Smoothing a Mossberg 500 Action

Delesoyby Dana Delesoy
Guns and Gunsmiths Contributor</p

Recently I performed an annual detail strip and clean on my Mossberg 500 shotgun. The gun was purchased four years ago, shot four times and put away in the safe. I brought it out now and again over the years for its annual cleaning and to practice working the action with dummy rounds.

During a cleaning it’s always wise to inspect along the way, and I did, as usual. I noticed several surfaces on the main moving parts of my gun were a bit bumpy and rough.

A new way of thinking

During the process of learning gunsmithing, I have developed a new outlook on things. I noticed this especially when reviewing AGI’s Professional Gunsmithing Courses for the second and third times, etc. The same proved true with subsequent looks at my own guns.

My gun has not been shot enough to have worn away any high points with rough areas. I knew that polishing key areas on the moving parts would smooth the action nicely, so this is exactly what I did.

A word of caution regarding smoothing and polishing

When smoothing and polishing surfaces be careful not to go too far, especially if parts are case hardened. My Mossberg does not have case hardened parts. Remove a little and check often. As a wise teacher once said; “It’s a lot easier to take it off than it is to put back on.”

For example: If the surface of a working part has high points with rough spots, there will most likely be “pits” in the low spots. The goal is to smooth the high spots. It is usually not necessary (or beneficial) to remove the pits. The pits could be .003″ deep from the high spots. If you polish all the high spots down flush with the “pits” and/or also remove the pits you could be removing more metal than necessary, leaving you with non-functioning parts.

This effect can be compounded when polishing two parts that rub along another. For example, if you remove .002″ off the bottom of one part and .002″ off the top of the part it rubs along, you have essentially removed .004″ .004″ is a lot of material to remove, and would in most cases be counter-productive.

Understand the function of the parts before going wild with polishing. Sometimes the part(s) are rough and removing any more than .002″, or even any at all, can be deleterious, leaving you with a self-induced problem needing repair.

Areas to be worked on

The main areas I smoothed were the surfaces of moving parts rubbing against another surface. Working the action slowly will amplify rough areas throughout the cycling of the action, helping you pinpoint them. Working the action quickly will hide rough areas. A good way to tell where contact occurs, and what kind of contact it is, is to liberally cover the clean parts with Magic Marker. Cycle the action vigorously a few times and then carefully disassemble it. The high points and corresponding contact points will tell a sad story on most factory guns.

Those parts smoothed are as follows: the elevator (cartridge carrier), action bars, extractors, cartridge stops, bolt body, disconnector, firing pin spring, safety connector on the rear of the trigger, and ejector. I will explain the function/action of each part, and provide pictures showing where each of these parts received their smoothing.

Tools used

The smoothing of the action did not require the use of files or sanding drums. I used a variable speed Foredom tool with foot pedal. 3M 400 grit sandpaper was wrapped around a mandrel and inserted into the chuck of the Foredom. I replaced the sandpaper as it tore or wore out. The cost of the smoothing project was about one quarter a sheet of sandpaper and a few cents of electricity.

Primary Extractor

The right outside of the extractor looked as if it had a coating of some sort (like chrome paint), and it was flaking off. Also the rear outside end of the primary extractor was sharp and galling the right inside of the receiver when the action was cycled. This is an area to look out for on any gun with an extractor that sits above/higher than the body of the bolt. I made sure to chamfer and polish any rough and sharp edges.

Primary extractor before polishing

Primary and secondary extractors

The hooks of both extractors are areas that are cammed outwards as they snap over the rim of a cartridge and enter their slots/cuts in the chamber end of the barrel. This action takes place just before the locking block fully seats in its cut-out in the barrel extension. The hooks of both extractors, as well as their edges (especially the bottom of the primary) were polished.

Primary (top) and secondary (bottom) extractors: “hooks” resting behind rim of cartridge

Primary cartridge stop

The rectangle tab just rearward and above the hook (by hook I mean the part that stops the cartridges from exiting the mag tube) on the primary cartridge stop were polished. When the action is cycled rearward this tab is the part the left action bar bumps into and cams outwards (leftwards) causing the de-activation of the primary cartridge stop. This action occurs just before the elevator bottoms out and is at the same point just before the action is cycled fully rearward.

I smoothed the underside of the primary cartridge stop’s hook as well because this is the area a cartridge rubs along when it is inserted into the magazine tube.

Primary cartridge stop; Left pointer shows shelf that holds rounds in the magazine tube; Right pointer shows the tab on the cartridge stop the action bar forces out of the way to de-activate the stop.

Secondary cartridge stop

As the action is cycled rearwards from fully locked up, the bottom right rear of the bolt carrier bumps into an inclined plane on the bottom rear of the cartridge stop. This forces the stop upwards at the front causing the stop to activate/turn on. This area received smoothing.

As the action cycles forward, a shelf on the bottom of the right action bar bumps into an inclined plane on the bottom of the cartridge stop (a couple inches rearward of the tip of the stop) which deactivates the stop. This area was rough, so it was smoothed.

Left action bar during rearward travel beginning the deactivation of the primary cartridge stop

The shelf on the cartridge stop where the hook holds the cartridges in the magazine tube when the stop is activated was polished to aid in a smooth releasing out of rounds out of the magazine tube as the stop becomes deactivated.

Both the inclined plane on the stop as well as the tip of the stop were polished.

Secondary cartridge stop: Left pointer showing shelf that holds rounds in the magazine tube; Middle pointer shows deactivation shelf; Rear pointer shows activation shelf.

Action Bars

The action bars move fore and aft along the tracks on the sides of the receiver. The sides of them did not need polishing but the top and bottom edges did.

The elevator (cartridge carrier)

The elevator is activated by the bolt carrier when the action is cycled. When cycling the action rearward the rear top of the bolt carrier bumps into an inclined plane on the bottom edges of the elevator causing the elevator to lower. The forward cycling of the action causes the top front of the bolt carrier to bump into and pass over an inclined plane on the bottom of the elevator causing it to raise. The entire bolt carrier is smooth from factory but the working surfaces of the elevator were rough so I smoothed them.

Elevator (cartridge carrier): The pointers show the areas polished. The left pointer shows the hump the bolt carrier strikes, forcing the elevator upwards; right pointer shows the hump the bolt carrier strikes, forcing the elevator downwards.

The ejector

The left side of the bolt has a track cut into the bolt body as a space for the ejector to clear when the action is cycled. As the bolt travels fore and aft the shelf on the ejector bumps into the slot in the bolt and rubs along the bolt body. The ejector was quite rough and sharp, so it got a good polishing.

Ejector: pointer shows the ejection shelf which is also the portion that tracks through the left slot on the side of the bolt.

The Disconnector

The rear right bottom of the bolt carrier bumps into the disconnector at a negative angle as the action is pumped rearwards from fully locked. I polished the top of the disconnector because it was a bit rough. The area of the bolt carrier that rubs along it was smooth so I left it alone.

The Bolt body

As the action is cycled from fully locked up to just beginning to unlock I noticed a slight sticking point. I found the top edges of the inside walls of the bolt body (on each side of the locking block) contained a burr. This was causing the locking block to slightly drag along those rough edges as it moved up and down during locking/unlocking.

The inside of the bolt body had burrs on the edges of the cut outs for the extractors and the holes bored for the locking block retaining pin. This caused the locking block to bind slightly as it traveled up and down. I removed the burrs and chamfered the edges.

Firing pin return spring

Newer models of Mossberg 500s have a firing pin return spring. They are robust and hard to compress. The cut off ends of the coils are sharp. The firing pin return spring is always compressed. It shifts slightly as the locking block moves up and down. One end of the spring rests against the body of the locking block and the other end of the coil rests against a washer that rests against the bolt body.

I smoothed both cut ends of the coils, in case the spring twisted ever so slightly during its slight movement (compression/ decompression), to prevent any possible hang-ups. The flats on each side of the washer for the rear of the spring were rough. It looked like a coating had begun flaking off so I polished the entire washer smooth.

Bolt body: Firing pin hole

When checking for freedom of movement and protrusion of the firing pin I noticed its hole in the rear of the bolt was rough inside and along the edge diameter. This caused a bit of friction as the pin moved. Since the firing pin return spring is very strong, any smoothing for the benefit of the pin’s movement is welcome.

Locking Block

The locking block has constant upward spring tension from the firing pin return spring. The head of the locking block drags along the top inside of the receiver as the action is cycled. I did not mess with the locking block. There was no roughness on it nor on its slot in the barrel extension so I left both those areas alone. I did however check for burrs in the screw holes in the top of the receiver because the locking block drags along them.

Note rub marks from locking block on top inside of receiver. Deburr and lightly chamfer.

Safety connector on the rear of the trigger

When the safety is placed in the “on-safe” position the safety connector on the trigger hits the safety when the trigger is pulled. Between the safety in the on-safe position and the safety connector this prevents any further movement of the trigger. This prevents the trigger from being able to move the sear clear of the hammer and fire the gun.

The top shelf of the safety connect for some reason was very rough, like someone had used a very coarse file. This does not make the safety any more difficult to operate because there is a slight gap between the safety and the safety connector. There is no friction because there is no contact between them. It is a different story if you have the safety on, the trigger pulled as far as it will go and then switching the safety off – there will be parts rubbing. I do not operate safeties in that manner. I polished the part because it was sharp and rough and should they rub against each other for whatever reason, those parts were smoothed.

In closing

Any areas I have not mentioned were already smooth from factory, i.e. the bolt carrier, chamber, and feed ramp on the barrel extension. All guns are not created equal and have character traits of their own. Your gun or your customers’ guns smoothness of parts (or non-smoothness) will likely differ from my guns, so proceed cautiously and accordingly.

Smoothing an action improves the function, can prevent premature wear and makes the gun much more enjoyable to shoot. It can be a time-consuming project but it is well worth it to take care of your investment. It is not unimportant that it also makes the gun much more fun to shoot . . . and your friends will notice.

The cost of performing the smoothing operation is next to none, other than the time it takes you. Be proud of your work. Show people your finished project, let them feel firsthand how smooth your gun functions and how smooth theirs could be by hiring you to work on it.

9 Responses to Smoothing a Mossberg 500 Action

  1. Excellent advice and well written………..patience and observation will also improve one’s ability as a gunsmith and learning curve of the particular gun.

    • Thank-you Ronald!
      It’s always a treat to bring out the guns for a good look over and feel now and again to appreciate them, especially since I do not get around to shooting much at all. In total accord with your comments, much appreciated!

    • Great to hear from you Dave! Thanks for reading and thanks for the comment.
      Hope all is well with you Sir!

  2. I’ve never been through a 500. I always kind of looked down on them honestly, but I see some similarities in parts with my beloved Winchester model 12.

    Well put together and if nothing else I have a good idea of how a 500 works now! Bravo sir.

    • Given the abundance of 500’s out there I’m sure you’ll run across a 500 someday that you’ll give a good look-over Dan. Actually a bit surprised you haven’t yet, although I understand. In any case your candor (regarding the 500 and all) is always appreciated and respected.

      I remember in AGI’s Pro-Course Bob Dunlap mentioned the locking block is not the most robust out there. Myself I’d prefer a steel receiver versus the alloy the 500’s have. What has plagued me with the gun for a long time now is the action bar lock has a very negative angle and also has too much of a gap (for my liking) between the action bar and the action bar lock. Will need to get a TIG welder in order to take care of those two issues.

      What I do like is the myriad of accessories for the 500’s, in particular the Fab Defense furniture. I also found really enjoy working on the 500’s and enjoy remedying their “Factory deficiencies” as I call them.

      My particular 500 is a JIC (Just In Case) Mariner with Marine-Coat finish all over it inside and out. It came in an orange PVC tube (along with some multi-tool with pliers and a separate knife or something). The tube has a robust, water tight cap so the whole affair will float if set into a body of water (that’s what is claimed anyhow, I haven’t tried that).

      I do not get the chance to see nor handle many guns the past 4 years (since moving away from my ol’ shooting range) other than mine, a friend’s and the occasional customer. With that I’ve never seen nor handled a Model 12 but I do have the AGI Winchester Model 12 Armorer’s course and have watched it a few years back. Awesome that you like your 12 so much!

      Thanks for reading and commenting!

      • We grew up shooting 12’s and 870’s. For all I knew that’s the only pump guns in existence. I’d say to Paul be careful with the dual action bars on the 870. They can sometimes get tweaked if you get a little out of sorts removing or reinstalling them into the receiver.

        My brother took his on a pheasant hunt to Oklahoma about 8 or so years ago. A heavy bag fell on his shotgun sleeve and impacted the forearm which bent the forearm tube assembly that attaches to the action bars. The gun was so bound then it nearly gouged the receiver and left some wicked scratches on the magazine tube when cycled and took a great deal of effort to do so. He was forced at that point to use my Browning Citori I sent with him. I know we all feel real sorry for him huh?!?! When he returned home we disassembled everything and the forearm tube assembly was crushed like an empty coke can. We acquired a new tube and got him back in service.

        I haven’t run an 870 that wasn’t relatively smooth. They have far fewer moving parts than the 500. I’d say in my experience making sure the action bars are perfectly straight and parallel and free of burs as well as the rails they ride in the receiver are what contribute a great deal to the feel of an 870. All guns (even the same model) are not equal. Do like Dana did. Take it apart and spend time fitting and fiddling the parts over one another and see how they all play together. Good Luck and thanks again Dana.

    • Paul,

      Unfortunately for me I’ve never seen nor handled an 870. I’ve dearly wanted one (or two, or three) for a few years now! It’s on my wish list!

      As per your question: If you can thoroughly disassemble and reassemble an 870 then go ahead and apply the same smoothing the action principles as I’ve explained here in this article. The principles here apply to many, many guns, including the 870.

      The most important principle to remember when polishing and smoothing parts is to fully understand the function of each part before doing any work on any part. I highly recommend AGI’s Remington 870 Armorer’s Course, and highly recommend anything else they have for 870’s!

      Also important when polishing/smoothing is only do a little bit at a time and check often! It’s easy to overdo it. It’s harder to put metal back on the parts than it is to remove it. Being relaxed, clear headed, taking your time and having fun with it all is also very important. Hope this helps.

      Would be awesome to read a “Smoothing a Rem 870 Action” article from you on G&G here!