MGI’s Hydra: Mutli-Caliber From One Lower–New Gun Review

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The 9mm barrel is on the rifle, but the 5.56 barrel can be put on in a matter of seconds.

The Hydra is a mythical creature with multiple heads.  It was a snake-like water serpent dragon thing best left alone.  Every time you cut off one head, two new heads grew back in its place.  The ancient creature is also the basis for one of the most exciting new takes on the AR platform.  The Hydra, from MGI, combines a multi caliber lower with an interchangeable mag well and uppers in various calibers.  Right now, MGI is making 5.56, 9mm, 7.62×39, and 300 Blackout.  That’s only four distinct heads for the Hydra, but there are other calibers in the works.

The method behind the madness is surprisingly simple.  The Hydra lower looks just like a typical AR lower, except the mag well has been chopped off.  The interchangeable mag wells then slides back onto the lower.  The upper can be altered by switching out the bolt, and adding a new barrel.  The forend has a clamp built in that secures the barrel inside the upper. The upper then secures the mag well, and so on.

For the novice, switching takes only a minute.  I’d recommend that you wait until the barrel cools down first.  When we were shooting video for this review, we were moving fast, and the barrel was awkwardly hot during the first half of the conversion.  Watch out for that.


This is the 5.56 mag well with a magazine in place. It feels odd off of its rifle.

The process is simple.  The clamp on the bottom of the forend has a steel bar that holds it secure.  Flip up that bar.  Slide the polymer piece down the forend, toward the muzzle.  That exposes two locking bars that are aligned parallel to the bore (which indicates that the barrel is locked in).  Move them in opposite directions until the rods are perpendicular to the bore.  At that point, the barrel can simply be pulled out through the forend.  The forend and upper will remain together as one unit and the upper can be removed like any other.

With the upper removed, the mag well slides off easily.  There is a detent pin in the front of the trigger guard that needs to be pushed in so the guard can be lowered.  Pull the mag well up and out, slide the new one on, and reattach the trigger guard. Switch out the bolt in the upper and pin the upper back in place.  Slide the new barrel down inside the forend, and reverse the steps above.  Make the locking bars parallel, secure them with the polymer piece, and lock that in place.  Insert a magazine, cycle, fire.


These rails guide the mag well onto the lower.

When the gun is together, it feels like any other similarly configured AR.  There is a bit of a rattle in the gun, especially when there is no magazine in the mag well, but the rattle is a component of the modularity.  Without a bit of play, getting the mag wells on and off the gun would be cumbersome.  With a magazine in the rifle, it solidifies and seems absolutely normal.

We have two different caliber conversions: 5.56 and 9mm.  The feel of the 5.56 was hardly distinguishable from any other similar rifle.  The 9mm well is a 5.56 mag well sized chunk that has been cut for the much smaller 9mm mag, so there is some more mass to it.


This soap bar looking piece is what holds the gun together.

The clamp at the base of the forend is flat and wide, and makes that part of the rifle feel unique, but is isn’t in the way of your support hand, at all.  The rest of the railed forend allows for the use of any sort of accessories.  It isn’t something you can swap out easily, because of the clamp, so love it or leave it.  For the benefit I see from the rest of the platform, I think I can live with MGI’s forend, no problem.  It has ample room and is railed on all four sides.

The grip and stock are interchangeable.  If you want to monkey with the design, that is easy.  The rest of the gun is typical AR, too, so you can make the typical modifications to triggers, stocks, charging handles, etc.

Up to this point in this review, the Hydra has been defined by the novelty of multiple calibers running from one lower.  Yet there’s more.  The gun actually has a lot to live up to.  It is one thing to simply offer greater modularity and a variety of calibers, but it has to shoot as well (if not better) than the competition if the whole platform is going to survive. Many of us not only own multiple rifles in different calibers, we own multiple AR-15s.  So just having one lower has to really fulfill a unique purpose.


This detent pin secures the trigger guard to the mag well.

All I can say up to this point is so-far, so-good.  This gun runs incredibly well.  We had absolutely no issues with .223 or the 9mm rounds we shot.  There were no failures, no failure to feeds, no light strikes.  And that’s a pretty incredible accomplishment.  Many of the guns we’re sent for review can’t do that with just one caliber, much less two.  Think if the difference in pressures between the typical .223 and 9mm rounds.  These piston system is adjustable, but the 9mm is blowback.  The 9mm and the .223 are also being driven by the same hammer, with a spring tension that isn’t adjusted between calibers.

The 9mm uses a Colt style double stack magazine.  They are easy to feed into the well, but we had to pull them from the well when they were empty.  A good, sharp flip of the rifle worked too.  The 5.56 mags worked well.  We tested our typical battery of aluminum, steel, and plastic magazines.  No issues, there, and mags dropped free predictably.


The Aimpoint PRO is a great option for this rifle, or for any AR. And point of impact was consistent between the two calibers.

We ran an Aimpoint Pro on top of the gun for most of the testing, and shot up silhouettes and steel.  The flat shooting of the 5.56 upper made shooting while moving as easy as it is with any AR.  The barrel is capped with a typical A2 style flash hider.  On full sized targets, the gun is as fast and reliable as any typical AR, and maybe more reliable (in the long run) because of the piston system.

And we didn’t experience much deviation in point of impact from the 5.56 to the 9mm.  We didn’t have to tweak the Aimpoint at all.  The range where we shoot has a couple of long bays for rifle work, and a steel challenge course set up along the side.  I wasn’t about to shoot steel with the 9mm barrel from typical steel challenge distances (at least not until I saw how far it slung lead), but from about 50 yards out, the plates were no challenge at all.  Even though the 9mm barrel isn’t ported and has no muzzle device, the weight of the barrel holds down the rise and it shoots incredibly well.  The weight of the package is balanced nicely, so moving between targets happens easily.


We emptied a .223 magazine and then changed the barrel out for the 9mm.


From the same position, the 9mm barrel showed the same basic shot placement.


The 9mm barrel operates with simple blowback pressure.

The barrels on these are all 16 inches.  For most typical 9mm rounds, that equates to 1,300 to 1,400 FPS.  The 5.56 pushes out closer to (or above) 3,000 FPS.  Both are respectable, and serve distinct purposes. But there are other options.  The piston 5.56 version sells for $1599.  A typical gas operated 5.56, 7.62×39, 9mm, or 300 Black Out would sell for $1,299. If you want the MGI Survival Package, which includes all four calibers, the price is $2,799.

The main difference between the options has to do with three variables: caliber, gas block, and piston (or blowback, or gas operated).  The piston barrels require unique bolts.  If you simply want one rifle in one caliber, than the choices are fairly easy.  The additional kits (barrel, mag well, and bolt) sell for $550.  Individual mag wells sell for $275.

In the end, I’m left with two questions.  How will the various components of the Hydra stand up over time?  And how will the wear on the firearm affect its accuracy?  These aren’t easy to answer in the time we typically have to do a review, but we will be doing some follow-ups.


The 9mm mag well. How well will these pieces handle the abuse of recoil? They should be fine, and they’re easily replaced.

And there are lingering questions for the platform, too.  We’ve got a long history of having different guns in different calibers.  Lately, multiple caliber platforms have become more common.  I have friends who don’t have gun loving spouses.  Some of them have artificial, absurd limits on the numbers of guns they can own.  I’m emailing all of them this review.  If the ATF considers this one gun, than a spouse must consider it one gun, too.  That’s the way it works.

Yet here are also serious benefits for training .  If you can’t afford to shoot 300 Black Out, or 5.56 even, you may be able to pick up plenty of 7.62×39.  Or if you want to keep a 9mm carbine set up for home defense, have another option for play, and another for hunting, all working off of a similar set of controls, with the same exact feel, then the Hydra is for you.  I would suggest running frangible ammo in the 9mm, if you are going to run it inside, as 115 grain bullets traveling at 1,300 will punch through walls.

I’m excited about the platform.  I saw it at the NRA Show this year, but didn’t get to put any rounds down range.  Now that I’ve shot it, I see that it is much more than a novelty.  I’d like to see the concept taken further.  At the very least, I’d like a .22LR upper and a 12 gauge upper.  I know I might be dreaming with that last one, but so what. I never thought I’d be able to swap a 5.56 upper for 7.62×39, either.  Check out the Hydra.  Their Survival Package is conveniently packaged with all four of their calibers.  Four heads on one fire-breathing dragon.  Not bad for $2,799.


One great option from MGI is a piston driven 5.56 upper. This keeps the guts cleaner, which makes switching between calibers much easier.


The mag wells simply slide down milled rails in the lower.


The 9mm uses a long stick magazine. It will hold twenty rounds, easy, and empties much faster than it loads.


The forend is fixed to the upper, and the barrel slides inside and locks tight.


For the most part, the lower and upper feel just like typical AR parts, but they offer so much more flexibility.


The detent pin is easily pressed in with the point of a .223.


Even the engraving on the lower speaks to the modularity. And it can frustrate your FFL, who has to mark the caliber.


The gap where the mag well connects to the lower shows the relaxed tolerances that make assembly easy and truly modular.


The piston system is just one option for the 5.56 upper. You can get one with a traditional gas system, too.


One of the best parts about the Hydra is that the entire gun breaks into a very small package. The barrel fits in this case, too.


Extra barrels will come packaged with mag wells and a magazine.


The Survival package comes with all four currently available calibers.

{ 15 comments… add one }
  • Army127 September 19, 2014, 8:24 am

    Did everyone forget about the LMT LM308MW. . . Tat has quick change barrels as well and you can get I think 6 different calibers for that weapon or more can’t remember right now, just got off work. So the Hydra is new in that it changes the lower mag but not in that it changes calibers, and for reliability and quality (not price though LMT is very expensive with all those calibers, and even with just one caliber) I would right now go with the LMT if I had the money. I think this system needs more time to be proven in the long term with reliability and survivability. It does seem like it could be a good weapons system if it works in the long term, and isn’t breaking all the time. I am a wait and see on this and not sure it would ever be for me, as I am fine with owning my 3 different AR’s I have now in 5.56, 6.8 SPC, and .308.

    • Administrator September 19, 2014, 11:36 am

      This is a different product. It has mag wells that match the host magazine.

  • Lee Dingle July 24, 2014, 4:44 pm

    I like the concept, but I think $550 is too much for additional kits. I guess I see the point if you file SBR paperwork then you only have to do that once.

    Lastly I was so excited to see one in person at the NRA convention, and they were less than helpful.

    I think MGI has a while to go before it is worth while.

  • Craig Brown July 14, 2014, 10:28 pm

    I’ll take the Robinson Arms XCR anyway over this.

  • otisbohannan July 11, 2014, 2:48 pm

    I have I had an 18 73 colt 45I called the cops to see if they would the train colt 45 I’m Jerry why did stole the gunand stole the guns in my house I’m so now the cops set to antween Jerry Winans and the cops and they have the gun so I don’t know what we need them to where the gun is at gun was given to me for a gift as init was just not a ordinary colt 45 use a special colt 45 HeyJohn Wayne Colt 45 solid nickel platingI’ll tell you how bad is the cops are that Jerry why didn’t she stole from his mom Man City soul is moms car and pick up Steven you doing about it

  • otisbohannan July 11, 2014, 2:48 pm

    I have I had an 18 73 colt 45I called the cops to see if they would the train colt 45 I’m Jerry why did stole the gunand stole the guns in my house I’m so now the cops set to antween Jerry Winans and the cops and they have the gun so I don’t know what we need them to where the gun is at gun was given to me for a gift as init was just not a ordinary colt 45 use a special colt 45 HeyJohn Wayne Colt 45 solid nickel platingI’ll tell you how bad is the cops are that Jerry why didn’t she stole from his mom Man City soul is moms car and pick up Steven you doing about it

    • Benjamin July 14, 2014, 6:28 pm

      A Democrat on Meth or Weed?????

  • Johnnylee July 8, 2014, 8:26 pm

    Still waiting for .308 Nato………………………………………………………………………………..

  • KENT Visnaw July 7, 2014, 11:56 pm

    Please forgive my ignorance. What is a 300 Blackout? Thanks, Kent

    • Administrator July 8, 2014, 8:56 am

      It is a fairly new caliber that can be loaded into AR receivers. It can be loaded both subsonic and supersonic.

  • James July 7, 2014, 8:59 am

    The only benefit I see with this kit is that the 7.62×39 adapter uses AK mags. Other than that, you can swap out your upper for less money than this rifle.

  • Augest west July 7, 2014, 7:48 am

    I just don’t see the functionality being worth it, Having to change out the bolt with every caliber. Maybe it’s a good idea but for the laymen who just shoots for target sport and does not hunt I cannot see spending that kind of money to go from a 9mm to a 300 and inbetween the 223/308 The total cost for one of these kits must be tremendous. OK maybe I’m wrong. The kit I just looked up was 2,999.00 and comes with everything you need. Now I also think I am wrong about the bolt set up and you only have to change for 9mm. That being said I take back my earlier comment and say if you can afford a 3,000.00 dollar AR style rifle you might as well get four for more of you buck.

  • James Dudley July 7, 2014, 7:06 am

    Good idea but for the price I would need it in 308 and 12ga. Anyone with a starndard ar lower can buy the uppers in the calibers they offer. Good start but still lacking.

    • EB July 16, 2014, 11:07 am

      The Hydra actually has over 40 calibers from .22LR to .50 Beowulf, including .308.

    • Beast December 29, 2014, 7:27 am

      Yes you can do this same task with different uppers on a traditional AR lower. But think about how much you would spend customizing each different upper with your favorite optics. Or how much frustration there would be in re-zeroing your only optic when you moved it between your different uppers. With this platform, your optics stay put. So can you get by cheaper? …. perhaps. But at what cost in frustration.

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