Mossberg 500: Cut, Crown and Choke Tube Installation

The shotgun featured in this post is a beat up Mossberg 500 I purchased for $120.  It came with a 30″ barrel and a fixed full choke.  I’ll be converting it over to a home defense type gun. First step is to cut and crown the barrel to 18.5″ and install interchangeable choke tubes.  Additionally, a new bead sight needs to be installed to replace the one that is cut off.

For this project, Brownells provided the following:

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

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My beater Mossberg 500.  30" full choke barrel.  This gun has seen better days.

My beater Mossberg 500. 30″ full choke barrel. This gun has seen better days.

The barrel is removed from a safe and empty gun. I like to cut my barrels a little on the long side to be careful.  I mark this one at 18.5".

The barrel is removed from a safe and empty gun. I like to cut my barrels a little on the long side to be careful. This one is marked at 18.5″.

A quick pass with a chop saw and the barrel is significantly shorter!  A bandsaw, hacksaw or even a reciprocating saw would work.

A quick pass with a chop saw and boom- way cooler looking! No chop saw?- A band saw, hacksaw or even a reciprocating saw would work.

Screw in choke tubes offer a great deal of flexibility.  Since I have Rem Choke tooling, I decided to install a Rem Choke system on my Mossberg (I’m going to give this poor gun an identity crisis).

Prior to installing a choke system, the barrel wall thickness needs to be evaluated to make sure it is thick enough.   This is critical in the case of the Mossberg 500 since the barrels are made thinner than those of Remington 870.  The Rem Choke tap is .814″-32 TPI.  The barrel is .858″ in diameter.  .858″-.814″=.044″. .044″/2= .022″.  So assuming the bore is centered in the barrel (which it may or may not be) the barrel wall should be .022″ thick after installation.

More information on determining if a barrel wall is heavy enough can be found in my posts Choke tube installation and Removing a fixed choke from a shotgun barrel and installing a choke tube system.  Additionally, Brownells provides great information on measuring for screw-in choke tubes here.

The high-speed steel reamer used here will not cut a chrome lined barrel  (which isn’t a problem with a Mossberg).  If you think a barrel is chrome lined, see if it takes cold blue.  If it does, it isn’t lined.  If it doesn’t it most likely is and you’ll need to remove the chrome lining prior to machining.

I'll be installing interchangeable choke tubes in the barrel.  In this case the Rem Choke style.  This is a Rem Choke reamer made by Dave Manson.  A bronze interchangeable pilot ensures it cuts true to the bore.

This is a Rem Choke reamer made by Dave Manson. A bronze interchangeable pilot ensures it cuts true to the bore.

The proper bushing is selected to match the barrel's bore diameter.  In this case a .728" bushing fits perfectly.

The proper bushing is selected to match the barrel’s bore diameter. In this case a .728″ bushing fits perfectly.

The pilots are retained on the reamer with a clip.

The pilots are retained on the reamer with a clip.

A look at the initial set up on the lathe.  The chamber end of the barrel is gently held in a three-jaw chuck.  The steady rest is located behind the barrel lug and the tail stock is used to align the barrel between centers.

A look at the initial set up on the lathe. The chamber end of the barrel is gently held in a three-jaw chuck. The steady rest is located behind the barrel lug and the tail stock is used to align the barrel between centers.

Grease is paced on steady rest bearing surfaces to prevent marring.  I adjust the steady rest so it gently touches the barrel when it is between centers.

Grease is placed on the barrel where the steady rest will contact it to prevent marring. The steady rest is adjusted so it gently touches the barrel when it is between centers.

One more view of the barrel in the lathe.  With the steady rest adjusted the tail stock will be retracted.

One more view of the barrel in the lathe. When the steady rest adjusted the tail stock will be retracted.

The reamer can now be aligned with the bore by using a dead center in the steady rest.

The reamer can now be aligned with the bore by using a dead center in the steady rest.

The reamer is coated in Viper's Venom cutting oil and inserted into the barrel.  A tap handle is attached to the reamer. The handle will rest gently against the lathe's bed.  The lathe is run at 70 RPM and the tail stock is used to gently advance the reamer.  I frequently stop to clean off the reamer and check progress.

The reamer is coated in Viper’s Venom cutting oil and inserted into the barrel. A tap handle is attached to the reamer. The handle will rest gently against the lathe’s bed. The lathe is run at 70 RPM (slowest speed on my lathe) and the tail stock is used to gently advance the reamer. I frequently stop to clean off the reamer and check progress.  While I am showing a tap handle in this picture, I normally secure the reamer in a large drill chuck.  Tool and die makers may not like using a chuck to hold a reamer, but I find it works well for this application.

Some guys will hold their reamer's with a regular wrench or pliers.   This is a picture I took of the method using a Knipex plier wrench.  I do not like this method.  I think it is unsafe to hold the tool in such a fashion and can potentially lead to injury.

Some guys will hold their reamers with a regular wrench or pliers. This is a picture I took of the method using a Knipex plier wrench. I do not like this method. I think it is unsafe to hold the tool in such a fashion and can potentially lead to operator injury.

The reamer does an excellent job making the choke cut.  It also squares the muzzle in the same operation.  A little abrasive cloth break the outside edge.

The reamer does an excellent job making the screw-in choke cut. It also squares the muzzle in the same operation. A little abrasive cloth is used break the outside edge of the muzzle.

The choke threads are cut with this Manson choke tap.  It is guided by the same bushing used with the reamer.

The choke threads are cut with this Manson choke tap. It is guided by the same bushing used with the reamer.

To tap the threads, the barrel is secured vertically in Multi-Vise.  Unlike a conventional tap, the Manson Rem Choke tap is turned continually in the same direction until it reaches full depth.  A heavy coating of Do-Drill helps the tap work efficiently.

To tap the threads, the barrel is secured vertically in a Multi-Vise. Unlike a conventional tap, the Manson Rem Choke tap is turned continually (don’t back it off to break the chip) in the same direction until it reaches full depth. A heavy coating of Do-Drill helps the tap work efficiently.

After cleaning out the threads with some compressed air, I test fit a choke tube.  Looks great.

After cleaning out the threads with some compressed air, I test fit a choke tube. Looks great.

A quick note on the order of operations shown here.  While I started with the screw in choke tube installation and then installed the bead, I would suggest reversing these steps.  The thin wall of the barrel after you machine it can be easily deformed.

To install the Mossberg front bead (I am using the original one from the gun) I'll be using a top dead center punch, 5-40 tap, tap handle and number 43 drill (not shown).

To reinstall the original bead from the barrel, I’ll be using a top dead center punch, 5-40 tap, tap handle and number 43 drill (not shown).

The shotgun is leveled in the vise.

The shotgun is leveled in the vise.

With the gun level, the top dead center punch is leveled and tapped with a hammer.

With the gun level, the top dead center punch is aligned and tapped with a hammer.

The barrel is now secured in a bench vise and I use a #43 drill to drill the witness mark from the punch.

The barrel is now secured in a bench vise and I use a #43 drill to drill the witness mark from the punch.  The choke tube is removed prior to drilling.

The 5-40 tap cuts the threads.

The 5-40 tap cuts the threads.

A quick test fit of the bead shows everything is working as it should.

A quick test fit of the bead shows everything is working as it should.

The barrel wall is thin at the end of the barrel from the choke tube system installation.  I gently hold the bead with some bronze jaws and use a file to shorten  its threads.

The barrel wall is thin at the end of the barrel from the choke tube system installation. I gently hold the bead with some bronze jaws in a bench vise and use a file to shorten its threads.

All done.  The bead looks great!

All done. The bead looks great!

The choke tube system looks good as well.

The choke tube system looks good as well.

Mossberg 500 18.5 inch barrel file

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