Chambering a rifle barrel.
In this article, we thread and chamber a barrel blank to install on the action we previously blueprinted. We will reference an action worksheet developed by Brownells, which is available for download here. Note: I stopped using this worksheet. I normally cut a .010″ bolt nose clearance that decreases to .008″ when the barrel is tightened.
All lathe work is conducted on a Grizzly gunsmith’s lathe.
This article is presented for information purposes only and provides an overview of the process we used. If you would like to learn more about chambering, the Viper bench-rest.com video on chambering a Remington action is highly recommended to learn more about the process.
Brownells provided the following tools for this project:
We ordered the following from Viper’s Bench Rest:
We ordered a sine bar and lug installation fixture from Holland’s Gunsmithing.
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The blueprinted action is secured in the Holland sine bar and lug fixture (left). The lug, is secured to the action with the plug (bottom right).
The bottom of the lug is aligned with the machined surface to ensure the lug is in the proper orientation prior to drilling.
The entire fixture is secured in the mill’s vise and the supplied cobalt drill is used to drill the hole to the proper depth.
Once the hole is drilled, we remove the lug and deburr the hole in the receiver, then reattach the lug and drive the supplied pin home.
Prior to working on our barrel, we use a depth micrometer to carefully measure the distance from the front of the receiver to the bolt face, bolt nose and bolt lugs. This information will be used with our barrel tenon worksheet.
This barrel chambering worksheet developed by Brownells, provides an excellent framework for calculating the dimensions of the tenon as well as the bolt nose recess depth.
Barrel makers mark relevant information on the chamber end of the barrel blank. In this case “Shilen” is the manufacturer, “308” is the bore diameter, “6” is the contour, “S”denotes stainless steel and “10” is the rate of twist (1:10).
Shilen recommends cutting off 1″ from each end of the blank. We saved the ends for future reference.
This fixture is secured in the four jaw chuck. The set of screws on each end of the fixture are used to align the axis of the bore. The PTG “Grizzly” rod, front, is used to indicate the bore. The bushings at the end are interchangeable to securely fit.
The “Grizzly” rod (top) accepts different bushings (center) available in .0002″ increments. Shown here are bushings from .2992″ to .3008″. The bushings are secured to the rod via a set screw.
A dial indicator is used, in conjunction with fixture’s screws, to align the bore with minimum run out. We labeled the sides of our fixture so we could keep track of the opposite sides when performing this task.
With the bore dialed in, we make a cut across the rear face of the barrel to ensure that it is perpendicular to the bore, then we begin to cut the tenon to length. We used high-speed steel (HSS) insert tooling from Brownells and Viper’s Venom cutting oil. The combination of HSS tool and oil provides a smooth finish on stainless steel. The tenon will be initially cut to the diameter of the recoil lug.
These are the high-speed steel cutting tools used. The 1/2″ threader (left) is used for threading the tenon. The 35 degree profile bit (center left) is used to cut the recess in the tenon shoulder so the recoil lug will fit flush against it. The right hand cutting tool (center right) is used to cut the majority of the tenon and face the breech end of the barrel and the last bit (right)is used to cut the groove between the threads and cut shoulder that supports the lug and chamfer the end of the tenon prior to threading. Note the slight radius at the end of each cutter.
The insert tooling has a slight radius. Once the tenon is cut to diameter, the radius prevents the lug from firmly seating against the shoulder. Note the slight space in between the barrel shoulder and recoil lug shown here.
By plunging the cutter in slightly, the lug is now able to fit securely against the shoulder.
Once the tenon is the correct diameter for the recoil lug, a groove is cut between the threads and the area the tenon will rest on. This begins .005″ less the thickness of the lug from the shoulder. Layout fluid is applied to the tenon to aid in the process.
An indexable HSS threading tool from Brownells is secured in the tool holder. The compound is set at 29.5 degrees for threading operations and the center gauge is used to ensure that the threading tool is properly aligned.
After a light pass is made, a pitch gauge is used to verify that the lathe was set up properly. We take multiple light passes with our threading tool, advancing the compound slowly for a quality cut.
With the threads cut, anti seize is applied to the threads and then the recoil lug and action are test fitted to check our work.
This counter bore tool will be used to cut the bolt nose recess. We calculated the depth of cut earlier on the worksheet. The tool is secured in a floating reamer holder and guided by a pilot.
The counter bore is thoroughly lubricated with Viper’s Venom oil and cuts are taken .025″ at a time. We stop the lathe, retract the cutter, clean, lubricate and advance it to within a few thousandths, start the lathe and repeat the process.
To measure the depth of cut, we attached a spring clamp to our tail stock and used a dial indicator in a magnetic base.
Here is the finished bolt nose recess cut.
To ream the chamber, we secure our reamer in a floating reamer holder. The PTG MARS, or Micro Adjustable Reamer Stop, is secured to the reamer’s shank. This tool provides a means to accurately adjust the depth of cut.
Initially, we adjust the depth of cut short of our “go” gauge.
This is our reaming set up. The tail stock is tightened with a torque wrench to ensure consistent alignment. The floating reamer holder is lubricated. The chamber reamer is thoroughly coated in Viper’s Venom oil. We advance the reamer .025″ and make our cut after which we stop the lathe, retract, clean and lubricate the reamer. The reamer is advanced to within a few thousandths of the cut, lathe started and the process repeated.
When the MARS hits the rear face of the barrel the reamer is removed and cleaned.
Anti seize is applied to the barrel threads. The action and lug are threaded into place with the “go” gauge in the chamber. Once the action is tight against the gauge, feeler gauges are used to see how much deeper the chamber needs to be cut. The reamer stop is then adjusted to half of this depth and process repeated until the “go” gauge fits. When the “go” gauge fits, a “no-go” gauge is then used to verify the chamber hasn’t been cut too deep.
Here the “no-go” gauge is inserted into the chamber. Notice that the bolt handle does not close.
The barrel is secured in the barrel polishing fixture, The fixture, made of aluminum, has two nylon centers on each end that hold the barrel without damaging it.
The barrel is secured in the polishing fixture and the 6″ hard felt wheel is treated with Polish-O-Ray. A gloved hand is used to slow the rotation of the barrel as it is polished.
Since the barrel will eventually be blasted with aluminum oxide media and coated with Cerakote, it is only polished at 140 grit. the polished barrel left, is compared to the factory finish.
The chambering, in this case .308 Win, is stamped onto the 9 o’clock position of the barrel. The stamping guide provides a precise way to align the stamps during this process.
Once the barrel has been cut and crowned, it is secured in a barrel vise and then an action wrench is used to torque the barrel into place.
After we cut and crown the barrel, reassemble and install the barreled action into the stock, we will once again verify the “go” gauge fits and the “no-go” does not.
For gunsmithing parts, tools and accessories, check out Brownells. For more information on how to chamber a barrel, check out Viper bench-rest.com’s video.