What is “caliber”? What is it a measure of?

“Whats a bigger caliber .40 or .380?” “Whats the difference between a 9mm and a .357?” “What does {caliber} mean?”

Caliber is basically the measure of the diameter of the projectile being fired. It is also a direct measurement of the diameter of the bore of a particular firearm. This measure can either be in fraction of an inch (i.e. .380) OR in millimeters (i.e. 9mm).

Common handgun calibers comparison

One common thing to remember in regards to caliber is that SOME calibers are not EXACT measurements. For instance, the .44 Remington magnum is NOT exactly .44 of an inch. It is closer to .429 of an inch than .44 of an inch. Another thing to remember about calibers is that we commonly see SAME caliber bullets stuffed in different size casings. For example, take a look at the figure above. If you did a little math, you will see that the .357 and the 9mm are the same diameter. The .40 S&W and the 10mm are also the same diameter. Let’s observe how this looks in a rifle example.

From left to right .308 Winchester, 30-06 Springfield, .300 Winchester Magnum (Missing is the Winchester 30-30, and the 300 Remington Ultra magnum)

These 5 cartridges are all in the “30 caliber family”. The projectile of ALL 5 of these cartridges measure’s .308 of an inch. These are essentially THE same SLUG stuffed in a different casing. “So which one is more powerful?” Take a wild guess between the 3 in the photo as to which one is more powerful. If you picked the one on the far right, you are correct. GENERAL RULE OF THUMB. Look at the size of the casing. The LARGER the casing, THE MORE gunpowder its going to hold. The MORE GUN POWDER the casing holds the faster the bullet is going to travel. The faster that bullet travels the more kinetic energy its going to have etc..

GRAIN-The STANDARD unit of Bullet weight measurement

You may hear or read of different bullets being 200 grains (often abbreviated “gr”), or 180 gr. This is the WEIGHT of the bullet/slug/projectile. For example, The .40 S&W is available in 135 gr, 155gr, 165 gr and 180 gr. Lets ask ourselves a few common sense questions and establish a few GENERAL ground rules (there are a few exceptions with high performance ammo manufacturers) 1. The LIGHTER the projectile, the higher the velocity (bullet will move faster through the air) but will not penetrate as deeply as a heavier projectile. 2. The HEAVIER the projectile the slower it will travel, but will work wonders when it comes to penetrating surfaces and retaining momentum. SOO..if you had a .40 S&W pistol with FMJ’s and you wanted to PUNCH THROUGH A WINDSHIELD (or sheet rock, or sheet metal, or plywood) and knock out a target in the front seat, which weight would you use? 135 gr or 180 gr? The 180 gr choice would be more practical. The slower moving 180 gr will retain MUCH more of its kinetic energy than the much FASTER moving 165 gr.

“Sooo…since a 230 gr .45ACP is a bigger bullet than the .357 Magnum which weighs 158 gr, is it more powerful?” The Answer is NO. Take a look at the figure below. 

.357 S&W Magnum (158 gr) on left .380 ACP (95 gr) in middle and .45 ACP (230gr) on right.

Obviously the .45ACP bullet diameter is wider than the .357 magnum. The .45 ACP also weighs more. SO…which one generates more power? To answer this question, Remember the GENERAL rule of thumb we previously discussed. That .357 magnum has a LOT more powder in its casing. This allows the .357’s 158 gr projectile to travel a LOT faster than the 230 gr .45 ACP. This considerable advantage in velocity allows the .357 to generate over TWICE the energy of the LARGER .45 ACP. GENERAL RULE OF THUMB…A LARGER DIAMETER CALIBER IS [NOT] ALWAYS MORE POWERFUL.



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