Back bore a shotgun

How do you back bore a shotgun barrel?

In Understanding shotgun back bore, I discussed what back bore was and how it affected patterning of shotguns.  In this post, we will look at how to back bore a shotgun on a lathe.

The easiest method would be to send it to a competent gunsmith, but where’s the glory in that?  In Tactical Shotgun Build: Back Boring a Barrel and Lengthening a Forcing Cone to Increase Performance, I discussed the process of back boring a shotgun barrel by hand.  As documented in that post, the results of our hard work increased the performance of the barrel.  In this post, I’ll be using a metal lathe to drive the back bore reamer.  Both methods end with with a functional barrel when completed properly.

All lathe work is conducted on a Grizzly 4003G gunsmith’s lathe.

The contents of are produced for informational purposes only and should be performed by competent gunsmiths only. and its authors, do not assume any responsibility, directly or indirectly for the safety of the readers attempting to follow any instructions or perform any of the tasks shown, or the use or misuse of any information contained herein, on this website.

Any modifications made to a firearm should be made by a licensed gunsmith. Failure to do so may void warranties and result in an unsafe firearm and may cause injury or death.

Modifications to a firearm may result in personal injury or death, cause the firearm to not function properly, or malfunction, and cause the firearm to become unsafe.

I’ve been trying to develop a solid set up for back boring a shotgun barrel for the past few years.  I’ve read that some old timers used a milling vise on the lathe’s carriage to secure the barrel so I gave it a try.  The milling vise only held the barrel at one point and lacked the rigidity I wanted.  Eventually I found a few pictures of the set up Tactical Ordnance, in Ontario, Canada, uses to back bore their barrels.  I decided to make a fixture similar to the one they use.

back bore set up labeled

I bought the least expensive 6″ 3-jaw plain back lathe chuck I could find on Amazon.  I mounted it to a piece of 1″ thick aluminum that bolts directly to the carriage using the two threaded holes the follow rest attaches too in the lathe’s carriage.

back bore set up rear view

An aluminum clamp is attached to the tool post.  This makes for a rigid set up prior to machining the barrel.  To mount the barrel in the fixture, I place the barrel between centers, close the fixture’s chuck on the chamber end of the barrel and use the live center in the tail stock to align the barrel while I adjust the aluminum clamp attached to the tool post.

The entire fixture cost me about $160 to make.  The most expensive part was the chuck, which cost about a hundred dollars.

I use Manson back bore reamers sourced from Brownells to back bore my barrels.  A reamer extension is needed to drive the reamer and Do-Drill cutting oil is used to lubricate it.  A Remington 870 18″ Police barrel is shown in the pictures on this post.

A word of caution, a high-speed steel reamer, like the one shown here, will NOT cut a chrome lined barrel.  If you have a chrome lined barrel the plating needs to be removed first.  This can be done with a hone, see below.  An old trick to determine whether or not a barrel is chrome lined, is to use some cold blue.  If the bluing takes to the bore, it isn’t chrome lined.  Cold blue will not take on chrome but will on bare steel.  (Thanks to David Manson, Manson Reamers for this tip).

back bore set up 3

The reamer is attached to the extension with a roll pin.  It is lubricated with a generous amount of cutting oil and inserted into the barrel.

back bore set up 4

I’ve run my spindle speed at 70 and 200 RPM feeding between .002-.005″ per revolution.  I’ve settled on a spindle speed of 200 RPM and a feed rate of .005″/revolution.  I clean and lubricate the reamer frequently.

back bore reamer at the muzzle

Deciding where to stop the reamer depends on what you are trying to do.   Since I am normally trying to tighten the choke on a barrel, I’ll stop the reamer from cutting before it passes through the end.  The reamer has a gradual taper, so this will form the choke.

To determine the stopping point, I normally insert the reamer into the muzzle of the shotgun and mark where the muzzle diameter matches the reamer.  I then measure the distance from the front edge of the reamer to the mark.  This is how far the reamer needs to protrude from the muzzle of the barrel.  To figure out when to stop, I either zero the digital read out (DRO) on my lathe when the reamer is flush with the muzzle and stop cutting when I get to that measurement, or, I use a small scale (ruler) to measure how far it has protruded.  On the barrel shown above, I stopped cutting once the reamer protruded 1″ from the end of the barrel.

When run on this fixture in the lathe, the reamer leaves a bright shiny finish. The surfaces can either be left as is or honed.  Honing is an absolute requirement if you do this work by hand.  Hones come in two types, one looks like a brake hone used in automotive work (click here to see), the other is a flex hone.  The flex hone is a series of abrasive balls that are run on a shank with a drill to polish the interior surfaces of the barrel.  Flex hones use a specialized oil to create a slurry which leave a really nice finish (more flex hone information can be found here).

flex hone shotgun barrel after back bore

Note the construction of the flex hone in the picture above.  Flex hones are available in different grits depending on the amount of material you need to remove.

Once the interior surfaces are honed, the barrel is ready to be cleaned and reinstalled on the shotgun.  Over the years, the best product I found to clean a shotgun barrel is a Bore Tech jag.

bore tech jag shotgun 1

The jag costs around $7 and is worth its weight in gold.  The red tip spears the patch and the four black finger expand to fit the shotguns barrel.

bore tech jag shotgun 2

Here is a patch before cleaning.

bore tech jag shotgun 3The patch after a pass through the barrel!  I really like the Bore Tech jag.

For more information, please see:

  1. Understanding shotgun back bore,, a good read for anyone who wants to understand back bore and chokes
  2. Tactical Shotgun Build: Back Boring a Barrel and Lengthening a Forcing Cone to Increase Performance,, my first attempt at back boring a shotgun by hand.  Includes test targets after the work is performed.
  3. Removing a fixed choke from a shotgun barrel and installing a choke tube system  Demonstrates the use of an expandable reamer to remove a fixed choke prior to the installation of an interchange choke tube system.
  4. SAAMI Shotshell standards
  5. Choke Adjusting, from Brownells
  6. Manson Reamers, manufacturers of back bore reamers
  7. Flex-Hone for Firearms, information about Flex-Hones