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.


  1. I’ve enjoyed your articles on this topic and appreciate the work put into it.

    What would your opinion be of the various not-a-shotguns shotguns that makers like mossberg and remington are pumping (hah) out if pressed into this role?

    • I was going to mention that in the last part. Guns like the Shockwave or Tac-14 came out a decade after the military started this project.
      In some ways, one of those new guns would be preferable. Smaller, lighter, etc. Add a sling mount like the Ergo one in the MEK kit and it would be handy.
      Downsides of these other weapons are two fold. The inability to quickly mount a stock makes them less ideal if pressed into the role of a primary weapon.
      The major downside for using something like a shockwave for breaching is the lack of a standoff. It would be harder to keep the muzzle of a Shockwave or Tac-14 the right distance from a lock and it might slip while you try and hold it in place. You run the risk of busting the barrel on the Shockwave or Tac-14. Because the ring the barrel uses to attach to the mag tube is much closer to the muzzle you might render the gun non-operational if you burst the barrel.
      That said, there was a limited run of Shockwaves with a breacher barrel.
      The MEK upgrade was pretty simple and commercial “off the shelf” parts that were readily available. Had they been doing that today, they might have gone with something closer to the Shockwave.


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