Mastering Extreme Accuracy in Gunsmithing: A Detailed Approach to Precision Chambering Rifle Barrels
Updated: 7 minutes ago
A chamber that is not concentric with the bore cannot be corrected later. So we cut it true the first time. The bore is dialed in to within .0001 to .0002 inch just ahead of the throat, the chamber is bored with a boring bar before any reamer touches it, the finish reamer runs in a fixed holder so the machine holds the alignment instead of the pilot bushing, and the bushing comes out for the last stretch of the cut so it never rides on the bore in front of the finished throat. Every barrel leaves the lathe under .0005 inch runout, verified by indicator, with headspace set between .0005 and .0025 inch. The rest of this article is why each of those steps is there, and what goes wrong when one of them is skipped.
A chamber is one of the few things on a rifle you cannot fix with a better load, a better scope or a better trigger. Everything else can be adjusted, replaced or tuned. Two axes that are not in line with each other stay out of line until the barrel is set back and recut or replaced. That is why this operation gets the attention it gets.
Understanding the Role of Chambering in Rifle Accuracy
The chambering process is crucial because it directly impacts how the bullet enters the bore and aligns with the rifling. Even a minor misalignment can cause the bullet to wobble, degrade accuracy, and impact consistency. Traditional chambering methods often leave room for these errors. After years of meticulous testing and refinements, we have developed a method that overcomes these traditional challenges, ensuring perfect alignment and maximum accuracy for precision chambering rifle barrels.
Where Traditional Chambering Methods Leave Room for Error
Traditional methods like “chambering between centers” or the “dial both ends” approach have limitations:
Bore Curvature Issues: All barrels have some degree of bore curvature. When using the "dial both ends" method, where the bore is aligned at both the throat and crown ends, the curvature between these two points can cause the bore to enter the chamber at an angle. This misalignment affects how the bullet aligns with the rifling and exits the barrel, impacting accuracy.
Runout and Misalignment: Inconsistent support and alignment during the chambering process can result in measurable runout (misalignment of the bore axis with the chamber axis). This issue can cause the bullet to enter the bore at a slight angle, reducing accuracy.
How We Chamber a Barrel, Step by Step
To address these issues, we use a chambering method that focuses on aligning the short section of the bore just ahead of the throat perfectly with the chamber. Here’s a detailed breakdown of our process:
Action Preparation and Bolt Lug Fitting
Before starting the chambering process, we ensure the rifle’s action is correctly prepared to support maximum accuracy:
Bolt Lug Contact: Proper bolt lug contact is essential for benchrest-quality accuracy. We ensure all lugs have at least 50% contact with their seats in the receiver, reducing action flex and vibration during firing, which can affect accuracy.
Bolt Play Reduction: We reduce bolt play by sleeving the original bolt to enlarge its diameter or by applying a thin layer of epoxy inside the bolt bore raceway. This step minimizes unwanted bolt movement, enhancing consistency and accuracy.
Ensuring Smooth Operation: We make sure the firing pin and all bolt parts operate smoothly without any binding. This includes checking for proper firing pin travel and spring tension to ensure reliable ignition. Even small issues in these areas can affect the rifle's performance.
Lathe Setup and Initial Bore Alignment for Precision Chambering Rifle Barrels
The first step involves setting up the barrel in the lathe. We use a modified "through-the-headstock" method:
Barrel Support: The chamber end of the barrel is supported in a 4-jaw chuck, while the opposite end is supported by a "spider" with four opposing screws on the outboard end of the headstock spindle. This setup allows us to make precise adjustments to align the bore.
Both of those bolt operations have their own articles. Lug contact is checked and corrected here:
And sleeving a bolt to take the play out of it is here:
Shimming for Precision: To ensure the barrel pivots correctly during alignment, we use small L-shaped aluminum shims between the chuck jaws and the barrel. This setup allows for fine adjustments and prevents the barrel from slipping, maintaining alignment accuracy.
Precision Dialing-In Process
The key to our method is precise dialing-in of the bore section just ahead of the throat:
Indicator Setup: We use a Mitotoyu .0001" dial test indicator with a 1.5" long tip to measure runout directly in the bore grooves. This indicator is extremely sensitive and allows us to detect even the slightest misalignment.
Bore Alignment: We start by aligning the bore within .001" at the chamber end and pre-drill the chamber. Then, we fine-tune the alignment, adjusting the bore to run true to within .0001" to .0002". This precision ensures the bore is perfectly aligned with the chamber, eliminating any angle that could affect bullet trajectory.
Chamber Boring and Reaming
After achieving perfect bore alignment, the next step is boring and reaming the chamber:
Boring the Chamber: Using a small boring bar, we machine the chamber hole true, aligning it dead-center to the bore. This step ensures that when the chamber reamer is used, it follows the trued hole perfectly, maintaining the alignment from the chamber to the bore.
Reaming the Chamber: The chamber reamer is carefully fed into the bored hole, ensuring it aligns perfectly with the bore ahead of the chamber. We use slower RPMs (300 to 500)and a slow feed rate initially to avoid reamer chatter, which can cause imperfections in the chamber.
Measuring and Minimizing Reamer Chatter: During the chamber-cutting process, we constantly monitor the depth and quality of the chamber to detect any signs of reamer chatter. To measure chatter, we use a dial test indicator, which provides precise readings of any surface irregularities. To prevent chatter, we employ a technique where wax paper or a slit patch is placed over the reamer nose. This method effectively dampens vibrations, ensuring a smooth, chatter-free finish. Additionally, we use headspace and feeler gauges to accurately measure the chamber’s depth, guaranteeing that it meets the exact specifications required for optimal performance.
Why We Never Run a Roughing Reamer
Most shops rough the chamber with one reamer and finish it with another. We do not. We pre-drill the chamber slightly undersize and short of the shoulder, then run a boring bar in to open it up to a true, round, straight hole. The finish reamer is the only reamer that ever enters the barrel.
The reason is tool condition. A drill is not rigid and it follows the hole that is already there. A boring bar cuts straight. When the finish reamer enters a bored hole there is very little metal left for it to remove, so it stays sharp far longer than a reamer that has had to hog out a rough hole first.
That matters more than it sounds like it should. A sharp reamer cuts cleanly. A dull one chatters, and it smears metal in the throat instead of cutting it. That smear is what a lot of shooters spend the first hundred rounds of a barrel's life burning out without ever knowing it was there.
That first hundred rounds is not free. A throat that was smeared rather than cut fouls harder and wears differently, and the shooter pays for it in cleaning time and in a barrel that never quite settles.
Why We Do Not Use a Floating Reamer Holder
This is the part that surprises people, and it is the key to everything that follows.
A floating reamer holder lets the reamer follow the pilot bushing riding in the bore. That works, and it is what most shops do. But it means the pilot is doing the aligning, so the pilot has to stay in the barrel for the entire cut.
We use a fixed holder instead, and we take the alignment out of the pilot and put it into the machine. The tailstock is set to run exactly true to the centerline of the lathe, left, right, up and down, and it is torqued down so that setting holds. With the bore already dialed in to that same axis, the reamer runs true the whole way in because the machine is holding it there, not the bushing.
Once the machine is doing the aligning, the pilot becomes a backup rather than the mechanism. That is what makes the next step possible.
Removing the Pilot Bushing for the Final Cut
The pilot bushing rides in the bore ahead of the cutting edges. As the reamer feeds deeper, the bushing sweeps forward through the barrel. Everything it touches early in the cut sits behind where the throat will end up, and the reamer machines straight through it as the chamber forms. The only marks that survive into the finished barrel are the ones in the short stretch of bore the bushing occupies at final depth, immediately in front of the throat.
So we pull the bushing for the last stretch of the cut, equal to the length of the bushing itself. Bushings vary from maker to maker, so the rule is the bushing you are holding, not a number on a page. Pull it for its own length and it never occupies the bore ahead of the finished throat at all.
Speedy Gonzalez teaches the same principle. In his words the pilots come off to prevent scuffing on top, as the throat terminates onto the top of the lands, and that scuffing typically leads to early copper fouling in the part of the barrel you least want it. Bartlein makes the same observation from the other direction, noting that an oversized pilot scores the tops of the lands in front of the throat.
That is a small piece of bore that most shooters will never see and never think about. It is also the exact piece that decides whether a new barrel fouls hard for its first hundred rounds or shoots clean from the start.
Bore finish is the other half of this. What a lap can and cannot do to a bore is covered here:
Final Chamber and Bore Inspections
Precision is crucial throughout the chambering process. After reaming, we perform several inspections to ensure everything is perfectly aligned:
Borescope Inspection: We use a borescope to inspect the chamber and throat for any imperfections, such as burrs or rough spots. Even a small burr can affect accuracy by tearing the bullet jacket as it enters the bore.
Runout Measurement: Using a .0001" long-reach dial test indicator, we check the runout throughout the chamber, neck, and freebore. We aim for zero or near-zero runout, indicating perfect alignment.
Headspace Setting and Final Adjustments
Proper headspace is crucial for safe and accurate firing:
Headspace Measurement: We use Go and No-Go gauges to set the headspace. Our goal is to achieve a minimum headspace of .0005" to .0025", depending on the type of action (custom or factory). This precise headspace ensures the bullet is properly seated in the chamber, reducing the risk of excessive pressure or misfires.
Final Alignment Check: After setting the headspace, we double-check the chamber and bore alignment to ensure everything remains perfectly true. Any deviation can be corrected with slight adjustments, maintaining the precision needed for extreme accuracy.
Throat Cutting and Crown Finishing
The final steps involve customizing the throat and cutting the crown:
Throat Customization: Depending on the customer’s needs, we may cut a larger diameter neck or a longer throat. We use separate neck and throating reamers to achieve the desired dimensions, providing flexibility to meet various customer requirements.
Crown Cutting: The crown is cut while ensuring the last 1.5" to 2" of the bore is aligned perfectly straight and true. A precise crown is crucial for accuracy, as it ensures the bullet exits the barrel evenly, preventing gas blow-by that could affect trajectory.
Final Polishing and Quality Checks
After chambering and fitting the barrel:
Polishing: We lightly polish the chamber and radius the chamber mouth to ensure smooth feeding and ejection of cartridges.
Final Quality Checks: Every rifle undergoes rigorous final checks to ensure all components operate smoothly and meet our accuracy standards. This includes verifying the smooth operation of the firing pin and ensuring there is sufficient firing pin travel and spring tension for reliable ignition.
Lug fitting, firing pin travel and spring tension are action work, not barrel work, and they are worth understanding on their own:
Why This Method Produces a Straighter Chamber
By focusing on the critical alignment of the bore just ahead of the throat with the chamber, we eliminate the common issues found in traditional chambering methods. Our process ensures that the bullet enters the bore perfectly straight, minimizing any potential for accuracy loss. The result is a rifle capable of delivering exceptional accuracy, whether it’s for long-range benchrest, tactical, or hunting applications.
What You Can Check Before You Pay for a Rechamber
A rechamber is not a cheap operation and it is not always the answer. Before anyone spends money on one, there are things a shooter can check at their own bench.
Look at the fired brass. A case that comes out of the chamber with an uneven mark around the shoulder, or that needs noticeably different force to extract at the same charge, is telling you something about the chamber. Look at the throat with a borescope if you have access to one, or have a shop look. Chatter marks in the chamber walls show up as fine rings. Copper that builds in one short stretch just ahead of the throat and nowhere else is the signature of a bushing scuff or a smeared throat.
Then rule out everything cheaper first. Mounts, rings, bedding, the scope itself and the ammunition are all faster and cheaper to test than a barrel. A rifle that suddenly opened up almost never did it because the chamber changed, because chambers do not change. A rifle that has never shot well since it was new is a different conversation.
This is the diagnostic that decides it, and what it looks like when the answer turns out not to be the barrel:
If you want us to settle it, call the shop at 507-677-6007 or start from the services and pricing page. We would rather tell you the chamber is fine than sell you a barrel you do not need.
Frequently Asked Questions
Why does chambering affect accuracy so much?
The chamber and throat are where the bullet transitions from sitting still to engaging the rifling. If the chamber axis is not aligned with the bore axis, the bullet enters the rifling at an angle. That shows up as inconsistency at distance long before it shows up at a hundred yards.
What is a pilot bushing and why would you remove it?
A pilot bushing is a hardened sleeve on the nose of the chamber reamer that rides in the bore ahead of the cutting edges and keeps the reamer centered. We remove it for the final portion of the cut so it never rides on the section of bore that ends up immediately in front of the finished throat. That is the section where bushing scuffing would otherwise stay for the life of the barrel.
Does removing the pilot bushing make the chamber less accurate?
Not with our setup, because the pilot is not what holds our reamer on center. The tailstock is trued to the lathe centerline and the reamer is held in a fixed holder, so alignment comes from the machine. In a shop that uses a floating reamer holder the answer would be different, because there the pilot is doing the aligning.
What is the difference between a floating and a fixed reamer holder?
A floating holder lets the reamer move to follow the pilot in the bore. A fixed holder makes the reamer follow the machine. Both can produce a good chamber. The difference is which one you are trusting, and that determines whether the pilot can come out.
Why do you not use a roughing reamer?
Because a boring bar does the same job better and leaves the finish reamer with almost nothing to remove. A reamer that is not hogging out metal stays sharp much longer, and a sharp reamer is the difference between a cut throat and a smeared one.
How much runout do you hold on a chambered barrel?
Every barrel chambered at Redleg Company runs to under .0005 inch runout, verified by indicator before it leaves the lathe. We dial the bore in to within .0001 to .0002 inch ahead of the throat before the chamber is cut.
Can a rough or crooked chamber be fixed later?
A rough chamber can be polished or stoned, and if the chatter is too deep the barrel can be set back and recut. A crooked chamber is a different problem. That is two axes out of line with each other, and no amount of polishing moves an axis. The fix is to set the barrel back and rechamber it, or to replace the barrel.
Will fire lapping fix a bad chamber?
No. Fire lapping never touches the chamber. The abrasive is on the bullet, which is already past the case mouth before it starts working. It can help a rough bore. It cannot help a rough or misaligned chamber.
Does a well cut chamber reduce the need for barrel break in?
That is our position, and the barrel makers say much the same thing. Bartlein states that the only thing you are breaking in is the throat area of the barrel. The throat is cut by a reamer after the barrel maker has finished lapping the bore, which means the throat finish is the gunsmith's work, not the barrel maker's.
How long does a chamber reamer last?
Bartlein puts it at roughly forty to fifty chambers before a reamer needs resharpening. That is one of the reasons we bore the chamber first rather than roughing it with a reamer. Less metal removed per chamber means more chambers per reamer, and a sharper cut on every one of them.
What do you check after the chamber is cut?
Runout through the chamber, neck and freebore, a borescope inspection of the chamber and throat for burrs or rough spots, and headspace against go and no go gauges. We also verify that threading and counterboring did not move anything, because those operations put more force into the setup than the reaming does.
Do you chamber barrels for customers who supply their own blank?
Yes. Call the shop at 507-677-6007 and we will talk through the chambering, the throat, and what the barrel is going to be asked to do.
Chambering is one input out of many. The whole system, in order of how much each part actually moves a group:
And if you want to know what your bore is actually doing before you decide anything, slug it:
What This Buys You
At Redleg Company, our commitment to quality and precision is evident in every rifle we build. Our advanced chambering process is just one example of the meticulous care and attention to detail that goes into crafting a rifle that performs at the highest level. If you’re looking for a rifle that delivers extreme accuracy shot after shot, we’re here to help.
Have a Barrel You Want Chambered
We chamber barrels for custom builds and for customers who supply their own blank. Call the shop at 507-677-6007, or read the standard and the pricing first on the services page. If you want to talk through the throat, the freebore and what the barrel is going to be asked to do, that conversation is free.
Redleg Company, 430 Main Avenue, Chandler, Minnesota. Phone 507-677-6007. Email info@redlegguns.com.
Related Reading
What a chambered barrel is and is not promised to do.
The threads that hold the whole thing together are their own craft.
Reloaders: our free reloading sheets are here.
Disclaimer:
The information provided in this blog post is intended for educational purposes only and should not be construed as professional advice or instruction. Modifying or working on firearms, including chambering processes, requires specialized skills and experience. Redleg Company Inc. does not assume responsibility or liability for any harm, damage, or injury resulting from the use or misuse of the methods and techniques described. Firearms modifications can affect the safety, performance, and reliability of a rifle. We strongly recommend that any chambering work be performed by a qualified and experienced gunsmith. Always adhere to proper safety protocols and consult a professional before attempting any modifications. Redleg Company Inc. disclaims all liability for any direct or indirect consequences arising from the use of this information.
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