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What Size Is It?

Ammunition terms can be confusing for both new AND experienced shooters. We aim (pun intended) to take some of the mystery out of these terms.


There are a broad range of calibers available; .22, 9mm, .45, 10mm, etc., but what does it mean? Caliber is simply a measure of the diameter. Many people refer to it as the diameter of the round, but technically speaking, it is the diameter of the barrel that the round is designed to fit into.

A .45 caliber barrel has a diameter of .45 inches. Whereas a .22 caliber barrel has a diameter of .22 inches. What about 9mm though? It is still the diameter, just measured using the metric system.

Wait a minute! Can't you shoot a .38 round through a .357 revolver?

Lands and grooves in a barrel
Lands & Grooves In A Barrel

Yes you can! This often creates confusion when it comes to the caliber designation. There is a long history behind this irregularity, but we will sum it up for simplicity sake.

There are two different ways to measure the diameter of the barrel. One way is to measure from groove to groove, while the other is to measure from land to land. You will get different measurements depending on which method you choose.

What about .357 Magnum and .357 Sig OR .45 ACP and .45 GAP, are these rounds interchangeable?

While they have the same diameter, they ARE NOT interchangeable. This is because they are not the same length. They must be used in the firearm that is chambered for THAT particular caliber.

Can a .40 S&W only be shot through a Smith & Wesson firearm?

No, the S&W represents the company that first manufactured that particular caliber. It is similar to the .45 ACP (Auto Colt Pistol) round. It is basically a trademark to credit the company that created and/or developed it. You can shoot this round through any firearm that is designated to handle that particular caliber.


Another confusing term is "grain". Many people mistakenly believe that this refers to the number of grains of primer found in a cartridge. Grain is actually a measure of weight. In fact, 1 pound equals approximately 7,000 grains. Typically the lighter the bullet is, the faster it will travel and the heavier it is, the more impact it will deliver.

Understanding these terms will help you to understand how to select the best ammunition for your needs.


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