DIY Bullpup Kit for Mossberg 500 & Remington 870

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Bullpup Unlimited isn’t a house hold name. They haven’t been around for 100 years. They have not revolutionized the firearms market–at least not yet. But what they have done is worthy of a closer look. Bullpup Unlimited builds kits that allow you to optimize your existing shotgun into a tool that is more compact, and more suitable for defensive use.

The BPU-500 kit for the Mossberg.

The BPU-500 kit for the Mossberg.

Hardly a beauty queen, but functional. If you need a more compact option, this one works.

Hardly a beauty queen, but functional. If you need a more compact option, this one works.

Why would you want make your own bullpup? Aren’t there enough bullpups on the market? Kel-Tec’s KSG, the Standard Manufacturing DP-12, the UTAS? I can see your point. And I own two of these three. Yet there are many out there who won’t trust their lives to new technology. No matter how many reviews come out praising a new design, there’s still something to be said for guns that have been around forever. Guns that have gone to war, and come back, and gone again.

And that’s the appeal of the Bullpup Unlimited kit. Designed around the Mossberg 500 and Remington 870, these DIY options take that old, beat-up, timeless pump shotgun and transform it into a compact powerhouse. Built as a non-permanent conversion, these kits are easily installed and even easier to remove if they don’t float your boat.

The polymer top rail offers a lot of space for the addition of sights, lights, lasers, shell carriers, etc.

The polymer top rail offers a lot of space for the addition of sights, lights, lasers, shell carriers, etc.

Features of the BPU-500

  • AR-style grip and safety
  • Split-trigger built in safety
  • Railed forend for accessories
  • Contoured slide release area
  • Rugged shell deflector for left handed shooters
  • Beveled magazine tube loading port
  • Tools included for conversion
  • ambidextrous sling points
  • Field strips with tools included under butt pad
  • Made from glass filled nylon
  • non-reflective surface texture
  • Overall thickness: 2.75, width: 8, length: 28
  • Conversion can be easily reversed
  • MSRP: $359.00
The AR style safety selector work well, too. That's an attractive feature of this build.

The AR style safety selector work well, too. That’s an attractive feature of this build.

How does it Shoot?

As this is not a gun, but a bullpup kit, the gun can only truly run as well as the gun you put inside it. Start with a functional 870 or Mossberg 500 and you should see good results. There are millions of these out there, and most of them work as well as they did the day they left their respective factories.

Because of the extra width of the BPU kit, a vertical grip may be useful to muscle the slide.

Because of the extra width of the BPU kit, a vertical grip may be useful to muscle the slide.

So what happens when you take a fully functional design and twist it up, figuratively speaking? You may think this thing won’t run. You’d be wrong! This kit is built around the Mossberg 500 and the bullpup runs flawlessly. Lock-up of the action is solid, the action racks smoothly and recoil is managed nicely. In essence, the only thing that has changed is the new shorter length and the adaptation of and AR15 style safety and pistol grip. And that’s exactly how it should be.

Running the gun through my normal testing and evaluation paces, we found the BPU-500 to be extremely controllable and easy to shoot. Recoil is dampened by the stock’s butt pad and the polymer enclosure shields your hands and face from all hints of heat, blast, and sharp edges. The controls are easily accessed and manipulated and, most notably, the trigger is a 2-piece (reminiscent of a Glock) design that adds an extra level of safety to the gun.

I’m not suggesting there is anything wrong with the old standby Mossberg 500, but when its dressed up like this… it makes you wonder why Mossberg hasn’t resurrected the old Maverick 12-guage bullpup shotgun.


Ergonomic Bullpup. Oxymoron? The constraints of the design don’t always make for gracefully operated guns. Ergonomics are sacrificed for the sake of size and maneuverability. With the BPU-500, this isn’t the case. The addition of features like the AR style safety and the AR style pistol grip allow for the shooter move seamlessly from America’s rifle into America’s shotgun. Beyond its controls, the BPU-500 also has the option of running railed panels on the handguard, giving the shooter places for a vertical grip, flashlights, or lasers.

Beyond the forend, the BPU-500 also features 3 ambidextrous sling mounting points as well as a full length 1913 top rail. The rail is made of polymer, but (considering that this is a scatter gun) it should hold a good enough zero to get the job done.

This gun is completely encased in polymer. When it comes to comfort, this is good. There is no hot or cold metal to touch your skin. There is less recoil energy transferred to the shooter. Where this becomes a negative is aesthetics. Plastic can feel somewhat cheap. The kit adds a very plastic look and feel to what has always been a very rugged and timeless firearm.

Some may like the looks of the BPU-500, but I personally don’t find it very attractive. However, it is very functional so I will bite my tongue on this note. That’s what actually matters. And I think I may be unfairly comparing it to the looks of the 870 and 500. Those two guns are classics. They’re exquisite. But not of the commercially available bullpups is winning a beauty contest.

Ejection happens high on the stock, behind the grip. In all of the rounds we fired, we had no issues with ejection.

Ejection happens high on the stock, behind the grip. In all of the rounds we fired, we had no issues with ejection.

But loading the gun itself isn't as easy as it is on a typical 500. Actuate the safety, flip it upside down, and fill it up.

But loading the gun itself isn’t as easy as it is on a typical 500. Actuate the safety, flip it upside down, and fill it up through that open cut behind the grip.

Reloading presented some new issues. Topping off a traditional shotgun is easy and intuitive. There is nothing easy or intuitive about reloading a bullpup shotgun and the BPU-500 is no exception. The best way to remedy the strange ergonomics of the the BPU-500 reload is to completely flip the gun vertically into your off-hand and load shells into the tube one by one.

If you can get past the strange reloading procedure, the gun is actually easy to reload due to the beveled loading port. I’ll give the bullpup shotgun design a C+ on reloads and the BPU-500 a B+ for making it easier than it normally is on an unconventional pump guns.


Installation of the BPU-500 kit is fairly easy. Though that statement is subject to your skill set, I will say if you can field strip your shotgun you will have little problems installing this kit. We have included the official Bullpup Unlimited installation video below for those of you who are interested in seeing the process or need help doing it yourself.

As far as tools need for installation, Bullpup Unlimited has made sure to include everything you need in the box. The conversion even comes partially assembled, making the install as easy as field striping the kit and inserting your gun.

I want to make it clear this is not a complete firearm and your success with the platform is subjective to the gun you are building it around as well as correctly assembling the firearm.

The rear end of the shotgun is secured just behind the thick buttstock.

The rear end of the shotgun is secured just behind the thick buttstock.


The BPU-500 is a quality kit that updates the pump-shotgun design. It gives the other bullpups a run for their money, all while avoiding the high price tag found on the competition. You can build a BPU-500 off an existing gun for $360. Or you can buy a Mossberg for $350 and the kit for $360 and have around $700 invested in the whole build.

{ 45 comments… add one }
  • Chuck February 9, 2018, 12:16 pm

    Thought it was funny during the first run, one of the rounds vibrated and fell out of the side saddle shell carrier.
    Looks like an interesting option, albeit a little bulky. I’m not sure that I would want to invest all that you could pay for a decent to good semi auto now days, but like he said, if you’ve got an old / extra laying around, it might be interesting / fun.

  • Dan Barnes January 25, 2017, 11:29 am

    my mossberg 500 has an 18 inch barrel, but it has a vent rib on it. will this fit within the bullpup confuguration?

  • darrenp1976 December 13, 2016, 6:15 pm

    Will this work on a 870 super mag, and were again do get those shell holders on your vest, very neat.

    • Paul January 30, 2018, 5:06 pm

      Did you ever find out where to get that shell holder?

  • Doug December 13, 2016, 11:31 am

    I’m just a little curious about the pricing. You compared a couple other models to this, and it would seem that price-wise they end up being somewhat comparable (within a certain $ range), but does that kit price fall into what most people would call a “reasonable” price range? I have a 500 that I think would be fun to convert, but $$ can be a bit of an inhibitor.

  • Ed December 12, 2016, 6:44 am

    Very old Winchester 20 gauge pump, works great but would love to turn it into a bullpup.

  • Rem870 October 27, 2016, 3:41 pm

    I spent some time playing with the Remington 870 Bullpup kit. It is fun to transform your standard 870 into a futuristic gun. Bullpup 870 is very compact and easy to use. But this kit is expensive and action is pretty stiff.

  • Craig March 1, 2016, 12:06 am

    Bought the kit, & a Mossberg Persuader last week. Really easy to assemble. Took to range over the weekend. Put 300 shells through it with zero issues. I have two complaints.
    1. The trigger, although it never failed, takes getting used to. It is a bit stiff, but once you realize it, & adjust for it, it easy to deal with.
    2. Although the butt pad absorbs most of the recoil for your shoulder, the hard polymer grip can be rough on your hand. After about 80 rounds, I put on my shooting gloves, & it felt great. I am going to order a Hogue grip to see if that solves the problem.
    I also purchased the shell holder seen in the video, which can be found at the link below. I put on iron sights & a red dot sight as well. All in all, I really like it!!!

  • Craig February 21, 2016, 9:38 am

    I too would also like to know where to purchase the shell holder. Could you add a link?

    • Dan March 15, 2017, 2:32 pm

      I would also like to know where you bought the shell holder.
      It looks like it has the Bullbup logo on it but I don’t see it on their site.

  • Travis Pitt February 14, 2016, 7:36 pm

    Great video and article, enjoyed it very much. I have several Mossbergs that would work with this. Maybe a stupid question…how does this complicate breaking down and cleaning the firearm after shooting?

  • Mike L February 12, 2016, 4:05 pm

    Great instructional Video. Nice NEW Application, good looking Shot Shell Vest What kind is it, and where can I get one just like it? Mike L

  • Ed Harrington February 11, 2016, 5:50 pm

    The shell holder on the Bullpup Mossberg 500 conversion can be found where and how much?

    • Craig February 24, 2016, 8:23 pm

      I asked this question also, but, it appears that the guy who made the video, only responds when someone posts something negative about his video. I did some digging & found the answer without his help. It;s a two piece system. the 1st piece is the Bullpup Unlimited 90-degree Side Rail. Here is the link:
      The 2nd piece is a picatinny side shell holder. They are available on the same site, but I got mine for a lot less at Amazon.

      Hope this helps.

  • Rick P February 9, 2016, 8:37 pm

    Will this kit fit onto a 590 or M590A1 Mossbergs?

    • Paul Helinski February 9, 2016, 9:52 pm

      When I spoke to them they said no. The best gun is the straight Persuader.

  • John February 9, 2016, 2:22 pm

    Will this conversion also fit the Mossy 590 A1?

  • Quincy February 8, 2016, 8:13 pm

    Got one of these kits on my 870……rock solid and works flawlessly…….always get people coming over at range and ask questions after watching me run it…

  • Don Barnes February 8, 2016, 4:46 pm

    Thanks for the video, I have a question you mentioned sob gear, can you post a link to the shotgun pouches and cards. Thx, db

  • Randy February 8, 2016, 3:30 pm

    Very well done and informative review.

  • Brian February 8, 2016, 12:58 pm

    With this work for a moss 590?

  • wilson February 8, 2016, 12:20 pm

    Will this work with the H&R Pardner? “They” say it’s a carbon copy of an 870.

    • Fletcher April 26, 2017, 10:41 pm

      I’d like to know the same thing

    • Gracie November 2, 2017, 12:25 am

      I have a Smith and Wesson model 3000, that is also a carbon copy of a 870, and was wondering if this would work. The gun works well but the wood is beat to hell.

  • Mike February 8, 2016, 12:03 pm

    Had me right up until he said Remington 870 mossberg

    • Paul Helinski February 8, 2016, 12:29 pm

      You try doing this on camera dickhead.

    • Yancey February 8, 2016, 4:38 pm

      yep, now I DEFINITELY don’t want this badass gun because the presenter made an insignificant error in his speech!

  • leonard olmstead February 8, 2016, 11:59 am

    Love this! However the reason I bought the mossberg is an ambidextrous safety. . I’m left handed is the AR safety available lefty?

    • DG February 8, 2016, 2:17 pm

      If it use’s AR parts which it looks like it does, you can get an AR15 ambidextris safety for $20 online. I use Rock River Arms, but CMMG and I think DPMS makes them also.

  • GrampyTom February 8, 2016, 11:05 am

    The original…… 28 years old…. … The new one looks to be far superior considering the availability of sights and accesories. My original has laughable sights in the carry handle. But I still have a special place in my heart for it. There is nothing like an original. This kit will only fit on a new modular Mossberg. We old dudes can’t mod one of our old 500’s to a new bullpup. Too bad.

  • Steve February 8, 2016, 10:38 am

    Since the conversion appears to cover the existing shotgun, how is the serial number of the gun visible for examination by a law enforcement officer, as required by law?

  • Matthew Mays February 8, 2016, 10:14 am

    Would it work on a mavrick I have been told they are nearly identical to the 500?

    • Chris Baker December 12, 2016, 7:19 am

      My 500 does not have a serial number. It predates the requirement. So, what do you do if you’re in that situation?

  • Ranbo February 8, 2016, 9:39 am

    Does this work for the 20 ga 870 as well?

    • Michael T Patzkowsky October 18, 2016, 1:31 pm

      I would like to know if it works for 20g also

    • bmaverick December 12, 2016, 8:27 pm

      Ditto. I love my Rem 870 20ga. It would be sooooo cool to do the bullpup kit conversion.

  • Jes February 8, 2016, 9:38 am

    Want to be taken seriously as a firearms and accessories reviewer? Get rid of the wind noise. It shows a great sense of disrespect for the viewer. As if they should watch the video just because you bothered to make it. It is annoying and unprofessional. If you can’t afford a piece of foam to cover it, make all your videos inside. That having been said, please keep sending emails, just don’t expect me to watch anymore videos offered by guns america.

    • Tim February 8, 2016, 12:42 pm

      No disrespect there yes maybe unprofessional in the eyes of someone who has nothing to do but go through and bash others, why don’t you make the video yourself since you are a expert in nothing, if you don’t like it don’t watch it. I respect the people who take the time to shoot videos to help others. Thanks to all who are doing that and thanks to viewers like jes who has nothing to do but bash others who are trying to help us

      • JLS February 8, 2016, 2:21 pm

        I agree with Tim I love these videos a little wind noise is ok with me just keep making the videos.

        • Michael February 8, 2016, 9:48 pm

          I’m hearing impaired though the wind noise wasn’t too bad. Very interesting concept by buying a second hand 870 (my preference) and use the kit to make a real CQC weapon that no one in their right mind would want to challenge. There’s lots to add and personalize. Good Video GOA, keep ’em coming, Michael SGT (Ret)

    • Jes' Vagina February 8, 2016, 4:45 pm


      I’m Jes’ entitled vagina. This video isn’t perfect so I figured I’d cry about it on a public forum and see if anyone gives a shit. As it just so happens, much to my chagrine, NOBODY gives a shit! I will continue crying until my boyfriend gets home and can console me and hopefully bring my fragile psyche back in line after this horribly traumatic experience. For now, please remove me from your list as I can’t bare to receive another video that has a slightly annoying sound in it!


      Jes’ smelly vagina.

  • Jerryj February 8, 2016, 7:28 am

    Wonder how this works with Remington 870 with a 28″ barrel and a XMax ???

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