Made from a variety of materials, the most common are brass and mild steel.

Several factors dictate cartridge case design. Among these are the ignition system, the pressure generated by the propellant charge, the type of firearm action it’s designed to be used in, the headspace point’s location, and the kind of projectile it’s designed to accommodate. These are:


The rim or extractor flange is larger than the diameter of the case body. It may also be used to headspace the cartridge case, controlling the seating depth. It can be rimfire or centerfire.

Rimmed cartridges are commonly seen in older rifle cartridges, such as .30-30 Winchester, and revolver cartridges, where the rim is acted on by the extractor star to unload the cylinder.


The rim is the same diameter as the case body, and the case head has an extractor groove. In the rimless design, the headspace point must be sought elsewhere, such as the mouth or shoulder.

Examples of rimless cartridges include the 9×19mm Luger, .45 Auto, and .223 Remington.


The rim protrudes minimally beyond the case body’s diameter to provide a headspace point without interfering with the cycling of semi-automatic firearm actions.

Not many shooters use semi-rimmed cartridges. Some types you may find available include the .25 ACP (6.35×16mmSR), .32 ACP (7.65×17mm), and .38 Super.


The rim is smaller in diameter than the case body, which can be useful for caliber conversions. Examples of rebated include the .50 Action Express, .50 Beowulf, and .458 SOCOM — the latter 2 share a bolt face with the .223 Remington/5.56mm NATO cartridge.


In high-powered rimless cartridges with an insufficient shoulder to headspace on, the belt is a circumferential band located above the extractor groove that stops the cartridge’s forward movement.

Some common types sold are the .300 Winchester Magnum and .375 Holland & Holland Magnum.

Ballistics: Internal, External, and Terminal

Ballistics is, broadly, the study of projectiles in motion and related effects. This category is further divided into the following subfields:

Internal Ballistics

Internal ballistics is the study of projectile motion inside the firearm. The sequence of events that leads to the firing of a modern metallic cartridge begins with the primer being struck by the firing pin and continues through propellant ignition, obturation of the breech, and propulsion of the bullet through the bore and out the muzzle.

External ballistics

External ballistics is the study of the projectile’s motion and behavior in flight after it leaves the muzzle, but before it strikes the target. Some elements related to external ballistics include:

  • Drag or air resistance, which has the effect of slowing the bullet.
  • Ballistic coefficient (BC), which determines the aerodynamic efficiency of the bullet. The higher the BC, the more effectively the bullet overcomes drag.
  • Gravity, which exerts a downward force on the bullet.
  • Minute of Angle (MOA), an angular measurement that is used to describe accuracy or precision. One MOA corresponds to a group size of approximately 1” at 100 yards, 2” at 200, and so on.
Terminal Ballistics 

Terminal ballistics is the study of the behavior of the projectile once it strikes its target. As a field, terminal ballistics applies to both the effects of ammunition on material targets, such as vehicles and personnel. However, a subdivision of terminal ballistics explicitly concerned with bullets’ impact, explosive shell fragments, and other living human and animal tissue projectiles is called wound ballistics.

  • Permanent wound cavity: The volume of tissue permanently crushed and destroyed by the bullet’s passage and the resulting hole left behind.
  • Temporary wound cavity: The volume of tissue temporarily stretched and displaced radially outward by transferring kinetic energy. Temporary cavitation is especially damaging to inelastic soft tissues such as the liver and fluid-filled organs.
  • Fragmentation: When the bullet’s structural limits are exceeded, the bullet may break apart into multiple fragments. While not a reliable phenomenon in handgun ammunition, high-velocity rifle bullets, particularly those of light construction, are more apt to fragment.

Final Thoughts

At We The People Holsters, we know that ammunition is critical to any self-defense weapon and the exercise of your Second Amendment rights.

If you’d like to discuss the best way to carry spare ammo for your concealed-carry weapon, call us at (866) 998-6191. We’d be happy to discuss your options. In addition to our durable IWB and OWB Kydex holsters, we also offer magazine pouches made from the same custom-molded thermoplastic, so you’re always prepared for what lies ahead.