Choke tube installation

February 2, 2013 Bill Marr Shotguns, Tutorial 0

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Interchangeable choke tubes offer shotgun shooters increased adaptability.

Recently we decided to install “Rem-Choke” style removable choke tubes on two of our barrels.  In our “3-Gun Competition Shotgun Build” article, we installed the tubes manually, without a lathe.

All lathe work is conducted on a Grizzly gunsmith’s lathe.

Note: It is important to verify the barrel you are working on is NOT chrome lined.  HSS reamers will not work on chrome lined bores.  An easy way to check is to see if the interior finish will take cold blue.

For this project, Brownells provided us with the following:

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Measuring the wall thickness of the barrel you are working on is critical when installing choke tubes.  All chokes, like the “Rem-Choke”, require a minimum wall thickness to be safely installed.  Thinwall choke tubes are available for applications in thinner walled barrels.  Brownells has more available on this topic here, explaining the minimum requirements for different gauges and tubes.

We are working on two 18″, Remington 870 barrels.  These have thick walls.  You will also notice that both barrels have Scattergun Technologies front sights installed on them.

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Barrel measuring tools.  The Manson wall thickness gauge (top) is used to measure the barrel wall thickness. The Skeets bore gauge (bottom) is used to determine the bore diameter. This helps determine whether or not the barrel has a fixed choke as well as which bushing to use when reaming and tapping.

While it is possible to measure the inside and outside diameters of the barrel and divide by 2 to estimate final wall thickness, this process assumes the bore is concentric to the outside wall of the barrel. We prefer to measure the wall thickness in multiple locations as an added precaution.

 

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To use the bore gauge, a standard ring is placed around the bearing at the bottom of the instrument.  In this case, the standard is .700″.  The dial indicator at the top of the instrument is then zeroed.  Adding the reading, for instance .029″, to the standards diameter of .700″ results in the actual bore diameter,  .729″ in this case.
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The tool is then inserted into the bore of the barrel that is being considered for choke tube installation.  Bore diameters are taken at the muzzle and a few points behind it towards to the breech to determine what size pilot is needed and whether or not a fixed choke needs to be removed.

If the barrel decreases in diameter by the muzzle, the fixed choke needs to be removed.  An adjustable reamer, shown in the parts list above, can be used to accomplish this.  In this case, neither barrel had a fixed choke, eliminating the need for this step.

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We then take the time to ensure that the barrel wall thickness is appropriate  by taking a number of readings at various places in the barrel.

We suggest referencing “The Remington M870 and M1100/11-87 Shotguns: A shop manual” by Jerry Kuhnhausen for further reading on chokes.  In addition to a complete overview of choke tube installation, the manual details two different lathe set ups for reaming barrels.

To install the removable Rem-Choke system you will need a reamer, tap and bushing. Choke tube reamer (center), tap (bottom) and bushing (top)
To install the removable Rem-Choke system you will need a choke tube reamer (center), tap (bottom) and bushing (top)
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Kuhnhausen uses an arbor in both of his reaming set ups.  Not having one (and since the barrels were Cerakoted), we elected to chuck the breech end of the barrel in the lathe’s 3-jaw chuck.
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The reamer is secured in the tailstock of the lathe and equipped with the appropriate sized bushing.
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This is what our set up looks like prior to running.  We set our lathe on the slowest speed and wore eye protection.
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Prior to stating the lathe, we thoroughly coat the reamer in cutting oil.  In this case we are using Viper’s Venom cutting oil.  We then turn the reamer in until it makes contact and back it off a few thousandths.  We start the lathe and slowly advance the tail stock.  We cut a short distance and then turn off the lathe, retract the reamer, remove the shavings, and re-lubricate. This process is repeated until the full depth of the cut is reached.
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The reamer will make a final facing cut on the barrel when it reaches the final depth.  Once the barrel is removed from the lathe, we take the time to verify that the wall thickness is still adequate. Never take a chance with safety.
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The barrel is cleaned of debris and secured vertically in a padded Multi-Vise.  The tap, with the appropriate bushing installed, is coated in Do-Drill cutting oil and the threading cut is made.
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The tap is run continuously and not backed off per the manufacturers instructions. The threads are cleaned and a light coat of oil is applied.
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We install a choke tube, verifying that the opening was reamed true, ensure that the tube does not protrude into the bore and that the fit is snug.

 

 

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Here are the Remington 870 barrels with their new choke tubes installed.  The tooling will pay for itself in a few jobs.

 

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