Removing a fixed choke from a shotgun barrel and installing a choke tube system

Removing a fixed choke and installing an interchangeable choke system on a Remington 870

February 19, 2014 Bill Marr Gunsmithing 0

Examine an older shotgun and you are bound to encounter a fixed choke.  While less versatile then an interchangeable system, they worked well in their day.

Before we get too far into the specifics of this project, let’s do a quick review of shotgun chokes.  In a shotgun, the choke is the difference between the bore diameter and the exit diameter of the muzzle (see table below).  As the difference in these dimensions increase, so does the density of shot patterns.  This is typically expressed in percentage of shot in a 30 inch circle at 40 yards.  For instance if the bore diameter is .729″ (standard for a 12-gauge) and the muzzle diameter is .689″, the barrel has a .040″ or full choke.  The shooter with this barrel would expect approximately 70% of his pattern in a 30 inch circle at 40 yards.

Constriction

(inches)

Name

Percentage of shot

in a 30 in circle

40 yd

.000

 Cylinder

40

.005

 Skeet

45

.010

 Improved Cylinder

50

.015

 Light Modified

.020

 Modified

60

.025

 Improved Modified

.030

 Light Full

.035

 Full

70

.045

 Extra Full

.050

 Super Full

While most modern sporting and hunting guns have interchangeable choke systems, many defensive guns still used fixed chokes.

In this project, I need to remove the fixed choke from my Remington 870 Police in order to install an interchangeable choke system. The choke on my barrel was .715″, too tight to allow the pilot bushing on my choke reamer to fit.

If I was working on an older gun, a similar method would be used to open up a fixed choke. The reamer would be used to adjust the muzzle diameter to the required diameter and then the surfaces would be honed.  Since I am installing a Rem-Choke style system, I won’t be honing any of the cut surfaces.

I ordered the following tools and supplies from Brownells:

All lathe work was conducted on a Grizzly 4003G lathe.

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I begin by determining the size of the bore using a bore gauge. This gauge has a .700" standard that the indicator is zeroed too. The reading on the indicator is added to .700" for the bore diameter. In this case the bore was .729" (.700"+.029") and the choke at the muzzle was .715".
I begin by determining the size of the bore using a bore gauge. This gauge has a .700″ standard that the indicator is zeroed to. The reading on the indicator is added to .700″ for the bore diameter. In this case the bore was .729″ (.700″+.029″) and the choke at the muzzle was .715″ (.700″+.015″).
I'll be removing the choke tube with an angle expanding choke reamer. The reamer uses two bronze bushings to guide it in the bore of the shotgun.
I’ll be removing the choke with an angle expanding choke reamer. The reamer uses two bronze bushings to guide it in the bore of the shotgun.
The reamer is adjusted with two opposing screws over an angled guide. As the blades are moved towards the rear of the reamer, the reamer expands. As the blades move towards the front of the reamer the cutting diameter decreases.
The reamer is adjusted with two opposing screws over an angled guide. As the blades are moved towards the rear of the reamer, the reamer expands. As the blades move towards the front of the reamer the cutting diameter decreases.
Here is a view of the setup used to remove the choke. The two bushings are secured in place using e-clips supplied with the reamer. The barrel is secured in a vice with bronze jaws. The reamer, coated with Do-Drill oil is slowly turned to remove the fixed choke.
Here is a view of the setup used to remove the choke. The two bushings are secured in place using e-clips supplied with the reamer. The barrel is secured in a vise with bronze jaws. The reamer, coated with Do-Drill oil is slowly turned to remove the fixed choke.
The reamer is driven by a large t-handle. The set up is quite long, suitable for the longest sporting barrels.
The reamer is driven by a large t-handle. The set up is quite long, suitable for the longest sporting barrels.
I take light passes, cleaning away the chips and lubricating the reamer after each pass.
I take light passes, cleaning away the chips and lubricating the reamer after each pass.
After the final pass the choke is removed.
After the final pass the choke is removed.
I am now able to fit the bronze pilot for my choke tube reamer into the bore.
I am now able to fit the bronze pilot for my choke tube reamer into the bore.
Here is my reaming set up on the lathe. The barrel is driven by the four-jaw chuck and secured in the steady rest. The tailstock feeds the reamer.
Here is my reaming set up on the lathe. The barrel is driven by the four-jaw chuck and secured in the steady rest. The tailstock feeds the choke tube reamer.
After the barrel is reamed for the choke tubes I use a tap to thread the barrel.
After the barrel is reamed for the choke tubes I use a tap to thread the barrel.
The choke reamer squares the muzzle of the barrel when the cut reaches the proper depth.
The choke reamer squares the muzzle of the barrel when the cut reaches the proper depth.
Success, the choke has been removed and the choke tubes installed.
Success! The choke has been removed and the choke tubes installed.

More information on choke tube installation can be found here: Choke tube installation

 

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