Ammunition: Understanding Cartridge Composition

Project ar15 Sep 09, 2023
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Ar15 ammunition 5.56 green tip

Understanding Ammunition Composition and Terminology

Ar15 enthusiasts enjoy a nearly unlimited selection of ammunition in a shining array of calibers to choose from. Even within a particular caliber, options of projectile weights and configurations are so many, it's important to chose the right cartridge for your specific needs. 

In this section we will review the parts of the cartridge, projectile diameter(caliber), casing length, the projectile weight and different casing metals. 

What are the parts of the cartridge?

The Casing

The casing is the part of the cartridge that contains all other parts; The primer the projectile and the powder. Most commonly made out of brass, casings are also commonly made out of aluminum and steel as well. When fired from the chamber, the casing expands and seals the gasses to propel the projectile forward and out the barrel. It is important to note the casing length of the ammunition to ensure proper chambering and subsequent firing. Example: 7.62x39 and 7.62x51

 Note the difference in the casing length can be seen in the second number of the projectile description.

 Ar15 Ammunition 5.56 green tip

The Primer

Priming compound is a mixture of lead styphnate, antimony sulfide, barium nitrate, and other chemicals. Primers are constructed of a brass cup filled with lead styphnate, a combustible compound. Lead styphnate explodes on impact. Primers contain a mixture of substances that perform three basic functions: First, an initiator, which is an explosive that starts the process when the firing pin hits the primer; secondly sensitizer, which helps in the ignition process; and finally a fuel, which sustains the flame and ensures adequate time to light the powder. 

Ar15 ammunition Casing primer

The Projectile 

The projectile or bullet, is the part of the cartridge that is propelled out of the barrel. These are usually made of lead surrounded by a copper jacket. However, projectiles with a steel tips or a steel core are available. These provide enhanced penetration of the projectile and are sometimes referred to as "armor piercing". Some more modern bullet constructions employ the use of polymer inserts to ensure controlled expansion of the projectile upon impact. The diameter of the projectile is generally referred to as the caliber, (Example; .223 Remington, 9mm etc.) with some exceptions. Projectiles also come in various configurations and weights. Common configurations are "ball" ammunition. Having a rounded shape at the nose, ball ammo is typically used for target practice. "Hollow point" ammunition.

This projectile type has a concave cavity in the nose of the projectile to create a controlled deformation of the projectile thereby imparting more of its kinetic energy to the target. They are commonly used for self-defense. The weight of the projectile is referred to as "grains". There are 7000 grains in one pound, and 437.5 grains in an ounce. Common weights for 5.56 NATO cartridges include 55gr. 62gr and 75gr. Heavier projectiles carry more energy but have a more arched trajectory over distance. Lighter projectiles shoot flatter and recoil less. 

Ar15 Ammunition 5.56 Hollow Point


The "charge" of the cartridge comes from the explosion of the propellant within the casing. Modern propellants are not the same black powders used in muskets. More refined and emitting far less smoke, powders today come in many formulas and strengths. 

AR-15 Calibers

As mentioned in our article AR15: General Specifications and Terminology (Jargon) ( by far, most ar15s are chambered in 5.56 NATO, the original caliber of the platform was designed for. Though producing less pressure, the outer dimension of the .223 Remington cartridge are nearly identical to that of the 5.56 NATO. Allowing it, (the .223 Remington) to be fired from ar15 barrels chambered in 5.56 NATO. But not the other way around. Do not attempt to fire 5.56 ammunition out of a barrel marked .223 Remington, as they are not designed for the additional pressure and forces of the 5.56 NATO cartridge.  Though there are many calibers available for the Ar15 platform, the beauty of the design allows users to change calibers by swapping the upper, magazines and in some cases the bolt carrier group. All of which are parts that can be obtained without the need of FFL involvement.