primer of a bullet

How A Bullet Fires And What It’s Made Of


The idea behind the way a handgun bullet fires, and what materials make it do so, have changed considerably throughout centuries of innovation across a few countries.


Historically , small lead balls were loaded down smoothbore barrels, a separate combustible black powder was ignited and, boom, projectile.


However, the matter changed with modernized, rifled barrels — which have grooves measured and etched into the interior of the barrel that spin a bullet to improve velocity, range and accuracy — and advancements in ammunition technology from black powder to nitrocellulose and smokeless powder.


The anatomy of a bullet is fairly straightforward. Here’s a quick overhaul on what’s in it and how its measurements affect its intended outcome.


A Run-Down On What’s In A Bullet Casing And How It Fires


bullet casing

Most people have seen movies or video games that depict bullet casings flying from the character’s firearm and clattering to the ground nearby.


But, there’s a sequence of interconnected parts before that happens in a handgun. When a trigger is pulled, a firing pin is released and the primer (a small metal cup with a bit of chemical explosive and an anvil) is struck, causing a miniature explosion at the base of the casing.


Where the firing pin strikes matters because that is a point of difference between types of bullets and their respective calibers (more on calibers in a moment).


For example, centerfire ammunition has its primer in the center of the bottom of the cartridge, which is the sum total unit of ammunition, and rimfire ammunition is struck on the bottom near the edge (or rim).


A cartridge, or round of ammunition, contains the primer (or a percussion cap) at the bottom of the casing, a propellant that is ignited by the primer and the projectile (bullet) mounted at the top of the casing.


Once the propellant ignites, there is a rapid reaction that releases heat and gas, placing pressure on the base of the bullet and on the sides of the casing. The propellant releases while the bullet is traveling forward and out of the barrel.


The previously mentioned rifling, or spiraled grooving cut inside the barrel, will twist the bullet as it travels through the barrel. Gyroscopic inertia is applied, which in a basic interpretation means that while the bullet spins it will be harder for it to be deterred, affected of deflected on its path from A to B. Of course, forces like gravity, wind, physical hindrances and so on will all take their toll on the ultimate shot placement.


When the bullet leaves the barrel with all that gaseous and heated force, the firearm as a whole kicks backward as a result of Newton’s third law of motion — for every action there is an equal and opposite reaction. That backward motion is recoil.


To build on that, there is perceived recoil, what is felt by the shooter based on a number of factors, and actual recoil, the forced measured in Newton metres or foot pounds.


Recoil and the ultimate damage of the bullet are driven by caliber, but note, of course, that caliber is not the de facto criterion here.


How Caliber Plays A Role In Things And What Is It?


bullet calibers

Think of bullet caliber as a unit of measurement. It is the diameter of the bullet within the cartridge, and it will approximately match that of its internal, respectively sized handgun barrel.


Caliber may be presented in either inches or millimeters, depending on preference, application and handgun type. For example, .45 ACP ammunition is measured as .45 inches, but as with other ammunition the resulting actual diameter is also affected by bullet type and composition, which can present potential variability by thousandths of an inch.


An example of a caliber typically measured with the metric system is 9x19mm Parabellum.


However, and this is key for those who are new to understanding how ammunition works, there is a wide variety of measurements, and they can often be mismatched or misunderstood as meaning one type of ammunition or another. This is seen with 9mm Luger and .380 ACP, which has the same diameter as 9mm Luger but comes at a shorter length.


Without getting into the specifics of every single caliber, know that diameter size and shell length will be some indicators of the strength of the ammunition and the intensity of its recoil, but also note that recoil is mitigated by other criteria within the handgun. Another indicator of strength, although it’s in many ways subjective, is stopping power.


Stopping power boils down to the capacity of a weapon to cause enough trauma to consequently incapacitate a target. In the context of this article, it can be interpreted to mean the ability for a certain type of ammunition to actually stop a lethal threat. A 9mm full metal jacket, a popular type of ammunition with a soft core metal like lead encapsulated in a harder metal like copper, is considered to be effective, but often is not the first choice for defensive rounds, as opposed to 9mm hollow point ammunition, which is made to expand or mushroom upon impact, thereby causing a wider wound as it travels and stops within the intended target.


It’s important to pause here and mention that this is not an end-all description of calibers and types of bullets, but rather an informational introduction for the uninitiated. There is a vast array of information on both categories, which the entry-level firearms enthusiast will definitely come into contact with as they spend more time on the range.


Beyond caliber, the measurement of mass is another consideration for the efficacy and power of a type of bullet.


What Is Bullet Grain?


what is bullet grain

Bullet grain is another means of measurement for ammunition, in this case mass instead of dimension, and it is a standardized unit in the firearms industry. There are 7000 grains, abbreviated to gr., in a pound.


Bullet grain will refer to the weight of the projectile, and the options will vary in each respective caliber. For reference, a few common grains in 9mm are 115gr, 124gr and 147gr. Grain is also used to measure propellant, but is primarily a consideration for projectiles (bullets) unless one is assembling ammunition or reloading used ammunition.


Some shooters will find that their handgun model is more accurate with a specific grain within its respective caliber. Others may disagree. It depends on comfortability, gear and training.


A general rule of thumb is that heavier grain bullets will travel slower, but transfer more energy, while lighter grain bullets will travel faster. While debated, it’s a topic of consideration when looking for a preferred choice in ammunition.


The weight of the bullet can play a role in accuracy and efficacy, but is definitely not the only determining factor.


Jake Smith 

About The Author


Jake Smith (@notjakesmith) is a copywriter in his final year of studying public relations and apparel at the University of Idaho.

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