Build an 11 Shot Mossberg 500 – Detachable Mags, No Gunsmith Required!

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This is the Adaptive Tactical Sidewinder Venom kit installed on a FLEX Mossberg 500 6 shot shotgun. The 5 round straight mag is on the gun and the 10 round rotary mag is on the mat.

This is the Adaptive Tactical Sidewinder Venom kit installed on a FLEX Mossberg 500 6 shot shotgun. The 5 round straight mag is on the gun and the 10 round rotary mag is on the mat. The Sidewinder forend has a multiposition grip that screws into a steel insert. After wrestling with the installation, I am really impressed with the overall reliability and solid performance.


Adaptive Tactical Sidewinder Venom Kit
From $199: http://adaptivetactical.com/sidewinder

I have come to realize that some products are worth figuring out. That was the case on my newest trunk gun project, adding an Adaptive Tactical Sidewinder Venom kit to one of my Mossberg 500 shotguns. The kit did not go on easy, which I’ll explain, and I even had to send my gun into the company. But it turned out that the issue was my misunderstanding of the directions, and a miscue of information on the phone. If you have considered buying a KSG, or some other high cap shotgun, yet you have an otherwise unused Mossberg 500 in the safe, I would seriously consider this kit, which starts at $199 and goes to fully decked out at $399. My review is of the base model with a standard forend, but as you’ll see on the company website, there is now a version with an optics rail forend, as well as some current camo pattern options. The Adaptive Tactical Venom Sidewinder is a detachable magazine kit for the Mossberg 500/Maverick 88 shotguns, with the 6 round mag tube. And after some unnecessary trepidation on my part, it works marvelously.

The bottom package is our test kit. It came with one rotary magazine. We also tested a single stack 5 round. On the top is my old Knox Sidewinder package from the 90s that is still unopened. The product has gone through a few owners and iterations over the years. This is not a new idea or prototype product.

The bottom package is our test kit. It came with one rotary magazine. We also tested a single stack 5 round. On the top is my old Knox Sidewinder package from the 90s that is still unopened. The product has gone through a few owners and iterations over the years. This is not a new idea or prototype product.


The Sidewinder Venom is not a new product. It has been around for decades and was previously manufactured under at least a couple other monikers. I actually have an original Knox Sidewinder kit from the mid 90s that I never installed. I bought it from a friend (Hi Cory!) during the ban era and never got around to figuring out why I thought I need it, but after installing this new version of the kit I now understand. It turns an already excellent self defense weapon into a absolute beast of a self defense weapon.

The “why do I need this” question immediately arises for anyone who knows defense shotguns, because a regular Mossberg 8 shot Persuader, and the military 590A1 Tactical versions of the 500 are incredible firearms. But they are somewhat unwieldly due to the length, and all of the weight of the shells in the front part of the firearm. As a trunk gun or home defense gun, I have always kept my 6 shot 500 with a pistol grip at the ready, as opposed to my 8 shot guns (I have 3), because it is quick, light, and has a lot of punch in 6 rounds. Then, since I reviewed the Mossberg FLEX, with the breacher barrel (which I decided to buy from Mossberg), I have switched to that as my goto gun instead of my original 6 shot 500, which was one of the first firearms I ever owned.

So it was with great trepidation that I decided to convert this gun into a Sidewinder mod. And at first I was disgusted with myself because it initially didn’t go well, as I’ll explain. But first let me explain some things about the Sidewinder that I was incorrect about.

  1. It is not for NFA “Short Barreled Shotguns” – I had actually almost given my old Knox kit that you see here in the pics to a friend of mine who has a registered Mossberg 500 SBS. Those guns usually have a cut down 3 round magazine, which to me, coupled with the ridiculous muzzle blast, is completely useless. A detachable magazine would be a great aftermarket accessory, but the Sidewinder replaces the 6 round mag tube, and can’t be shortened.
  2. The rotary mag can be loaded either way and mounted on the magwell facing left or right.

    The rotary mag can be loaded either way and mounted on the magwell facing left or right.

  3. This is not a fringe nerd product – These days there are far more firearms and accessories out in the LE and military field than are designated “official use.” I know for a fact that the Adaptive Tactical kit is a military product, because when I sent my gun to them UPS **lost it** and it would have otherwise never have been found. But…Adaptive Tactical is a military supplier, so when the notified UPS that they would have to report it to FBI on those grounds, it was found in less than a week.
  4. I like it better than my KSG – This is really hard for me to admit, because I absolutely love the KSG. But my hands have been used to the Mossberg 500 for decades. When the KSG first came out, I was an early reviewer and I fell in love with the gun, which I also bought. But as time went on, I found that I felt more comfortable keeping the Mossberg as my goto. I don’t need to always carry 15 rounds in my shotgun, but being able to snap on an extra 10 round mag is a great feature.

Perhaps the biggest advantage to the Sidewinder kit (besides the price because the KSG is still seldom discounted below MSRP), is that you don’t have to keep the gun “loaded” in your car or house. In many states, having a loaded shotgun in your car violates state hunting laws, and in some states it is even a felony, even if it is in a case. If you live in a state where the ammo can’t be in the same car compartment as the gun, the Sidewinder allows you to keep the gun empty, yet have 10 rounds and more mags at the ready in your center console. If you have kids in the house and you want to keep one gun out of the safe for home defense, it’s the same thing. You can keep the mag in a separate location, and click pump, she’s good to go.

Getting the magazine tube off of your Mossberg 500 is not easy. I didn't have a vice set up so I used a pipe wrench and thick piece of leather to hold the receiver and some channel locks on the mag tube, which did get boogered up a bit. I don't plan to take the sidewinder off this gun, so the mag tube was going in the garbage anyway.

Getting the magazine tube off of your Mossberg 500 is not easy. I didn’t have a vice set up so I used a pipe wrench and thick piece of leather to hold the receiver and some channel locks on the mag tube, which did get boogered up a bit. I don’t plan to take the sidewinder off this gun, so the mag tube was going in the garbage anyway.


Now onto the installation. What an ordeal, but I genuinely think it was my fault.

To start, you have to take apart your Mossberg 500 as normal, including the two pins to take out the trigger assembly, and the bolt rails, plus the two metal “finger” shell retainers that keep the shells in the tube while pumping. Remember those darned shell retainers.

Then you have to unscrew the mag tube, which will not be easy, especially if you don’t have a vice set up, which I don’t. But I was able to use a piece of thick leather and a plumbing wrench in the same way, with channel locks to bread the tube free, because I don’t plan to reuse the tube. If you want to keep your tube and receiver both pristine, it would be a good idea to use a vice and probably thinner leather with channel locks or a pipe wrench to gently break the surprisingly stubborn tube free.

Beware that you have to screw the Sidewinder mag tube into the receiver hole only until it gets to the edge, then back it out and sand off some metal. They give you a piece of sandpaper to use. I didn't understand this simple instruction and muscled the tube on, swaging the end of it so that a shell couldn't pass. I was thankfully able to dremel this lip off. What a bonehead huh?

Beware that you have to screw the Sidewinder mag tube into the receiver hole only until it gets to the edge, then back it out and sand off some metal. They give you a piece of sandpaper to use. I didn’t understand this simple instruction and muscled the tube on, swaging the end of it so that a shell couldn’t pass. I was thankfully able to dremel this lip off. What a bonehead huh?


Then you screw the Sidewinder assembly into the mag tube hole, but do so very gently, and watch the front of the hole, where the shells eject from the tube, to see where the Sidewinder tube comes to the end. DO NOT FORCE THE TUBE TO GET IT TO LINE UP WITH THE RECEIVER. That was my first mistake, because I didn’t understand this. Mossberg does not have a standard thread pattern on that threaded hole. It starts where the person working that day decided to start it, so the Siderwinder magazine well will not line up to straight at exactly the same depth every time. And since straight is straight and can’t be modified, what has to be modified is how much threaded tube you screw in.

Adaptive Tactical includes a piece of sandpaper to sand away some of the magazine tube, so that it lines up straight without forcing the tube in further than it needs to go. So when you get there, if your mag well isn’t straight, back it out, sand some off, then try it again. And do that until it is straight and the tube is clearing the edge.

This was my other issue, for which I actually sent the gun into the company. It clearly says in the directions to leave this shell stop part off of the gun when you reassemble it.  The result was a gun you had to shuck twice for each shot. Yet another bonehead move.

This was my other issue, for which I actually sent the gun into the company. It clearly says in the directions to leave this shell stop part off of the gun when you reassemble it. The result was a gun you had to shuck twice for each shot. Yet another bonehead move.


I didn’t do that. I forced it, and that swaged the end of the tube into a cone that would not allow the shells to pass. My remedy was to dremel away the swaged portion, and it worked out fine. But this was only the first of my problems. See the pictures. This issue is easily avoided by being patient, and backing out the tube, then sanding, until it fits correctly.

My next issue was that I didn’t understand one of the key directions. There are those two metal fingers that hold then release the shell from the magazine as you pump. The directions, which have been rewritten over the decades, are very clear that you only install one of them, but for some reason, it didn’t register what a “shell stop” actually is. So I had this installed kit, but the shells just weren’t feeding correctly. A phone call to the company didn’t resolve the problem, because I didn’t call the little metal fingers “shell stops” so they just couldn’t figure out what was going on with the product. Then, after sending the gun in (and having UPS lose it and find it), they were like “oh yea that’s the most common issue we deal with when gunsmiths try to install the kit.”

In trying to figure out what was wrong, I confused how this wedge fits in as well But fortunately the system is mature and well designed, so I figured it out. You should have no problem with yours if you follow the directions.

In trying to figure out what was wrong, I confused how this wedge fits in as well But fortunately the system is mature and well designed, so I figured it out. You should have no problem with yours if you follow the directions.


You don’t need a gunsmith to install the kit! You just need to actually read and understand the directions, and it would help to have a vise handy. In trying to figure out my issue, I tried to turn the inside shell follower around, and the retainer shim became confusing, but I never had to mess with that at all.

Now that the kit is installed correctly, I took it out today and put several boxes of both low brass bird shot and high brass buck shot through the gun. It didn’t budge, and though I would say that the 500 is maybe a little more lubrication sensitive (I usually keep my 500s dry), the Sidewinder Venom worked without one failure. I was able to test the $35 straight 5 round magazine, and the $90 drum 10 round magazine, which can be clicked in either left or right leaning. I didn’t get the $50 10 round straight mag, but I doubt they ever have problems either. The entire system can be bought in black at base prices or in ATACS, Multi-Cam, or Desert Digital camo for a little more.

My only real complaint with the system is that I have on several occasions laid a magazine down and nicked the magazine emptying button, which is on all the mags. The shells just flow out onto the ground. I think it should have a stiffer spring so that it is harder to accidentally release.

My only real complaint with the system is that I have on several occasions laid a magazine down and nicked the magazine emptying button, which is on all the mags. The shells just flow out onto the ground. I think it should have a stiffer spring so that it is harder to accidentally release.


My only beef with the system is that the magazine emptying button on the Adaptive Tactial magazines.

If you are a fan of the Mossberg 500, but you’ve wanted more rounds without having to thumb rounds in one at a time, the Sidewinder Venom is a really good investment in a gun that will never be worth less than you put into it. A lot of big box stores and gun dealers carry the kit at slightly discounted prices, and the mags are readily available right now in this slow market directly from the company and other places online. Converted Mossberg 500s and Maverick 88s show up on GunsAmerica from time to time as well. Check out the new forend on the Adaptive Tactical website before you jump on buying one. Maybe they’ll send me one and we’ll return to this product for a second round, and a second gun! I miss my FLEX forend, but oh well. 🙂

This is the system laid out so you can see what goes where. I no longer need my FLEX forend assembly and magtube.

This is the system laid out so you can see what goes where. I no longer need my FLEX forend assembly and magtube.

{ 37 comments… add one }
  • Brian Neagle April 8, 2017, 6:39 pm

    Paul, Thank You for your insight and experience in dealing with the trails and tribulations you encountered during this conversion. I too have Mossberg 500’s in my home. It is one of my two H/D choices. I am comfortable with it being a good choice. It has proven to be a great gun through the years and very reliable. My other is a Smith & Wesson wheel gun. The answer to why is because neither fail. Many may disagree with my selections and that is fine. I will not berate them or question their choice. I may ask them why that was their choice but that would be as a learning experience and perhaps sway my own. Not as a point on contention. I enjoyed reading your post. I appreciated your honesty in detailing and sharing your difficulties.

    Thank You.

  • Bill F. September 2, 2016, 8:06 pm

    Paul, great article! I carried a 870 for 15 years, but was given a Mossberg 500 many years ago. I love that shotgun, as well as other Mossberg weapons I have since purchased. I respect the fact that you took a bullet for the rest of us considering this modification. You laid it out nicely, and your honesty with the challenges you encountered is much appreciated. We aren’t all black belts in gunsmithing like a lot of the know it all critics that posted comments. Thanks again!

  • Mehrlol June 4, 2016, 2:09 pm

    Hello, thanks for the article.
    Is the system works with a ventilated fairing on the barrel? And is it possible to put some picatiny rails on the rifle even with the kit conversion?

  • L.F. December 16, 2015, 1:05 am

    how many rounds do you need to get the friekin job done ??? Yo Cowboy !

  • Switchblade December 15, 2015, 6:36 am

    This Rube Goldberg setup is going to be way too heavy to be feasible as a fighting shotgun. Two hours time is too long to put on something like this for an amateur anyway. I don’t believe there is any logic to dragging around more than 8 or 9 rounds anyway, unless you truly believe in the zombie apocalypse.

  • Jeff December 6, 2015, 12:19 pm

    Looks cool.
    But why go to all that trouble and expense when you can just use the Aguila 1-1/2″ 12 ga shells and effectively increase the magazine capacity?

    • Peter December 16, 2015, 9:06 pm

      Have you ever tried those in a Mossberg 500?
      I have and doing so throws the reliability right out the window…fast!

  • William C October 31, 2015, 4:56 pm

    Ii just completed this conversion on a Mossberg 500 JIC Cruiser. It went very easy, but a vise would indeed make it much simpler.

    “It is always the people who think they know something that don’t read the directions and look on youtube for videos.”

    It’s a case of having too much knowledge for the task. Having never disassembled a shotgun, I read and re-read the instructions until I understood exactly what they were talking about and could identify each part. I took the shotgun apart, and then re-assembled it and did a function check. *Then* I proceeded with the conversion and can now visualize each step even though I only did it once.

    Good article. Thanks.

  • Al Martin October 22, 2015, 4:08 pm

    Is this adaptable to the .410 version of the Mossberg 500?

    • Administrator October 22, 2015, 6:32 pm

      no

  • skip June 27, 2015, 10:33 am

    Having read the article, and the comments, I am shocked that a man tries to help others, and some of the commentaries try to ridicule him for being helpful. He not only tried to be nice and helpful and took his time and effort to do so, he admitted he was not the sharpest knife in the drawer when it came to doing this . That takes a lot of courage and character, but shows he is a man and can admit to himself and others he makes mistakes and has no problems with his ego admitting it. I would suggest you that like to ridicule others use him as an example of sorts and maybe even take a good long look in the mirror before commenting in the negative next time. Bet if he had a cure for cancer and a member of your family had it, you would not be so easy in your criticism of him.

    • Donald Clinton January 25, 2017, 12:50 pm

      I find it better to learn from people that make mistakes during process. I like to know what not to do. What to do I can figure out. Great article. I know what to look forward to, when working on a firearm system similar to this now. Thanks!

      • SeaRick March 14, 2017, 9:48 pm

        Precisely. Ideally, we would always like to learn from a ‘perfect’ install but reality is that it is easy to make mistakes especially if you don’t speaka da chinese spanish french instructions found on so many products.

  • Steven June 23, 2015, 9:37 am

    Will the drum hold 3 inch shells, or is it 2- 3/4 inch only, as with the drums made for the Saiga 12?

  • noel p. June 22, 2015, 10:24 pm

    Cut the author a little slack. The article is good, well illustrated and the errors he admits to are common in gunsmithing and moreso in gun use. I believe many should go look into the mirror and see if they recognize the fellow who has never made a firearm mistake and we are not talking about gun safety here either. This may not be for anyone or everyone but if you love to experiment with guns and you have a Mossberg 500 lying around and you want a weekend or evening project go for it. I always like these types of articles. they have me playing around with my Swiss K-31s now and although my beech stock is maintained strictly as issued, I\’ve got plans for the other two and one is a scope thing. When we bought a case sized order back when they were about $125.00 we had so much fun that we immediately started fun matches (and try and beat that K-31 for accuracy) and playing around with the ideas that we read about.

  • Scotty Gunn June 22, 2015, 1:37 pm

    Too Tacticool. Seriously, just buy a Vespr or a Saiga. Then you get a 20 round drum. 11 shot pump? Yawn. The 590 is 9 rounds-isn’t that good enough? The Sidewinder kit has been around for years and years. Surprised they are still in business. $35 for a five round mag, $50 for a 10 round? Ouch.
    Hate to say it, but your converted gun looks very um “ghetto”…..

    • Mikial December 14, 2015, 6:54 pm

      @Scotty Gunn

      Got to agree with you there. I’ve had a 500 for a lot of years and used it a lot to hunt with. It resides in my stash with plenty of ammo, but these days my home defense shotgun is a Saiga 12 with a 12 round mag. And yes, all you naysayers, it ticks like a clock every time I shoot it. I keep it in a horizontal rack on the side of my mattress with a good light on it. Anyone coming into my home uninvited is in for a rude welcome, no matter what they are armed with. All these “Gee Whiz” tacticool guns are just a fad . . . an expensive fad at that.

  • Pops45 June 22, 2015, 12:52 pm

    Some harsh comments. Never did I see the author state he was a gunsmith, in fact he acknowledged the mistakes he made. No problem offering constructive criticisms or suggestions, but some of these comments sound like the bloggers on MSNBC.

    • Administrator June 22, 2015, 1:31 pm

      The funny thing is that they don’t even read the article. It clearly says that gunsmiths make that error all the time. They actually make both errors all the time. Gunsmiths are the most common customer support issues at AT. It is always the people who think they know something that don’t read the directions and look on youtube for videos.

  • John June 22, 2015, 12:28 pm

    Just heat the magazine tube up where it screws into the receiver. It breaks the threadlocker. You don’t even need a wrench to unscrew it. Dang. How much work do you actually do on firearms? And you’re a gun writer?

    • SeaRick March 14, 2017, 9:54 pm

      I think there is a difference between a gun writer and a gunsmith. From your post, you would expect a politician to be a journalist for politics, a Miss America to comment on make-up, or a grocery cashier knowing how to farm. In this case, the author acknowledged he made mistakes that were likely his own fault – a refreshing situation these days when so-called journalists are merely lying, shit-eating douche bags for the leftie snowflakes.
      I for one thought the video was well-done, credible and above all honest. Each to their own opinion, I suppose, since I am expressing mine about your post (not you but your post).

  • david June 22, 2015, 11:00 am

    RTFM

  • Bo June 22, 2015, 10:50 am

    Looks simply TOO ninja for me.

  • Joe June 22, 2015, 9:58 am

    This would be a good idea for someone already in possession of the gun it is intended for, except with a real butt stock instead of that silly wrist rattler pistol grip.
    I would most likely go buy a new Century automatic with the ten round mag myself if I already didn’t own two automatic shotguns .

    • Administrator June 22, 2015, 10:06 am

      Well this gun is a Mossberg FLEX, so the stocks interchange. That was the point of using it.

  • Dave Hicks June 22, 2015, 9:49 am

    12 gauge only ? I like my MOSSBERG”s , 410 and up but have been shooting 20 gauge because of less recoil. All those comments on the article author, ( Paul ) he’s a gun nut too so no more poking fun at is honest gumsmithing.

    • Administrator June 22, 2015, 10:07 am

      No look at the website. They do have other gauges.

    • Administrator June 22, 2015, 10:10 am

      There is a link out to the website so you can easily find the information page. 🙂

  • Luke June 22, 2015, 9:02 am

    I’m an AdapTac Authorized Dealer and I converted my old M500 with the Sidewinder Venom™ 10-Shot Rotary Magazine with the Wraptor™ Forend. The conversion took less than 2-hours, (because I actually understood the easy-to-follow instructions). The real fun started when I dumped 90-pellets down-range and was almost instantly ready to dump 90-more.
    The only improvement needed is a more shock absorbing rubber recoil pad because there is no shoulder conversion kit yet available… OUCH!

  • Tommy June 22, 2015, 8:55 am

    Will this work on the Mossberg 500 chainsaw grip?

  • Mahatma Muhjesbude June 22, 2015, 8:54 am

    Yes, finally a viable ‘fix’ for the historical tactical deficiency of all pump ;’shitguns’ the ease, or rather the lack of it, and speed in reloading an already unfavorably limited magazine.

    But why didn’t they make a kit for the most prolific pump shotgun in the world? The 870?

    • Dave Hicks December 14, 2015, 12:57 pm

      I’ll stick to the 5 round tube,for easy carry. This just seems to clumsy to me. There are too many Remington 870 varieties to mess with, is the most likely reason this kit didn’t come out for the 870.

  • Mas June 22, 2015, 8:18 am

    Seriously – a vertical grip on a pump shotgun? Boooo!!!!

    What kind of “gun writer” doesn’t even have access to a vise?

  • Rick June 22, 2015, 8:16 am

    I have never sanded metal, unless it was a burr or to simply dull a cut. How do I unsubscribe to such dribble? In fact I wonder how you got my email address. And the author himself mentions to how it took him years to install. I am guessing he wasnt to excited himself to try out this kit.

  • Christopher Billings June 22, 2015, 7:34 am

    Would this kit also work with the rifled slug barrel.

  • Scott miller June 22, 2015, 6:24 am

    I just installed my sidewinder kit yester day, I went for the upgraded version, it only took me two hours to install, and I am very happy with the result, it is an awesome kit looks great and works well

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