Guide to Slugging a Rifle’s Bore

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Dec 2, 2024
8 min read

Updated: Oct 22

Disclaimer

The information provided in this blog post is intended for experienced gunsmiths, firearm enthusiasts, and advanced shooters who are familiar with proper firearm handling and safety procedures. The process of slugging a rifle’s bore involves the use of specialized tools and techniques, and improper execution can result in damage to your firearm or personal injury.

Always ensure your firearm is unloaded and cleared before starting any gunsmithing work. Use appropriate tools, exercise caution, and work in a safe, controlled environment. If you are unsure about any part of this process or lack the necessary experience, consult a professional gunsmith or contact Redleg Guns for expert assistance.

Redleg Guns is not responsible for any damage to firearms, personal injury, or other consequences resulting from the improper application of the techniques described in this post. Follow all local, state, and federal laws related to firearms and gunsmithing.

When in doubt, seek professional help to ensure your firearm operates safely and effectively.

Precision Starts with Understanding Your Barrel

For precision shooters and gunsmiths, the journey to better accuracy often starts inside the barrel. One of the most powerful techniques for diagnosing a barrel’s performance potential or identifying issues is slugging the bore. Whether you’re working on a precision bolt-action rifle, restoring a vintage firearm, or evaluating a high-dollar custom barrel, slugging reveals the barrel’s true dimensions, tight and loose spots, and even potential defects.

At Redleg Guns, slugging isn’t just a technique it’s a critical step in ensuring every rifle we work on meets our standards for precision and reliability. Let’s dive into how to slug a bore properly and what insights you can gain from the process.

What Is Slugging and Why Does It Matter?

Slugging a bore involves pushing a soft lead slug through the barrel to measure its true internal dimensions. This process can reveal:

Tight or Loose Spots: These may indicate machining imperfections or wear.
Bore Erosion: You can feel throat and bore wear using this method, which helps diagnose the overall condition of the barrel. A bore scope will allow you to see some of this as well.
Bore Roundness: Measuring the slug in multiple directions lets you determine whether the bore is perfectly round or has oval or irregular spots.
Crown Condition: The crown is critical for accuracy, and burrs or imperfections here can affect measurements and performance.

By identifying these issues, you can decide whether a barrel can be salvaged with lapping, or if it’s so far out of spec that replacement is necessary. Even new barrels can benefit from slugging, as no two barrels are the same even from high-end manufacturers.

Tools You’ll Need for Slugging a Rifle Bore

At Redleg Guns, we’ve fine-tuned our slugging process with tools that make the job precise and safe for the barrel:

Neco Lead Slugs (Cut in Half): Pure lead slugs slightly oversized for the bore. These slugs are soft enough to fully engage the rifling without damaging the barrel.
Brass Rods: A variety of lengths and diameters to match different calibers and barrel lengths:
- Diameters: 3/16", 1/4", 5/16"
- Lengths: 3", 10", 23", 36"
20W-50 Oil: Used as a lubricant for both the bore and the slug to ensure smooth movement.
Micrometer: For measuring the slug’s dimensions across both the lands and grooves with precision. It’s important to note that a micrometer is essential for this process, as it provides the level of accuracy required to measure bore dimensions to within thousandths of an inch. While calipers are useful for many tasks, they are not precise enough for this level of measurement and may lead to incorrect conclusions about your barrel’s condition. For reliable results, always use a quality micrometer.
Cleaning Supplies: A clean bore is essential before starting the process.

Step-by-Step Guide to Slugging Your Bore

1. Clean the Bore: Before slugging, thoroughly clean the barrel. Carbon fouling, copper deposits, or debris can skew results and make it harder to detect true tight or loose spots.

2. Prepare the Barrel and Slug:

Apply a thin coat of 20W-50 oil to the bore.
Lubricate the lead slug.
Choose the correct diameter brass rod for your barrel and have different lengths ready for different stages of the process.

3. Start at the Crown:

We cut the Neco slugs to 1\2" to 5\8"
Place the lead slug at the crown (the muzzle end).
Using a mallet, gently tap the slug into the bore just enough for it to engage the rifling. Be very careful not to damage the crown, as even minor imperfections here can ruin accuracy.

4. Drive the Slug Through the Bore:

To begin slugging the bore, you’ll start by using the 3-inch brass rod to tap the lead slug just into the rifling at the muzzle. Be sure to tap gently and evenly, ensuring the slug is fully seated and engaged with the lands and grooves without damaging the crown. Once the slug is started, switch to a rod that matches the approximate length of the barrel to continue the process.

Marking the Rod for Control
Before pushing the slug through, mark the brass rod at the bore’s entry point (where it meets the muzzle). This will serve as a visual reference to help you stop before the slug is pushed completely out of the bore. This step is crucial to avoid accidentally driving the slug out too far, which would require starting over.

Pushing the Slug Through the Bore
We attach a shop-made handle to the back side of the brass rod for better control and ergonomics. A stubby screw driver that takes insert ends will work. Slowly and methodically push the slug through the bore, using steady and moderate pressure. As you do this, pay close attention to the feel of the slug as it moves through the barrel.
- Feel for Resistance Changes: Tight spots in the bore will require more effort to push through, while loose sections will feel easier.
- Work Smoothly: Avoid forcing the slug too quickly; let it move naturally under consistent pressure to prevent deforming it or missing subtle changes in bore dimensions.
By using the full-length rod and carefully monitoring how the slug moves through the barrel, you can identify and map out any tight or loose areas. This tactile feedback provides invaluable information about the condition of the bore, helping you pinpoint potential trouble spots that may require lapping or further evaluation.

5. Diagnose Throat Erosion:

To evaluate throat erosion, you can move the slug back and forth near the front of chamber. Erosion often feels like a “tighter” spot when pushing the slug forward.
Mark these areas on the outside of the barrel as you work through the bore.

6. Finish at the Chamber or Crown: Once the slug exits the barrel (either at the chamber or crown), you can remove it and prepare for measurement.

Analyzing the Slug

Now that you’ve slugged the bore, it’s time to interpret the results:

1. Measure the Slug:

Use a micrometer to measure the slug’s dimensions at several points around its circumference.
Look for differences in the land diameter (smallest measurement) and groove diameter (largest measurement).

2. Check for Consistency:

A good barrel will have consistent dimensions throughout. If the bore opens up toward the crown, it can destabilize bullets, degrade accuracy, and make the barrel unserviceable.
A bore that gets tighter toward the muzzle is generally acceptable, as it doesn’t typically hinder accuracy. However, fluting can sometimes cause bore dimensions to open up slightly due to the machining process. As long as the fluting does not extend into the last inch or two of the barrel near the crown, any impact on accuracy is usually minimal. The crown and the final section of the bore are critical to bullet stability as the projectile exits the barrel, so maintaining consistency in that area is key to precision performance.

3. Evaluate Roundness:

Rotate the slug while measuring to check if the bore is perfectly round. Barrels that aren’t round can create uneven pressure and lead to accuracy issues.

4. Identify Trouble Spots:

Tight spots, loose areas, and throat erosion should all be marked on the outside of the barrel for reference. This information is invaluable for the next step: lapping the bore.
If you encounter a loose bore, you may need to “bump up” the lead slug while it’s already in the barrel to ensure it properly engages the rifling for accurate measurement. This technique is especially useful for identifying variations in bore dimensions, such as areas that are tighter or looser than the rest of the barrel.

Here’s how to bump up a lead slug while it’s in the bore:
1. Position the Brass Rods: Once the slug is inside the bore, take two brass rods. Place one brass rod against the back of the slug (the chamber side) to hold it securely in place.
2. Apply Force to Expand the Slug: Using the second brass rod, gently strike the rod with a hammer. This creates controlled pressure that compresses the lead slug lengthwise, causing it to expand outward inside the bore. Be sure to strike with light, controlled taps to avoid over-expanding or distorting the slug.
3. Check Engagement: After bumping up the slug, push it slightly forward and back to feel whether it now makes proper contact with the rifling in the loose section of the bore. Repeat the bumping process if necessary to ensure the slug is snug enough to provide accurate measurements.
4. Measure Tight and Loose Areas: As you move the slug through the barrel, you may need to bump it up again in looser sections to maintain proper engagement. This is especially important when checking for transitions between tight and loose spots, as these areas provide critical insight into the condition of the bore.

At Redleg Guns, we slug every new barrel before chambering. High-dollar barrels, while better made, can still have slight inconsistencies. Tight barrels may require running the slug back and forth multiple times, while loose bores may need the lead slug “bumped up” for accurate measurement.

We also reject barrels that fail our standards:

Barrels that open up toward the crown.
Bores that show significant inconsistencies in diameter or roundness.

These measurements aren’t just about specs they’re about confidence. A perfectly slugged and measured bore sets the foundation for ultimate precision and reliability, whether you’re competing or hunting.

Final Thoughts

Slugging a bore is part art, part science. Done correctly, it gives you a window into your barrel’s soul its strengths, weaknesses, and true potential. Whether you’re assessing a factory barrel or evaluating a custom build, slugging provides the data you need to make informed decisions about lapping, chambering, or replacing the barrel entirely.

Stay tuned for our next post, where we’ll cover bore lapping and how to address the trouble spots you’ve identified. If you’re unsure about your rifle’s bore or want an expert’s touch, Redleg Guns offers professional slugging, lapping, and custom rifle builds. Contact us today to learn more!

Let’s Hear from You

Have you tried slugging your barrel? What challenges or surprises did you encounter? Share your experience in the comments below or reach out to us for advice.