Bullet Breakdown: How Diameter, Grain and Length Factor

Posted by Team Springfield

Sep 25, 2017 8:13:20 AM


Fairly new to the world of firearms and ammunition? Pistol newcomers and first-timers might have a lot of questions when it comes to factory ammunition and bullet specifications. What does caliber relate to? What does grain measurement and overall length refer to? And why do these specs matter?

Here's a breakdown of these bullet specs and what they mean for overall performance.

Diameter / CALIBER

This measurement refers to the diameter of the bullet or projectile (and the corresponding internal diameter of the barrel). Diameter translates to caliber, and it's the most important spec, as it roughly relates to the name of the cartridge, too. Caliber is the term you're probably most familiar with and the one you'll need to know when you buy ammunition.

We loosely use the term "caliber" to refer to the cartridges of our firearms.

A cartridge's measurement is listed in either hundredths of inches or millimeters. Let's start with 9mm. This round, the most popular handgun cartridge in the world, utilizes a nine-millimeter (or .35-inch) diameter bullet.

Here is a list of standard calibers and cartridges you are likely to come across:

Common Pistol Calibers (measured in millimeters)

  • 9mm
  • 10mm

Common Pistol Calibers (measured in hundredths of an inch)

  • .22
  • .380
  • .38
  • .40
  • .44
  • .45 

Most cartridges are properly named based on diameter (i.e., like the 9mm, the .45ACP bullet is actually .45 inches in diameter). Others are only close. The .38 special round, the most-common revolver cartridge, utilizes a .357-inch bullet diameter, which is actually closer to .36 caliber.  We know, it's a bit confusing. But wait—there's more.


Speaking of weight, let's set the record straight on grain. When it comes to buying ammunition, the "grain" indicated on the box actually refers specifically to the weight or mass of the bullet. Grain is a unit of measure. There are 7,000 grains in a pound and 437.5 grains in an ounce.

A 230-grain, .45-caliber bullet weighs .525 ounces, a little more than half an ounce. 

A 115-grain, 9mm bullet weighs .262 ounces, half the weight of the 230-grain bullet. 


To make it more interesting, European ammunition is measured in grams. That same 115-grain, 9mm bullet weight would be listed as 7.45 grams outside of the United States.

Common Bullet weight in Grains (.380 Caliber)

  • 95 gr

Common Bullet weight in Grains (9mm Caliber)

  • 115 gr
  • 124 gr
  • 147 gr

Common Bullet Grains (.40 Caliber)

  • 165 gr
  • 180 gr

Common Bullet Grains (10mm Caliber)

  • 180 gr
  • 200 gr

Common Bullet Grains (.45ACP Caliber)

  • 185 gr
  • 200 gr
  • 230 gr

As you can see, the heaviest, most commonly found 9mm round (a 147-grain bullet) is still lighter than one of the lightest-weight .45ACP rounds (185 grains). Depending on your personal preferences and what you are trying to accomplish with your pistol, you may choose a heavier or lighter bullet weight. 


Typically for a given cartridge, the heavier the bullet weight, the lower the velocity, and the lighter the bullet, the higher the velocity. Bullet grain varies based on caliber and manufacturer guidelines. But generally speaking, smaller caliber rounds use lighter-weight bullets and larger caliber rounds use heavier bullets.


We've hit diameter and weight—length is up next. Sometimes a cartridge is also designated by its case length. (The case is that empty shell that ejects from the semi-auto pistol after you fire a round.) 

For instance, that popular 9mm cartridge—the one everyone loves to shoot—is referred to as 9x19.

Nine is the bullet diameter, and 19 is the case lengthBoth measurements are in millimeters. This common cartridge is also known as 9mm luger and 9mm parabellum

It can become confusing, as other cartridges are also 9mm in bullet diameter, but the case length is different:

  • 9x18
  • 9mm short
  • 9x21
  • 9x23

The examples above are all very different cartridges. They are not interchangeable, as their length is different, but all are designated as 9mm.

Be sure you are purchasing the correct ammunition based on your pistol's specifications.  


We have only touched on the basic characteristics of ammunition, and there is so much more to learn. The ammunition options may seem endless and overwhelming as a new shooter, but with research, experience and testing, you won't be an ammunition amateur for long.

Shooting, testing and studying different types of ammunition can be fun, challenging and rewarding. It's called ballisticsthe science of the motion of projectiles in flight.  And there is a plethora of available information on the topic. 

You will quickly learn that all ammunition is not created equally. Within a specific caliber, the weight of the bullet, design of the bullet and velocity have a definite effect on the performance, recoil, accuracy, precision—and the terminal ballistics of the projectile.

Ballistic testing of factory ammunition is a learned skill that you will gradually develop. It may take time for newcomers to get it down to a science, but most shooters enjoy ballistics equally as much as shooting itself.  

For more info on choosing the right pistol ammunition, check out this blog post by Team Springfield™ member Kyle Schmidt—A Beginner's Guide To Choosing Pistol Ammo.

More questions? Reach out to your local range, shooting pro, gun shop or sporting goods store for additional help. And before you buy ammunition, always make certain you have the correct caliber for you model of firearm.

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Topics: First Time Gun Buyers, Accessories

Springfield Armory® recommends you seek qualified and competent training from a certified instructor prior to handling any firearm and be sure to read your owner’s manual.  These articles are considered to be suggestions and not recommendations from Springfield Armory®.