August 20, 2012

Working the Mossberg Model 500 Successfully

This budget pump-action is reliable and has many features found on more expensive firearms. Here’s how it operates.

Just as an automobile mechanic would rather work on a late-model Oldsmobile than an old Ford Pinto, we gunsmiths prefer to work on firearms of high quality. However, we can’t always do this, for just as the automotive market has a place for the Pinto, the Vega, and the Renault, the shotgun market has its entry-level products. Some are better than others; some are out and out dangerous. The better ones, however, are safe, inexpensive, and sold in great numbers by Wal-mart, Kmart, and a host of large retail chains.

One of the more popular of these inexpensive shotguns is the Mossberg Model 500 pump action. It is a reasonably strong and reliable repeating shotgun with many features normally found on much more expensive shotguns. It, of course, has it’s weak points, and the modern shotgun smith will do well to familiarize himself with the service and repair of this popular firearm.

Normal disassembly and cleaning of this shotgun is quite simple, and I’m sure that the gunsmith and hobbyist needs very little instruction in these procedures, but since there are a number of areas of special concern to be inspected during the maintenance of this shotgun, I will explain how we service, inspect, and repair the Mossberg 500 shotgun in my shop.

Basic Work

The barrel is removed from the frame of this shotgun by releasing the bolt from its locked position and pulling the slide handle rearward about a third of the way. A thumbscrew attaches the barrel to the magazine plug. This screw should be no more than finger tight, but you will often find this screw has been overtightened or has rusted. In this case, you will need to use a small set of water-pump pliers (padded of course) to break it free. Clean and inspect the barrel. If it has been used to fire slugs, you will no doubt have to remove a great deal of lead from the bore. The Mossberg’s bore is generally left relatively rough by the factory, and you can often see the distortion left by the welding of the vent rib posts.

American Gunsmith, Book of the Shotgun. Chapter 19

Courtesy, American Gunsmith

The Mossberg Model 500 pump-action shotgun is an inexpensive, medium-weight shotgun. It has many features normally found on much more expensive firearms; as such, it should be salvaged if it is malfunctioning.

Performance can be greatly improved by polishing the bore surface. This can be done with any of the available bore polishing systems. The easiest system I know of requires a 3-inch slit in one end of a 5/8-inch hardwood dowel (12-gauge diameter) and a piece of medium emery cloth inserted in the slit and wrapped clockwise around the dowel. Spin the dowel rapidly inside a well-oiled bore. This can be attached to a drill or can be spun in the lathe. If you are using this polishing method to remove lead from a shotgun bore, be sure not only to wear a particle mask, but also to do this work in a well-ventilated area.

Test the Safety

Slide it on and off to be sure that it is properly fitted. If it cannot be easily slid to its On position, it is possible that the safety tab on the back of the trigger needs adjusting. This is easily adjusted with a small file. Be careful not to remove too much material from this tab; remove just enough to allow the safety to slide off and on freely. Removing the trigger assembly is done by pushing out the retaining pin from either side of the receiver and carefully lifting the trigger assembly out from the rear. Cleaning this assembly is best done by solvent bath and compressed air. The trigger assembly housing is made of plastic and is easily damaged. Carefully inspect the tabs on the front of the assembly. If these tabs are damaged (by pulling the trigger assembly straight down or pulling it from the front), it will need to be repaired.

American Gunsmith, Book of the Shotgun. Chapter 19

Courtesy, American Gunsmith

Here the Mossberg 500 trigger assembly has been set up in the mill/drill to cut a slot across the front edge. The damaged retaining tabs will then be replaced with a piece of 1/8-inch aluminum bar stock. The most important part of this repair is knowing how this damage happened in the first place, so you can avoid doing it again.

When you find a trigger assembly with damaged retaining tabs on its front edge, these tabs will need to be restored before the shotgun can be used safely. This can be done by milling a 1/8-inch slot all the way across the damaged area and restoring the tabs by gluing a piece of 1/8-inch aluminum bar stock or plastic into the newly milled slot. Since this assembly is made out of plastic and cuts very easily, a drill press with a cross-slide vise and a 1/8-inch end mill can be used to make this cut. The new tabs will have to be fitted to the receiver. This is easily done by hand with a small file. Be sure that when fitting the new tabs to the shotgun’s receiver that the trigger assembly can be easily removed and installed. The most important part of this repair is learning how this damage occurred in the first place, and how to avoid damaging this part in the future.


Disassembling the receiver is done by holding the receiver upside down, pulling the action slide assembly rearward and lifting out the bolt slide. The action-slide assembly can now be removed from the magazine tube. Check the action slide tube nut and tighten it if necessary. A piece of bar stock held in a vise can be used to tighten the nut. With the safety in its On position, squeeze the legs of the elevator assembly and remove it. The bolt will now slide out of the front of the receiver.

Clean and inspect these parts, paying special attention to the firing pin, the extractors and the extractor springs. The extractors should operate freely, and they should have equal spring tension. Clean the receiver and the bolt thoroughly and oil both extractors and springs. The ejector is held to the left side of the receiver by the ejector screw. Check this screw for tightness. It is not a bad idea to remove this screw and apply some Loctite to its threads while you are servicing the gun. This will not only ensure that looseness of the ejector screw does not become a future problem, but it will also give you an opportunity to carefully inspect the ejector and replace it if necessary.

American Gunsmith, Book of the Shotgun. Chapter 19

Courtesy, American Gunsmith

The Mossberg 500’s forend nut should be checked for tightness, since the hardwood used in the shotgun tends to shrink a bit more than walnut stocks. Here an old piece of exhaust strap is used as a wrench.

The Mossberg Model 500 uses a safety button made of plastic. I suppose this causes very little trouble in shotguns used in the South, but in cold weather, this safety button becomes very brittle and is prone to breakage. The safety button is easily replaced by removing the safety screw while holding the safety lock inside the receiver. Be careful not to lose the detent ball and spring. Replace the plastic safety button with one made of metal.

These are readily available from Brownells and should be kept in inventory by any gun shop planning to service this shotgun.

The newer Model 500 shotguns come pre-drilled for a scope base. Mounting a scope base permanently on the Model 500 will present no problem, but many of your customers are going to want to remove their scopes during bird season, and you will find that the threads in the aluminum receiver do not hold up very well to constant removal and installation of scope base screws. Thus, when installing a scope base, use as long a screw as possible. This will spread the load over much more threaded area. If a threaded hole becomes stripped, a 146-48 screw can be used as a “first oversized” replacement screw. To install this oversized screw in a damaged hole in aluminum, you will not need to re-tap the threads. You will get a very tight fitting screw by lubricating and carefully threading the oversized screw into the damaged hole, allowing the steel screw to cut its own threads. If the threads are damaged severely, the holes will have to be enlarged to accept an 8-40 base screw. This repair will also require the oversizing of the holes in the scope base.

American Gunsmith, Book of the Shotgun. Chapter 19

Courtesy, American Gunsmith

A #52 (wire gauge) drill is being used to make a hole for a 1/16-inch roll pin. This is the best way to retain a vent rib if the solder at the chamber end has failed. I like to install pins at two posts near the center. In my many years as a gunsmith, I have never seen one of these repairs fail.

The hardwood used in the Mossberg’s butt stock and forend seems to shrink much more than the walnut found in more expensive shotguns, and I have run into so many Mossbergs with loose butt stocks that I recommend checking and tightening the stock bolt of any Model 500 that comes in for service.

The current Model 500 comes equipped with Mossberg’s Accu-Choke screw-in choke system. You will find these are compatible with Browning’s Invector system, chokes for which should be readily available. Older Mossberg Model 500 shotguns can generally be fitted with the Accu Choke/Invector style tubes, and this is a welcome upgrade on a fixed-choke barrel.

The vent rib on the Mossberg shotgun is retained only at the rear where it is soldered above the chamber. If this solder fails, the rib will slide about freely across the dovetailed rib posts. To repair this rib failure without relying on a solder joint at the rear, either retain the vent rib in the center by means of a long shanked “steel” 6 x 48 center bead, or drill the side of the rib at two of the rib posts near the center of the barrel, using a #52 drill, and secure the rib to the posts with 1/16-inch roll pins. I prefer the latter method. It takes very little time, and I have not seen one of these repairs fail. ★