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9mm Case Dimensions (Length and Diameter)

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The actual measurement of 9 mm (0.35 in) cases can vary. Reloading manuals typically list them as 0.754 inches (19.05 mm) long, but the acceptable range is known to be somewhere between 0.744 and 0.754. The case mouth’s diameter is usually 0.39 inches (9.91 mm), while the web diameter is 0.38 inches (9.65 mm).

To read a detailed and highly informative discussion on the topic, go here.

In this article, I’ll touch on whether these dimensions affect penetration. I’ll also discuss how case length and diameter matter in the real world, and how to measure them.

Do 9mm Dimensions Affect Penetration?

9mm dimensions do not make a significant difference in penetration. However, the weight of the bullet does. A heavier bullet will penetrate further than a lighter bullet.

This is due to something called momentum. Momentum is the mass of an object multiplied by its velocity. That means a heavier bullet usually has more mass and, therefore, more momentum.

When a bullet hits its destination, the shot’s momentum is transferred to the target. The greater the speed, the greater the force of the impact, and the deeper the bullet will penetrate.

However, the length and diameter of the case affect how much powder you can fit in it. The amount of powder you use affects the bullet’s velocity. So, if you want to increase the penetration power of your shot, you’ll need a longer case.

Related 5 Best Types of 9mm Ammo To Stockpile.

When Does the Length of the 9mm Matter?

Several 9mm cases on a brick floor

Manufacturers usually list the nominal case length, which is the industry-standard measurement. This is the length of an empty case with no bullet or powder. However, the length increases when you add pallets and powder to the case. This is due to something called case neck tension.

Case neck tension is the force that holds the bullet in place. It’s created by the pressure of the powder pushing against the back of the shell. The more powder you add, the greater the case neck tension and the longer the case becomes.

So, when a manufacturer lists the 9 mm (0.35 in) case length as 0.75 inches (19.05 mm), they are actually listing the nominal case length. However, these measurements aren’t always accurate, which may not be a big deal. The slight difference in size doesn’t affect the accuracy or anything else unless there’s something seriously wrong with your barrel or bullet.

The length of the 9 mm (0.35 in) case matters under two circumstances: when you are using a subsonic load and when you are using a cast bullet. As mentioned earlier, the length can also affect how much powder you can put in it.

You’re Using a Subsonic Load

Subsonic means “below the speed of sound.” In other words, it’s a load that won’t produce a sonic boom when fired. This is important because subsonic loads are often used with suppressors. Suppressors help reduce the noise of a gunshot, but they only work if the bullet is traveling below the speed of sound.

If the bullet travels faster than the speed of sound, the air will compress in front of it and produce a sonic boom. This boom will be just as loud as the gunshot itself, so there’s no point in using a suppressor.

You’re Using Cast Bullets

Another situation where the length of the 9 mm (0.35 in) case matters is when you’re using a cast bullet. Cast bullets are made by pouring molten lead into a mold. They’re often used for target practice because they’re cheaper than jacketed bullets.

When Does the Diameter of the 9mm Matter?

The diameter of the 9mm matters when it has to fit in the gun’s chamber. Manufacturers usually list the nominal case diameter, which is the industry-standard measurement and is the diameter of an empty case. The diameter increases when you add pallets and powder to the case. 

The increase in diameter is due to something called neck bulging. Neck bulging is the expansion of the case neck caused by the pressure of the powder pushing against the back of the shell. The more powder you add, the greater the neck bulging and the bigger the case diameter becomes.

How Is the Length of a Case Measured?

The length of a case is measured from the base to where the bearing surface ends. The easiest way to do this is to use a digital caliper tool or micrometer.

To use a caliper, get the case you want to measure, then follow these steps:

  1. Hold the case so the mouth is facing up and the base is flush with the top of the caliper.
  2. Close the caliper until it touches the case mouth.
  3. Read the measurement on the caliper scale.

It’s important to note that the caliper’s jaws shouldn’t clamp down too hard on the case wall, as this will distort the final measurement. Instead, let them touch lightly. You also want to make sure the caliper is perpendicular to the case to get an accurate measurement.

How Is the Diameter of a Case Measured?

The diameter of a case is measured using calipers. You will want to measure the diameter at the case mouth, but you can also take measurements at the base and shoulder. The diameter can vary depending on where you measure, so it is important to be consistent when taking measurements.

To use a caliper, you will need to zero the caliper. That means you place the caliper’s jaws on a flat surface and turn the knob until the pointer is at the zero mark. Once the caliper is zeroed, do this:

  1. Hold the case by the base and insert the caliper jaws into the mouth of the case until they’re touching the inside wall.
  2. Gently squeeze the caliper handles until they touch the case walls.
  3. Read the measurement.

Final Thoughts

Manufacturers use nominal case dimensions when they list the 9 mm (0.35 in) case length and diameter. This is the industry-standard measurement that everyone uses, but it isn’t particularly accurate. For starters, when you add a bullet and powder to the case, the length and diameter will change due to neck tension and bulging. These changes are usually small and don’t have a significant effect on penetration.

For more, check out Are Shooting Gloves Worth It? (Pros and Cons).

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