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The AR-15 is the most popular firearm design in the United States. The reason behind that fact is the gun’s ability to easily and quickly transform to meet the end user’s requirements. Everything from heavy-barreled long range precision to tiny stubby 9″ lightweight designs are possible, and swapping from one to the next can be accomplished using everyday tools and a little elbow grease. MGI recognized this fact and decided to take it one step further — why need tools at all? Why not make a gun where you can change barrels, calibers, and even magazine types easily and quickly on the range without any tools? And so they did, and dubbed it the Hydra . . .

There are two nifty enhancements that allow the Hydra to change so quickly from one configuration to the next: the barrel locking system and the magazine well.


On a traditional AR-15, the barrel is held in place using a barrel nut that holds the barrel tightly in place on the upper receiver. That barrel nut is a massive pain in the ass to work with though, and changing the barrel also generally means needing to cut the gas tube and re-install a new one afterwards. With the Hydra, the barrel nut has been replaced by two lever-operated bars. Turn the levers outward and the bars rotate, revealing a cut that allows the barrel to slide out of the rifle. Turn them back parallel and the cut is replaced by solid steel, tightly trapping the barrel in the upper receiver. Once parallel, a plastic keeper slides onto the levers to keep them from rotating and make sure everything stays nice and snug.

What’s particularly nifty about this design is that it doesn’t need any proprietary barrels or parts. Any standard AR-15 barrel and gas tube will work, and the gas tube can remain attached to the barrel the whole time. So while MGI sells a 5.56 NATO barrel kit, in reality you can buy whatever barrel you want and make it work yourself.

Changing barrels is cool, but changing magazines is even cooler.


The Hydra uses a custom-designed lower receiver that is actually two pieces. The serialized part is the fire control and pistol grip assembly, and the magazine well is a non-serialized part that can be replaced. In fact, MGI sells a kit that lets your rifle take AK-47 magazines and shoot 7.62×39 ammo, and it can be shipped straight to your door. The result is somewhat impressive.


I do have one gripe about this setup, namely that the parts aren’t keyed. An end user can quickly and easily swap things around on the gun, but there’s no system in place to ensure that the proper bolt is being used with the proper barrel. A 5.56 bolt fits in the gun when the 7.62×39 barrel and magazine well are installed, and vice versa. For those who pay attention and know what they’re doing it shouldn’t be an issue, but I would have liked to see some sort of failsafe built into the system. Heck, even just marking the bolt carrier with the intended caliber would have been nice, but nope. It’s a potential safety issue, and something to remember when playing with the gun’s guts.

The design works. With the push of a couple pins and the addition of some new parts, the gun can transform into a completely different beast. In fact, here’s a video showing how easy it is to swap from an American diet to some Soviet chow.

Before we get too much further into the review, I want to talk about the gun’s accouterments. The forward quad rail is big and bulky, but that’s a necessity for the quick barrel swapping nature of the gun. What isn’t necessary is the standard M4-esque furniture on the back of the rifle. MGI decided to stay with the standard carbine stock and A2 grip, and while their choices function properly they’re not exactly the most comfortable or fashionable choice. I would have liked to see some Magpul on the gun, but that’s just me.

That’s all well and good, but the real question is how well the gun shoots on the range.


The word “disappointing” comes to mind when we talk about the gun’s accuracy. The best group I got out of the gun’s 5.56 NATO barrel was a 2 MoA group at 50 yards, and that’s after trying a couple different brands of ammunition to find one the gun likes (Liberty Ammunition 5.56 — go figure). The 7.62×39 barrel perforated a group size nearly identical to the 5.56 barrel, so no change there. I get the feeling that this lack of accuracy is due to the mounting system for the barrel, namely the two rotating pins that hold the barrel in place. A barrel nut might be a pain in the ass to work with, but it holds things together extremely well. When you’re trying to shoot small groups, the key to accuracy is to make sure the barrel and scope alignment stays identical throughout the entire process. That doesn’t seem very possible with the mechanism in this rifle.

Another possible culprit for the accuracy issues: the trigger. IT IS TERRIBLE. I complain a lot about stock triggers in AR-15 rifles, but this one might just take the cake for worst stock trigger ever in the history of the world. Unless they were going for an 8 pound six-stage trigger, that is.

One thing I can’t fault the gun for is zero shift. Anytime I change even the slightest component related to my scope or barrel, I assume that my gun is no longer zeroed. With the MARCK-15, though, that assumption isn’t necessarily true. I spent a good half hour on the range doing nothing but shooting three rounds, removing the barrel, replacing the barrel, and then shooting three more rounds and the group never moved. I had one (large) ragged hole by the time I was done, and the rifle’s average point of impact never deviated more than a 1/4 inch from the point of aim.

Even swapping between calibers, the bullets flew straight and true. Moving from a 5.56 NATO to a 7.62×39 barrel, all I needed to do was spin the elevation adjustment turret a predictable and repeatable number of clicks and I was right back in the bull’s-eye.


The MGI Hydra MARCK-15 is a legitimately new gun. It takes the AR-15 platform and makes it even easier to swap around parts, and does it in a way that the average end user can make the changes without any tools in the field. The resulting product has its high points and low points in terms of manufacturing and design, but in general I like it. I’d just like it better with a little more attention paid to the details.

MGI Hydra MARCK-15 Piston

Caliber: 5.56 NATO
Barrel: 16″
Operation: Piston
MSRP: $1,599 (Website)

Special thanks to Alamo Tactical in San Antonio, Texas for being an awesome FFL.


Ratings (Out of Five Stars):

Accuracy: * *
Not particularly accurate, but removing the barrel won’t change the point of impact and swapping between barrels everything remains on the center line.

Ergonomics: * *
The stock and the grip are straight out of the 1980’s, and there’s a reason Magpul makes tons of money these days.

Ergonomics Firing: * *
The trigger is legitimately terrible. recoil is manageable even with the 7.62×39 kit though.

Reliability: * * *
No malfunctions, no complaints. However, getting the magazine to seat properly with the AK-47 magazine well is an issue.

Customization: * * * * *
I love the engineering that went into this gun. Great idea, well executed, and functional. Some gripes about the window dressing, but the ability to swap barrels on the fly is fantastic.

Overall Rating: * * *
The direct impingement version of this gun retails for $1,300, and I’d say it’s worth the price. The piston version is $300 more, and I just don’t see the value added. Then again, I generally don’t like pistons. For the rest of the gun, there are some high points and there are some low points. Use a better trigger (ALG Defense QMS anyone?), get some better furniture, and maybe work on the accuracy a bit and we definitely have a winner. But the concept is solid, and I hope to see it improve over the next few years.

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  1. Well I was reading this review and I was all giddy like “a TOTALLY modular AR-15 platform?!”
    Then I saw the accuracy. Huge letdown. Good review though, now I know to wait and see if they fix the whole accuracy issue.

    • The design has tons of potential. They just need to work out the kinks. And its not necessarily inherent design issues, a better trigger goes one hell of a long way.

      • If you’re in a situation where you need to takedown & conceal an AR15, 2 moa @ 50 yards isn’t going to be a factor..

        Unless you’re name is ….


        James Bond.

        The quick easy takedown is primary benefit – changing calibers is the cake..

    • I believe 2 moa meets the milspec accuracy requirement. I’m interested in the modular design.

  2. Looks pretty interesting but I already did the same thing (more or less) using parts from little startup companies. Granted, it was expensive, but I think it turned out well.

  3. Great review! As much of a PITA those barrel nuts are, they do their job. This gun kinda strikes me as an AR-15 version of the P320, with all the modularity and whatnot. Accuracy and trigger though we’re big letdowns, but fix those and what you have here is a great piece of engineering. It’s good to see some more actual improving on the AR-15 platform.
    PS It looks really cool with an AK magazine, and this might be the closest we’ll get to an AK for a while 🙂

  4. I can see the accuracy issue only being a problem if long range shooting is your need. For urban self defense use, where shots are going to be under 100 yards, this could be a great choice. And this concept is in the early stage of development. It should get better with the next generation, like most things do.

  5. I believe that Military Arms Channel has taken a look at this recently on a range trip video. Cool idea; good for spec-ops who want to shoot up somewhere and have it look like Terry with AKs whilst having 5.56 for official jobs.

  6. By the time you get a decent trigger, and Magpul accessories, etc., you will probably have a couple of grand or more tied up in this thing. It may be cheaper in the long run to just buy two separate AR’s, which would give you better accuracy.
    Myself, I cant see the value of a 2K AR that only gets 2 MOA.

    • I’d have to agree here. It seems like a neat toy but I’d personally have 2 neat toys that work better for the same money.

      I also can’t think of a single honest justification for this instead of 2 rifles.

      Having said the above…… I still want one!

      • Personally, I think this would be awesome to set up as a SBR, that way you could swap up calibers and everything pretty effortlessly, and less paperwork than doing multiple SBRs

        • You realize you still have to do the paperwork. If I buy two spare 10 inch barrels I still have two stamps.

        • Seans, the registered SBR is not the barrel, it’s the lower receiver, same as any other AR-15. A 10″ barrel is not an SBR, same as a 10″ upper is not an SBR.

      • They also have .22LR, 6.5 Grendel, 6.8SPC, .450 thumper, .458 SOCOM and .50 Beowulf
        plus the 2 x39 russian calibers 5.45×39 and 7.62×39

        Pretty much anything that fits into an AR magwell or an AK magazine will work.

  7. “The MGI Hydra MARCK-15 is a legitimately new gun”

    Yeah, It has been a new gun for at least the last years if not longer.

    • I’ve had my setup since 2008 or so. It’s an awesome concept. Too bad they haven’t gotten the glock 9/ 40 mag wells done yet.

  8. Very cool idea, and could very well be my next AR/AK. A lot of folks are perfectly happy with 4 MOA, but I’d like to see better. Heck, some AKs are 4 MOA all day.

    Of course part of the reason ARs are so popular is just by switching out uppers – and there isn’t any loss of accuracy.

  9. Nick I don’t understand why, if the trigger was part of the accuracy issue, you would not drop a better trigger in there and see how much that helps. If you were specifically trying to test the gun as shipped, could you run that test later? I’m curious about the accuracy potential of the system and it sounds like the trigger made that hard to determine.

    • We test guns as they come from the factory. The second you modify a gun it no longer reflects what you can expect from the factory if you buy your own.

      • I understand your policy on testing the gun as sent, however a follow-up review would be greatly appreciated and interesting. Perhaps you could test it with a nice trigger replacement or a completely different lower (non MGI) and see if the accuracy is improved. Does anyone buy an AR and leave it alone?

        • Sure they do. I bought a basic complete lower and an upper with a red dot included. I have literally not made a single modification to it.

  10. I guess this would be a little easier than carting around a spare upper or two, but really … Push out two pins, slide off old upper, place new upper, push pins back in, done.

    Unless you’re keeping the same caliber but only want to change barrels, I just don’t see the benefit other than maybe a bit less expensive. Given the possibility of using the wrong bolt, etc., however, I’m not convinced it’d be worth it.

    • If you want to run AK mags you’ll need a new lower too (hello FFL). Also for 9mm you’d need some sort of magwell block.

      • C mags has a new(ish) line of AR-compatible 7.62×39 magazines. (PWS specifically recommends them for use with their uppers and standard AR-15 lowers.)

        Given that I don’t have any AK mags to start with … Why would I want to use them?

      • The new CPD mags* are getting a lot of good reviews. Those are 7.62×39 and fitted for a standard AR magwell.

        I like the innovation here, but it’s adding more moving parts. To me that suggests more potential for fitment issues or wear. I much prefer dedicated uppers with zeroed optics and BUIS.

        * There was a company called C-Products that apparently made crap mags & went out of business. They’ve been bought out / rebooted as C-Products Defense and the reviews I’m seeing on those are very positive.

  11. Nick, given a rifle in the same cost range and optics, what sort of grouping would you achieve?

    • I actually was zeroing a similarly priced rifle with a lower power optic at the same time. I was holding sub-MoA groups with no issues. Triggers are a huge deal.

  12. $1500, a bad trigger and questionable accuracy…sign me up. Not. You can buy 3 AR’s now for $1500, not necessarily in 3 different calibers but still. And those CS throw levers…

  13. The price of a high-end AR.

    The accuracy of a low-end AK. (oooh, I went there!)

    The ergonomics of a brick.

    • …and not sure the Return on Investment is worth it. I’ll stick with swapping uppers or purchasing another rifle for another caliber. But that’s just me. This may be the bees knees to someone else.

    • An AR-15 with the accuracy of a AK-47. Modular indeed.
      For some intended purposes it may work well. At least with several calibers you would have options in an ammo pinch.

  14. It appears that one can stick a regular AR upper on their modular lower receiver (and you can get those separate). I think this lower + AK magazine well + regular 7.62×39 AR upper would be a real winner. No more mucking around with kludgey AR/AK hybrid magazines of questionable reliability. Could replace those AKs that are no longer being imported from Kalashnikov Concern.

  15. What gas piston kit is that? I think it rather amusing they put a bayonet lug on the thing when it appears to be a carbine length system (so the bayonet won’t fit properly when fixed….)

  16. I never understand why they don’t start these guns off in life as a pistol. That way you can always turn the gun into a rifle and then back to a pistol again (Like the Thompson Contender Kit). It’s always nice to be able to throw a compact barrel on the AR platform and now with the advent of the Sig Brace you can always go back and forth without any real loss of use of the platform.

  17. I’ve been looking at this system with quite a bit of curiosity. I like the AR platform, I’m just not incredibly fond of the .223/5.56 (I’ll keep my .22s rimfire). If I do get one, I’ll definitely just start with the stripped lower. And maybe just standard uppers.

    If the .308 shown on the MGI homepage works out, then I’m sold.

  18. Your 22 rim fire should last a long, long time considering how much ammo you will probably be putting through it.

  19. I own one in .45APC using Glock 21 mags. Makes a nice carbine combo with my Glock. I agree the trigger needs work but the adaptability and number of calibers dies make a fun system.

    • Hey Tim:
      I say your comment about the MGI in 45ACP caliber using Glock Mags. When and where did you purchase aforementioned Item. I’m a LE officer and have not found the kit that will convert my MGI hydra 15 into the 45ACP Platform? They don’t list it on their web site avaliability to purchase, many of the patrons that purchased this weapon have been waiting to purchase this conversion kit also listed myself with MGI CO to be notified when item is avaliable for purchase.

  20. Definitely not a 3-gunner though, Nick. Definitely not. Yet, at least. Really excited for Gen2 on this gun.

  21. Lol @ 6-stage trigger:

    1. Slack
    2. Slop
    3. Grit
    4. Creep
    5. Wait for it….
    6. Waaaait for iiiit….


  22. It shows promise. I think the barrel needs a better way to affix it into the upper receiver, and part of the accuracy issues are due to:

    – repeatability of the barrel mount into the upper
    – how the op-rd changes point of impact on the BCG
    – the trigger (but then, to me, most all OEM/”mil-spec” AR triggers suck rocks off the ground.

    Another issue: What’s the quality of the barrel? What chambering does it have? What’s the rifling look like?

    As for the barrel nut issue on AR’s: The way barrel nuts work on AR’s is much easier to change than screwing a barrel into a typical bolt action rifle receiver. The “crush” of screwing the barrel into a typical bolt action receiver can change your headspace, whereas the AR’s headspace is fixed by the barrel extension and bolt fit into the extension.

    And then there’s the torque issue of putting a conventional barrel into a bolt action receiver: Most of you haven’t seen what we gunsmiths do to take a barrel off/onto a receiver. It involves clamping on a wrench or vise onto the barrel cylinder, and a wrench is then applied to the receiver (or vise-versa). Sometimes, I have to put a 6′ cheater on the handle to get the barrel to break loose.

    Compared to a crusty old bolt gun’s barrel, the AR barrel nut is an absolute joy to deal with.

    The way I’d deal with the issue of the quick-change barrel would be different than the two camming surfaces on only two sides of the barrel extension. I’m thinking of something that is much more complicated, but would apply pressure against the extension uniformly into the upper all the way around the circumference of the extension, which isn’t done with this system.

  23. So, if they combine this replacable magwell system, and incorporate the improvements on the gen-II LRr308 upper, plus add a trigger and/or slightly upgrade the locking system to get under 1.5moa accuracy or better, and you’d have something approaching the be all, end all general purpose rifle.

  24. I have the hydra upper. My experience has been much better. Consistent moa from bench regardless of caliber shot so far. Red dot type sight is 3moa dot and no need for adjustment between calibers. Always in the zone.
    However off bench not so much.
    Must be the gun though. Because I never do anything wrong.

  25. Oh by the way. My cost to go to 300 blackout was only $275.00 takes 1 min.(cost of barrel, gas block, gas tube, roll pin, and flash hider .
    6.8 was like $225.00
    Next will be ?
    I like the concept. Just wish 7.62 mags worked well.

  26. I have one of these uppers, it sucks. I liked the concept, but now I realize all the concept flaws. Plain and simple you are better off with 2 or 3 or 4 guns, and for those who say it’s better for familiarity with a common platform, that’s crap, that’s like saying once you learn how to drive one car well you will suck at driving all other cars. I can operate an ar and an ak equally well and never get confused which one I am running, and if one gun breaks, I still have one.

  27. So I bought one of these two years ago and I agree 100% with the author. I have the DI version instead of the piston and have never had any real issues with it.

    The stock factory barrel and the trigger I believe are the main culprits for the authors accuracy issues. I got a new barrel from Bravo Company and then used a giessele trigger. Mine runs roughly 1 MOA now at 100 yards. Add some furniture that you like and you got a great rifle.

    The best parts though is changing from AK pattern to AR and back. Yes the bolts are hard to tell the difference but you can do your own work to tell them apart. My AR barrel has a different style muzzle break than the AK and I used a standard anodized AK bolt carrier and a shiny chrome or whatever it is for the AR.

    Also training is awesome. They have conversion kits tat can use Glock mags and a standard blowback bolt. Roughly 500 per conversion. The 9mm saves me gas and time from having to go to an outdoor range just to get some triggertime. I really like the Hydra man and they just keep getting better slowly but surely.

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  29. .22LR
    6.5 Grendel
    One Lower, One upper. “Drop in triggers work”<<<this is huge. Different Magwells, different barrels, and bolts. All modular.
    I am building mine! I can shoot whatever I have!

  30. I must have gotten a good one off the line. As a retired SWAT Commander I consider myself a little better than adequate with the AR platform.
    My experience with accuracy was not the same. I remove the barrel each time the firearm is cleaned and it is true that the reset has never changed the point of aim. I use a Konus multi reticle with tacticle turrets. Not a high priced site. I have had no luck with the flip up steel sights. I love it and have no regrets and no special upgrades.

  31. The MGI Hydra is a great alternate avenue to shoot pistol caliber rounds I bough mine in 9mm, with the intention of purchasing the 45ACP caliber that they stated it will use Glock magazines to be multi versatile from my Glock 45ACP weapon to the MGI System using the same magazine. Unfortunately its been over 3 years and they haven’t produced this conversion kit and they don’t have any urgency to produce this conversion kit for people that were so inclined to invest in their product. Sorry to say this company has put a sour taste in my mouth believing in the integrity of this company. Most likely will sell this weapon and purchase sig MPX, at least this company stands up to their promises to the Customers.

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