Why Bullet Diameter Matters: .355″ vs .356″ for 9mm Reloading

When it comes to 9mm reloading, small details can make a big difference. One of the most commonly overlooked specifications is bullet diameter.

You will often see 9mm bullets advertised as .355″ or .356″, and it can leave reloaders wondering:

Does that 0.001″ really matter?

The short answer is: yes, it can.

What Is the Correct Diameter for a 9mm Bullet?

A typical 9mm barrel has a nominal groove diameter of approximately .355″. Bullets are commonly manufactured slightly larger than the groove diameter so they can properly engage the rifling when fired.

This is why you will commonly find 9mm projectiles manufactured at approximately .355″, .356″ or slightly different dimensions depending on the manufacturer and bullet design.

The important thing isn't simply the number printed on the box.

Consistency is critical.

Why Does Bullet Diameter Matter?

The bullet needs to interact correctly with the barrel's rifling.

A bullet that is undersized may not engage the rifling as consistently as intended. It can also provide a less effective gas seal.

This can lead to problems such as:

• Increased gas cutting

• Leading

• Reduced accuracy

• Inconsistent velocity

• Poor bullet-to-barrel engagement

• In extreme cases, potential stability problems such as keyholing or tumbling

For reloaders chasing accuracy, this becomes particularly important because we're not only dealing with bullet diameter. We're also dealing with:

• Bullet weight consistency

• Bullet shape

• Seating depth

• Cartridge overall length

• Case tension

• Expander dimensions

• Crimp

• Coating thickness

• Barrel dimensions

All of these variables can influence the final result.

The Problem Can Start During Seating

Here's something many reloaders don't consider.

A coated bullet may measure .356″ before seating, but the finished cartridge can tell a different story.

If excessive seating force, insufficient case expansion, incorrect expander dimensions or other loading variables cause the bullet to be squeezed during seating, the bullet can potentially be reduced in diameter.

For example:

.356″ bullet → .355″ → .354″

That may only look like a tiny difference on paper, but you're now dealing with a projectile that has been physically altered during the loading process.

And that's where the expander becomes extremely important.

Your Expander Is More Important Than You Think

One of the biggest problems when loading coated bullets is getting the case mouth prepared correctly before seating.

Too little expansion can cause the case to scrape, shave or damage the coating and can require excessive force to seat the bullet.

Too much expansion, on the other hand, can reduce case tension and create its own problems.

You need the right amount of expansion for the bullet you're loading.

That's exactly why Bullet Corp developed our Expander Thru Plugs.

Bullet Corp Expander Thru Plugs

The Bullet Corp Expander Thru Plug is designed to properly prepare the inside of the case before the bullet is seated.

Instead of simply flaring the very top of the case, the expander works inside the case to create the necessary clearance for the bullet.

This helps address some of the most common problems reloaders experience when seating coated bullets:

• Bullet shaving

• Coating damage

• Excessive seating pressure

• Difficult bullet seating

• Inconsistent bullet seating

• Case deformation

• Poor bullet alignment

The result is a smoother, more controlled seating operation.

Why This Matters With .356″ Bullets

If you're loading a .356″ coated bullet, the inside diameter of your case needs to accommodate that bullet correctly.

If your expander isn't doing its job, you're asking the bullet to force its way into a case that hasn't been properly prepared.

That's not what you want.

A properly matched expander and bullet combination can make a significant difference to the loading process.

An Undersized Bullet Can Create Bigger Problems

Bullet diameter isn't just an accuracy consideration.

The bullet also needs to create an effective seal with the barrel.

If a projectile is significantly undersized for the application, combustion gases can get past the projectile more easily.

That can contribute to gas cutting and leading, particularly with lead-based projectiles.

Poor engagement with the rifling can also negatively affect accuracy and stability.

If you're seeing unexplained leading, poor accuracy or bullets that aren't stabilising correctly, bullet diameter is one of the things worth investigating.

Don't automatically blame the barrel or firearm.

Measure the bullet.

Why Bullet Corp Uses .356″ for Its 9mm Coated Bullets

At Bullet Corp, our 9mm coated bullets are manufactured at approximately .356″.

The goal is to provide reloaders with a consistently dimensioned projectile designed specifically for 9mm applications.

But manufacturing the bullet correctly is only one part of the equation.

Your loading equipment and process matter too.

That's why we don't just make the bullets.

We make the components that help you load them correctly.

Our Expander Thru Plugs are designed to complement the loading process and help solve the expansion and seating problems that reloaders commonly encounter.

Don't Confuse Bullet Diameter With Case Neck Diameter

Another common mistake is thinking that the bullet should simply fit loosely into the case.

It shouldn't.

The case needs to grip the bullet properly through case tension.

That's why the sizing and expansion process is so important.

The expander needs to provide enough clearance for the bullet to enter the case without unnecessarily damaging or deforming it, while still leaving appropriate case tension after seating.

What Should Reloaders Check?

If you're looking for consistent 9mm ammunition, don't focus only on the bullet.

Look at the entire loading process.

1. Bullet diameter

Measure your bullets and verify what you're actually loading.

2. Bullet weight

Check consistency if you're chasing accuracy.

3. Case preparation

Your sizing and expansion need to work together.

4. Expander dimensions

Make sure your expander is appropriate for the bullet you're loading.

5. Seating force

If you're having to force the bullet into the case, something may be wrong.

6. Seating depth

Keep your COL consistent and follow appropriate published loading data.

7. Crimp

Don't assume more crimp is better.

8. Finished ammunition

Measure what comes out of your press, not just what went into it.

Bullet Corp: Bullets + The Right Loading Equipment

At Bullet Corp, we don't just sell bullets.

We understand that the entire loading process matters.

That's why our product range includes both high-quality coated bullets and the equipment needed to properly prepare your cases for seating.

Our 9mm Match Pro bullets are manufactured at approximately .356″, while our Expander Thru Plugs are designed to help reloaders properly prepare the case before seating.

The goal is simple:

Better components. Better preparation. More consistent ammunition.

Final Thoughts

That tiny .001″ difference between .355″ and .356″ isn't something reloaders should automatically ignore.

Bullet diameter, case expansion, case tension and seating all work together.

If the bullet is undersized, you can potentially run into problems with gas sealing, leading, accuracy and stability.

If the case isn't expanded correctly, you can damage the coating, deform the bullet or make seating unnecessarily difficult.

Get the bullet right. Get the expansion right. Get the seating right.

That's where consistent ammunition starts.

Ready to Fix Your Expansion Problems?

Check out the Bullet Corp Expander Thru Plugs and our 9mm Match Pro bullets and give your reloading setup the components it deserves.

Bullet Corp — Built for Reloaders.

Important Safety Note

Always follow reputable loading data and safe reloading practices for your specific firearm and components. Never assume that a load developed with one projectile is automatically safe with another projectile, even if the bullets appear similar.

Bullet Corp recommends that reloaders perform their own load development and verify their ammunition carefully before use.