Mossberg 500 rebuild

Customizing an old shotgun on a budget

August 12, 2015Bill MarrGunsmithing,Shotguns0

In this post we will take a look at a budget shotgun rebuild/conversion. The goal of this project is bringing new life to a beat up old gun.

The shotgun featured is aMossberg 500 I purchased for $120. The pictures don’t do it justice, it was in really rough shape. It came with a 30″ barrel and a fixed full choke, but I’ll be converting it over to a home defense type gun.

For reference purposes, aMossberg 500 schematic can be found here.

Here’s the plan; the barrel will be cut and crowned from 30″to 18.5″, drilled and tapped for a bead sight, an interchangeable choke system installed, old wood stock replaced with a new synthetic one, metal finished in Cerakote and the standard plastic safety upgraded to an oversized metal one.

First step is to cut and crown the barrel to 18.5″ and install interchangeable choke tubes. Also, while we are working on the barrel, a new hole needs to be drilled and tapped for the bead sight to replace the one that is cut off.

For this project, I ordered the following from Brownells:

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

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The 30″ barrel was too long for just about everything except goose hunting. A quick pass with the hacksaw trims the barrel down to 18.5″ long. Looking at the cut off section of the barrel (above, bottom) next to the 18.5″ barrel (above, top), you can really appreciate how long it was.

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). If you don’t have access to a lathe and choke reamers, this step can be skipped, I juts prefer the increased versatility of an interchangeable choke system.

Prior to reaming and tapping for the choke, barrel wall thickness needs to be evaluated to make sure it is adequate. A critical step 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 tubeshere.

The high-speed steel reamer used here will not cut a chrome lined barrel (which isn’t a problem with aMossberg). 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.

Thechoke reamer requires a size pilot to follow the bore. These pilots are available in .001″ increments. The pilots are secured to the end of the reamer with an e-clip.

This is how I set up my lathe. The 3-jaw chuck drives the barrel. Initially I use the live center to align the barrel on the steady rest.

Thechoke reamer is aligned in the barrel with a dead center in the lathe’s tail stock.I don’t have a reamer holder big enough to hold a choke reamer. Instead, I use a large tap handle to keep it from turning. The tap handle rests against the lathe’s bed. I use a spindle speed of 70 RPM and apply pressure against the reamer with the tail stock’s quill. I frequently remove the reamer, clean and oil it.

Once the reamer has cut to full depth, I use some emery cloth to break the outside edge of the muzzle.

Thechoke tap (above), uses the same pilot as the reamer.

With the barrel secured vertically in a multi-vise, the tap is lubricated with a generous amount of cutting oil and inserted into the bore. Unlike a traditional tap, this one is turned until the cut is complete and then backed out. Backing off this tap may result in its damage.A test fit of the choke shows all works well.

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.

There are many ways to drill and tap a hole, most do not require a lot of equipment. In this case, I’ll be using thetop dead center punch (above, left) and a tap with handle (above, right).The barrel is attached to the receiver and held level in a vise.

The level on thetop dead center punch indicates the location of a new bead. A tap against the hardened punch marks the barrel.

A number 43 drill is used to drill the tap hole for the bead.

A5-40 tap is used to cut the threads.

Test fitting the bead ensures it looks good.

Since the choke installation process cut material away from the barrel, the bead threads need to be shortened. I do this with a file.

The barrel is now cut and crowned, reamed and tapped for a Rem Choke system, and the bead has been installed.

The refinish the gun, it needs to be completely disassembled. In addition to allowing proper coverage ofCerakote, disassembly will allow any parts that will not be coated to be detailed cleaned and/or repaired.

The barrel nut has seen better days. Someone took a pair of pliers to it.

A little work with a file makes the barrel nut look better.

One of the nice things about aMossberg 500 when compared to a Remington 870, is that most parts are easily removed. For instance, the ejector on aMossberg is secured with a screw. To remove it, the screw is simply backed out. On the 870, this is a major task that is both equipment and skill dependent since the ejector is riveted in place.

Another big difference, the magazine tube of theMossberg 500 simply unscrews from the receiver. It isn’t brazed in place like the Remington 870. The magazine tube needs to be removed in order to remove the magazine spring and follower.

When you take the slide assembly off aMossberg you can figure out why their forends always bounce around so much. Take a look at the detents on this assembly…

And look at the assembly in place of the magazine tube. The tube is significantly oversized, the dimples allow the tube to somewhat track the magazine.

Mossberg 500s and 590s both have two extractors, as shown in the image above. The machine work on aMossberg tends to be crude, take a look at the tooling marks in the bolt lug on the right (above).

I have to remove the safety to coat the receiver. Be careful not to lose the small detent or detent spring.

All metal parts are ready to be degreased. This is an important first step BEFORE the parts are blasted withaluminum oxide. Removal of dirt and oil residue prevents the media from becoming contaminated and prevents abrasive from getting stuck to the parts.

The receiver going into the blast cabinet.

Here it is after being blasted. Looks good as new.

The parts are degreased a second time. I prefer using sprayTCE from Brownells. This is the last critical step. I also blow off the parts with compressed air to remove any remaining aluminum oxide from the blast cabinet.

The parts are sprayed withCerakote stainless. I think this is a pretty cool looking finish. In many ways, it looks more like the Mariner finish you’d encounter on a 870 Marine Magnum. I apply it with theNICS baby sprayer (sorry, no pictures).

The parts cure in acuring oven (above). I’ve had great luck using theBrownells curing oven, however, a shop built curing oven would work well. My first was an old gun safe, coated in insulation and heated with an industrial heat gun. An oven this elaborate isn’t mandatory, home built ovens work well.

The newextended safety(above, top) is metal, to replace the factory plastic safety (above, bottom). Installation instructionscan be found here. I also upgraded to a newHogue overmolded stock set.

Once everything is back together, the gun looks very different.

This is always the most rewarding part, sitting back and looking at the finished product.

The gun looks as good as new. The stainless Cerakote makes it look really sharp. A quick note on the last picture above, I need to go back and paint the recess on the rear of the safety red. This indicates the safety is off and the shotgun is ready to fire.

Even though it is only a 6 shot bead sight shotgun, it still looks pretty nice. Not bad for $120 beater, right?

Not counting tools, the expense break down for this project including parts and consumables was, shotgun $120,stock set $60,extended safety $20,Cerakote $20,choke tube $17, andTCE $15. Total investment $252. for comparison purposes, an 18″Mossberg 500 Mariner with a electrolysis nickel finish has a street price of just under $500 new (at time of writing). This gun, is about half the money and now has a better stock, Cerakote finish and interchangeable choke system. Who knows, maybe this will turn me into a Mossberg guy?

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