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Upgrading the military shotgun – Part 4 – Stock and Sling mount

Part 1 HERE.
Part 2 HERE.
Part 3 HERE.

We started with the old M500M, pretty much a 6 shot Mossberg 590 shotgun.

Your generic riot gun. We made a few changes, but it is time to make the most often made change in military shotguns. A pistol grip.

Pistol grip only shotguns sell fairly well in the civilian world. All manner or cheap M500s, Maverick 88 Persuaders, and the like are sold to ignorant novices looking for a home defense gun. Experts and experienced gun owners tend to scoff at this as people tend to shoot pistol grip only shotguns very poorly.

USMC Breacher with pistol grip shotgun


In the above picture of a “SEAL Armory” you see two military M500 shotguns that have been retrofitted with pistol grips. They don’t even have the same model pistol grip on them. It makes me think these were retrofitted at different times.

So why would the miltiary be retrofitting these shotguns to pistol grip only if pistol grip only is considered so terrible in the civilian world?

Simple answer, these are now a breaching tool, not an offensive weapon system.

The full sized shotgun was intended by the military to be a proper weapon system. But our modern combat now often requires breaching locks. Shotguns excel at that, so the military pump shotgun has primarly become a breaching tool.

There was another picture I wanted to share but for the life of me I can’t find it. It clearly showed an individual with a carbine, shotgun, and pistol. The shotgun was clearly just an additional piece of equipment for breaching, not intended as a combat weapon as he already had a rifle and pistol.

Breaching with a full size standard shotgun sucks. So going to the pistol grip makes it a great deal handier.

With the M500A2 MEK, the military went with the Mossberg FLEX system. I am told that the Flex system was developed at the request of the miltiary. Previous systems required tools to change stocks and/or had issues breaking.

Direct from Mossberg you can buy a Flex stock adapter, or a kit that includes the pistol grip. I opted to get the kit with the grip as it is cheaper than buying it seperately.

Inside you would find the adapter that attaches to the receiver, and a grip that slides on to it.

The miltiary also adds a sling mount plate. In this case an Ergo sling mount. This took me a long time to get as everywhere was out of stock. I ordered from Optics Planet that said they would ship it in a week and after a month I cancelled my order with them. I found one for sale by an individual online, bought it, and USPS decided to give it a tour of the US. But I finally got it.

The Ergo sling mount is machined from aluminum and has loops on each side allowing for ambidextrous use.

I had expected this sling mount to be thin stamped steel. I was rather surprised that it was a 1/4 inch thick aluminum.

When I went to assemble everything. I found that the bolt included with the Flex adaptor had greatly reduced thread engagement due to the 1/4 inch thick sling plate. While it likely would have been fine, I was worried about it pulling out under recoil and damaging my receiver. So I bought a longer bolt from the hardware store. (With my luck, it will probably be this replacement bolt that snaps in half and screws me over).

A quarter inch longer socket head screw from the local hardware store.

The Flex adapter bolts on to the receiver, and uses a belleville washer. You torque it to about 12.5 lbs/ft. Which isn’t really that much.

Then the locking mechanism is slid in from the top and a split pin holds that in place. It is fast and easy to install.

A tapered splined interface connects the Flex adapter on the receiver and the stocks or grips. I found this was very tight and secure. So tight I had to use a mallet or screwdriver to install or pry-apart the stock from the receiver. I would not call this quick change. I don’t know if it will loosen up with use, I just hope it doesn’t become sloppy and loose.

The end result

One thing I don’t like is how thick that Ergo sling plate is. It moves the grip 1/4 inch farther back from the trigger. I found I had to stretch my trigger finger to reach that trigger. But shooting was just fine. I also don’t like how much harder it is to hit the slide release on the left side of the gun while a pistol grip is installed.

I had to use a mallet to switch between a stock and the pistol grip.

By pulling up on the latch on the Flex adapter, turning it 90 degrees, then spending 5 minutes hitting things with a rubber mallet, I could switch between a collapsible stock and the grip.

The Flex stock is purchased by it self, not including the adapter that needs to be mounted on the receiver.

Now Mossberg offered a similar unit that attaches directly to the receiver. I just happened to have one around(not that I wanted it, I sold it after taking these pictures).

Flex left, direct attach right
Direct attach left, flex right.

The older style direct attach unit that screws to the receiver does not let you removed the trigger group on the Mossberg when it is installed. To completely disassemble the gun, you would have to unscrew a set screw, remove the AR15 style stock, unbolt the adapter, then you could fieldstrip the shotgun. That is terrible in my opinion. It has a longer length of pull. It appears to use a commercial spec receiver extension but I did not double check that. Neither stock could interchange on these two.

The direct attach model also was blockier in profile up near the top of the grip. I wonder if that would make it less pleasant against the webbing of your hand while shooting. But I didn’t try using it. I’ve also read that with these units if you removed the stock and attempt to use them pistol grip only they would sometimes break the attachment bolt. I don’t know if they mean the bolt attaching it to the receiver, or the bolt attaching the grip to the adapter.

I only fired a handful of shots with the pistol grip only, then with the collapsing stock. I had expected the pistol grip to be uncomfortable as it is hard plastic. Surprisingly (to me) it wasn’t unpleasent, but it wasn’t something I would want to shoot a great deal.

The collapsing stock with pistol grip also makes the slide release harder to hit. I was expecting to notice a great deal of difference while shooting it at different lengths, but I didn’t really notice much.

Personally, I’ll likely end up picking up the Flex quick detach standard stock for this someday and mainly use that.

The work and modifications are complete. I’ll talk a little about the history and though process of the “Military Enhancement Kit” in the upcoming last part.

Upgrading the military shotgun – Part 3 – A rail system

Part 1 HERE.
Part 2 HERE.

A Mossberg shotgun comes out of the box with “corncob” style plastic forearm that is completely functional and in a way minimalistic. It can get a little slick when wet, but not terribly so. About as bog standard as it gets.

The military decided to replace this forend with a tri-rail. A unit made by ERGO. https://www.ergogrips.net/

I gotta say I was wrong. In the past I’ve said that I didn’t get the point of railed handguards on a shotgun. Now I’ve never said anything bad about ERGO, but I never really was impressed with their grips. To me their grips felt too soft and the wrong angle and shapes. I was a little leery picking up an ERGO rail for my shotgun. I had a feeling that ERGO was kinda like TAPCO.

It is nice to say I was wrong about ERGO, their rail for the Mossberg is very nicely made.

First things first. Mossberg has two different lengths of action tube. Because of this, ERGO offers two different lengths of their forend. (ERGO now also offers a MLOK forend, might be a better choice for the average person should they want one) These military shotguns use the longer rail. There is an aftermarket threaded end cap that will allow the shorter tubes to accept the longer forend. That would be an excellent option for people who have the shorter tube who want to use a full length forend.

Another note, there are a couple versions of this rail. There is a commercial version with a relieved, not true M1913 rail. Then there is a bulk packed version with a proper M1913 picatinny rail.

This common photo of a M500A2 MEK shotgun shows the commercial rail, as you can see the notch milled down the center of the rail.

I bought a bulked packed rail from Pro Patria. ERGO part number 4865(BULK). The commercial version is part number 4865. MSRP is $96. I had a hard time finding it cheaper, so I am very grateful for the deal Pro Patria gave me of the rail and used barrel.

There is a threaded cap holding on the handguard on a Mossberg action tube. I found a sheet of metal that fit the notched and unscrewed the cap.

I’ve read horror stories of people taking off the plastic corn cob grip. People reporting they had to cut it off, or use an arbor press. Others make tools to take it off. I was able to hold the plastic and press the tube against a table top and it slipped right off. I used this oppertunity to coat the action tube with a thick layer of oil, and slid on the ERGO rail, and reinstalled the end cap with some Vibra-tite. Quick and easy.

Only complaint I would have with the install is that there is a fair bit of rotational tolerance, so I had to try and center the rail while I tightened the end cap. The end cap is the only thing keeping the rail from rotation a bit on the action tube. I think using a forward grip would easily provide enough torque to rotate the rail some on the action tube.

ERGO includes 3 ladder style rail covers with their rail that completely covers all the rail. I decided I wanted to try shooting this gun with the rail uncovered to see how it felt.

Here is where ERGO really surprised me.

Many cheap rail, and some expensive ones (like on the FN SCAR I got to use, and my Geissele rail) have sharp edges on them. Those are not too dissimilar to the “cheese grater” disparaging insult for rails systems.

This ERGO rail is perfectly deburred so that it feels just fine to use. It provides a much grippier surface than the plastic corncob forend. Shooting buckshot and holding the bare rail was not unpleasant. I had expected it to be painful. But the rail surfaces does grip your hand solidly during recoil. So I’m going to throw those rail covers on it.

There is a lot of wobble in a Mossberg action tube. The gun is designed with this leeway to allow for debris and still function. This means that this rail is completely unsuitable for any sort of aiming device.

So, why would the military want a railed forend?

Simple answer, to mount a light.

Unfortunately breaching is not often done in a bright well lit area. Military rails and room clearing often happen at night, or in dark indoor areas. It is critical for speed and efficiency that the breacher make their shots count. When they shoot to breach, it lets everyone know that we are here.

Why not a forend with an integrated light? Apparently the military looked at that option. Robert Clements, who developed this system, was able to get Insight to make a integrated light forend for the Mossberg.

Insight’s discontinued weapon light

Remington has an interesting solution. On the 870MCS, there are slots on each side of the handguard to install a little keyfob light.

Photo by DJX, posted on AR15.com

The military isn’t looking for some super bright high lumen alternative to the sun. They want a low intensity light that provides some discrete illumination for the breacher. Tools like the old SOPMOD Low Intensity Tactical Illuminator (LITE).

Photo from Pro-Patria

This railed forend allows for mounting a low intensity light for breaching. Or any other standard weapon light when the shotgun is used as a primary weapon system. That is why the M500A2 MEK has a tri-rail

Upgrading the military shotgun – part 2 – the barrel and history of use

Part 1 HERE.
Part 3 HERE.

There is at least 35,000 Mossberg shotguns in use by our military. Means there are at least 5 or 6 of these guns for every M1014 in use. So there are plenty of them stack away in various armories.

In the civilian world, the shotgun is seen as a very versatile tool. From hunting small game, big game, or self defense, we have a wide variety of options in ammo and configurations. The military not concerned about the hunting side of things. The shotgun was generally used for riot control, security forces, and breaching.

I’m told the Army has more Mossberg 500s for each Infantry Brigade than M240B machine guns.

Riot Control
Security Forces
Breaching

Using the Marine Corps as an example, in the past they used The Remington Model 11, Winchester Model 1897, Remington Model 10, Winchester Model 1912, and Ithaca 37 shotguns in the past. Eventually the Mossberg was adopted to replace all previous pump action shotguns.

It is often said that the Mossberg M590A1 is the only shotgun to pass the military torture test. This leads people to believe that the M590A1 model is the only one used by the miltary. Fact of the matter is that the M500A is used a great deal more than the 590.


Originally the military specified 3 types of shotguns for use:

The Type 1 was the classic trench gun with bayonet lug and capable of taking a heat shield. This is what we commonly see sold by Mossberg as the M590A1. Sources say about 1000 were purchased by the Army and USMC.

Type 2 shotguns are mostly 18.5 inch barreled M500 shotguns purchased by the Army. Older ones might be bright blued finished, later procured ones would be parkerized. Army choose to go with the plastic trigger group as they very rarely fail and it was decided it was cheaper to replace an entire trigger group than to attempt to repair or rebuild one. These are often standard M500A or M500 MILS shotguns.

Type 2 shotguns purchased by the Navy and USMC are M500 shotguns built to M590 specification using the 5+1 mag tube. Navy likes to buy 17 inch barreled models and the USMC often buys the 20 inch barrel length. These are often called M500A2 or M500M MILS shotguns. Heavy barrels, 590 style mag tubes, bead sights, metal safety and trigger group.

Type 3 shotguns are specified to have rifle sights. It is unknown if the military every bought any.

Navy 17 inch barreled M500
Army M500 where the stock has been replaced with a pistol grip

Let us not forget that the Mossberg shotguns share a common receiver. A 500 could be turned into a 590 by replacing parts, or the other way round. The majority of the parts also interchange with the Model 835 and Maverick 88.

With current use of military shotguns, they have primarily become a breaching tool. Functioning less as a weapon, but an additional piece of equipment for a soldier to carry.

I’m sure this guy just loves carrying two weapon systems.

You would think that breaching with a shotgun would be quick and easy. Shoot the lock, kick the door, enter.

Unfortunately there are any number of things that can go wrong. You need to be at the right angle and distance from the lock. You need to be able to see the lock. If there are multiple locks on the door, you need to be able to engage each one.

There are all sorts of things that can go wrong with breaching. When attempting to breach quickly and/or in the dark it can be hard to get the right stand off distance. Worst of all, you can end up bursting the barrel or having fragment come back and injure yourself or friendly troops.

On the left, a 20 inch M500 barrel, the right a 18.5 inch breacher barrel

That is why the military decided on a fixed breaching stand off. Tests were done on choke based or removeable standoffs but there were failures leading to the decision to go with a permanently mounted one.

This new barrel is approximately 16 1/4 inches in length with out the stand off. The permanently attached standoff brings the barrel to 18.5 inch overall length.

Now, there was a little problem trying to source this barrel. Mossberg is a major manufacturer of shotgun barrels. You can buy all manner of M500, M590, and 870 barrels from them.

590 style threaded cap on the left, 500 style screw in barrel on the right.

Your average Mossberg M500 uses a barrel that screws right in to the magazine tube. The average M590 uses a ring attached to the barrel that and a cap that screws onto the magazine tube. But, Mossberg has made 500s that use the 590 configuration, and 590s that use the M500 style barrels.

Generally, only “tactical” style barrels are available in the 590 setup. All the specialized hunting barrels are set up for the 500 system. Why Mossberg has two different styles of barrels is something I don’t understand. I would have thought they would have picked one system and discontinued the other long ago.

As I said, Mossberg makes all sorts of barrels. You can easily buy a M500 style breacher barrel from their website. You can even buy Mossberg manufactured 870 breacher barrel from their website. But, for some reason unknown to me, they do not list the 590 style breacher barrel for sale.

When I finally decided to pursue this project, I decided I would get the barrel first. I never imagined it would be such a pain in the ass to find one for sale. It finally got to the point where I was expecting to have to buy a whole additional shotgun that came from the factory with this barrel just to get the barrel. Fortunately I got contacted by Pro Patria, who sold me a used barrel as a good price. Around that time a used barrel of this model also showed up on ebay. There are a good many of them out there, but most all of them are mounted on guns.

Changing the barrel on the Mossberg shotgun is very easy. Unload the gun, open the action slightly. Unscrew the magazine cap (or on a M500 the screw on the barrel). Then slide the old barrel off. Slide the new on on and screw on the mag cap.

This new barrel is 1.5 inches shorter than the old barrel. While that is not much, it is enough now to let me case the gun in my 33in rifle case with out disassembling it. That is convenient.

The bead front sight is replaced with a white dot front sight. This sight screws in place of the brass bead and has a white plastic insert. I’m undecided if that is really a worth while upgrade or not. There is a tiny gap between the base of the sight and the barrel and I question if it is any more durable than the bead sight.

But, most of the bead sights on the shotguns I used in the Corps were broken off, so having a front sight is already an improvement.

In the next section I’ll talk about the rail system.

Upgrading the military shotgun – Part 1 – Pointless Rambling

Part 2 HERE.
Part 3 HERE.

Alright, I was going to wait until the project was complete and post a short and sweet, concise, article about it. But now the last part I ordered is indefinitely delayed, I’m going to drag this out.

Later on I am going to talk about equipment, military usage of the shotgun, and various possibly interesting things, but that won’t be in this post. This post is just me rambling.

I don’t like shotguns. I don’t like them so much that I only own five of them. If anyone ever needed any evidence that I don’t like shotguns, that should be damning enough in it self.

When I found out that there were some contract over runs of the Mossberg model we used while I was in, I picked up one. Hey, if I was going to have a shotgun, I want it to be something somewhat special.

Mossberg M500M MILS

Some would say that every red bloodied American should own a shotgun, and those people would be wrong. But if you are a gun nut, you probably want to have at least a pump shotgun around, and the 500M MILS fit that niche for me. Something I could pull out once or twice a year, shoot, and remember why I don’t like shotguns.

For example, recently I have been doing 50 yard shooting with buckshot. I would be left wondering where the hell all the pellets are going. Shooting at an 8 inch bullseye, I would be left with only a few two pellets strikes in the black after firing multiple shots down range.

Now, kinda off topic, like a bulimic, I binge and purge my collection. I’ll spend a while buying up stuff I think I will like, then at some point, realize I have a guns, optics, and accessories I don’t like or don’t use, so I sell off that stuff. I sell it off to get money to spend on other stuff I won’t like or use. You will hear more about that soon.

The M500M is a keeper as it fits a niche my other guns don’t. That is, being a pump action shotgun. It is also a keeper as it has sentimental value of the very rare times I used a shotgun in the Corps.

Back in 2017ish the USMC announced that they were upgrading their pump shotguns into the “M500A2 MEK”

I’ll go into detail about the kits and the parts later.

I saw this, and was kinda tempted to reproduce the kit for my Mossberg. But rational thought prevailed and I didn’t.

I kept telling my self that I would be spending a good bit of money on a gun I rarely use, to change it into a configuration that I would shoot even less. It would be silly of be to waste my money doing this.

For about 3 years I told my self that. But then I had a thought. A worrisome idea that repeated like an earworm. What if I was becoming one of those old fogies spouting off non-sense like, “my wood and blued steel”? What if I was bypassing a major upgrade because I was comfortable with my old and obsolete clunker? *Gasp* What if I was acting like a M14 fan?

Of course not, but now I had an excuse to waste my money on this ‘upgrade’.

The parts of the Military Enhancement Kit were mostly easily available. Any conversion would be reversible if I didn’t like it or wanted to return to the stock configuration. And most of all, I had just sold some gun stuff I didn’t like and was ready to blow some money on something else I wouldn’t like.

I figured the heart of this conversion is the barrel, so I started looking for a barrel. I’ll talk about the barrel in part 2.