Ammunition can be confusing even for those with experience. There are so many different names for calibers, types of bullets, grain weights, and variations in energy. It’s essential to know what ammo your gun will shoot, what types can be harmful, and what other names your ammo can be referred to so that you can increase your chances of finding it available. In any case, never use ammunition not specifically designated for use in your firearm. Failure to use the correct type or caliber of ammunition may cause your firearm to jam, fail to fire, or even generate excessive pressure which can damage or even rupture your firearm, causing personal injury, death, or property damage. To avoid overwhelming you with information, I’m only going to cover the most common handgun calibers in this post.

First, let’s define what a cartridge or ammunition is. Oftentimes people wrongfully call it a bullet, which technically speaking isn’t the correct term. The bullet is one of four components that make up a cartridge, which is the actual term to describe ammunition. A handgun cartridge consists of four parts: bullet, casing, primer, and powder. The bullet is the projectile that leaves the barrel of the gun when you squeeze the trigger. The casing houses all of the components. The primer is filled with an impact sensitive compound that ignites when struck by a firing pin. The powder is the fuel source that propels the bullet down the barrel.

caliber understanding

Now, some of the most common handgun calibers in terms of size and power, starting with the smallest to greatest are .22LR (long rifle), 380 Auto, 9mm, 38 Special, 357 Magnum, 40 S&W, and 45 ACP (automatic Colt pistol). There are many other calibers, but these are some of the most common handgun calibers you’ll see at stores. The numbers and letters, if we’re going in numeric order, make little to no sense to someone new to firearms. So why are calibers named the way they are? There are two main conventions of caliber names which use either millimeters or inches for the diameter in the name. Let’s take 9mm and 380 Auto for example, in principle, both are named after the diameter of the cartridge; 9mm is the metric diameter and 380 Auto refers to 0.380 inches.

In actuality, 9mm and .380 have the same bullet diameter, but are different calibers. 9mm is longer in both bullet and casing and has more power while .380 is shorter in both bullet and casing and is less powerful. Some of you who are more experienced with metric conversions may have realized that 9mm converts to around 0.354 inches, not 0.380. So, why do they have the same diameter, but a different measured diameter? When the calibers were originally named, the 9mm was named according to the diameter of just the bullet and the 380 was named for the diameter of the cartridge with the bullet seated in the casing. It’s a very small difference, but one of the first subtleties of caliber naming that causes confusion.

caliber understanding

This same convention of measurement causes confusion for two other popular rounds, the .38 Special and .357 Magnum. In this case, both use the same diameter bullet. A .357 Magnum is simply a longer and much higher-pressure cartridge that shares the same diameter bullet as the shorter 38 Special. Since the cartridge of the .357 is just a longer version, the lower pressure 38 Special is safe to fire in a .357 Magnum handgun. The reverse however isn’t an option, as the cartridge is too long. 357 Sig, an entirely different round, is a shorter bottle-neck cartridge, but it has a larger case diameter. Some people might confuse this for the same caliber and just assume it’s Sig Sauer’s version of .357 Magnum ammo. It’s not.

Returning to the 9mm cartridge, it should be addressed that 9mm is known by multiple names such as 9mm Luger, 9x19mm Parabellum, 9mm Nato, and 9mm Parabellum. All of these are the same caliber and are manufactured all over the world. This doesn’t mean that if you see a box labeled 9mm on it, that it’s compatible ammunition for your 9mm firearm. One 9mm you may find on the shelf that is not compatible with your 9mm pistol is actually 380, which is also referred to as 380 ACP, 9x17mm*, 9mm Browning, 9mm Corto, 9mm Kurz, and 9mm Short. *in a caliber name if there is an x after a first number the number after it indicates the cartridge length.

caliber understanding

As you shop around for ammo, you’re going to notice that even if you find the same caliber, sometimes the bullet, that projectile itself that shoots out of your barrel, looks different. Traditionally, there are two types of bullets you will commonly run into, round-nose or hollow-point. Round nose is used as training ammunition and can be a full metal jacket (FMJ) or cast from lead. These types of bullets are used for practicing with, due to the simple fact that they are more affordable to shoot. One thing to note, if the bullet is strictly cast lead, it will cost less but will foul your barrel. Some companies, such as Federal Premium®, offer coated variations like its Syntech® line. These bullets are made from bare lead but are coated with a polymer which creates a smooth surface that creates less friction in the barrel, causing less lead fouling.

Hollow-point ammunition is what you will want to select for self-defense. Hollow-point ammo is easily recognized by the cavity at the front of the bullet. This causes the bullet to expand in a predictable manner creating a mushroom effect, expanding when it hits the target. This serves two purposes that are important for self-defense; it causes more damage with the larger hole and it dissipates more energy into the target as well as imparting that energy to the target, it has less chance of going completely through the target and hitting an unintended target. It should be noted that you can shoot hollow-point ammunition on the range, but it’s not common due to the cost difference.

caliber understanding

Once you’ve decided on the proper ammunition for your firearm, test fire it. Not all ammunition and firearms cycle. You might think it’s a matter of the quality of the ammo or the gun, but this may not always be the case. Some brands, grains, etc. cycle better than others and at times it might prefer the less expensive ammo. It’s important to do this, especially with hollow point ammunition so that you can rest assured that a malfunction is less likely to occur in a self-defensive situation.