The MGI Hydra AR-15 Rifle

Exponentially expanded AR-15 modularity.

The MGI Hydra AR-15 Rifle
By: Steve Tracy

A multi-headed water serpent from Greek Mythology was called the Hydra. A multi-caliber carbine in firearms terminology from MG Industries (MGI) is also called the Hydra. Mack Gwinn Junior was a member of the U.S. Army’s Special Forces recon team in Vietnam and his experience led him to design, patent, and improve upon various weapons systems.  

His first company, Gwinn Firearms, became the very successful Bushmaster Firearms.  Gwinn eventually sold off Bushmaster, but remained involved with the world of firearms that fascinated him. His newest venture, MGI-Military offers his innovative Hydra rifle, which expands the modularity of the AR-15 platform exponentially.

The Hydra is unlike any other AR-15 rifle on the market. It separates itself from its rivals because, despite its benign appearance, the Hydra platform can transform between calibers and missions like no other. The Hydra offers a quick change barrel, swappable bolts, and an ingeniously interchangeable magazine well that allow the creation of a rifle or carbine to suit virtually any purpose.    

Quality Standard Equipment

The basic MGI model is the MARCK-15 (as in MACK/AR-15), but it’s better known as the Hydra. The standard 5.56mm NATO barrel is 16 inches long and features a 1:9 rifling twist rate for standard weight bullets (55-66 grains). The barrel is free floated inside MGI’s aluminum quad rail and sports a front sight tower (adjustable for elevation), mil-spec black phosphate finish, and M4-style flash suppressor. 

The Hydra operates via the proven direct impingement system, commonly used for decades on the AR-15, where gas from a fired cartridge is directed to act upon the bolt to cycle the action.

At a glance, the carbine appears like the many AR-15s on the market. The five-position collapsible, M4 style stock has MGI’s logo molded into the side. The pistol grip is a plain-Jane plastic edition and the magazine release, bolt release, left-side safety lever, and charging handle are all standard AR-15 fare. The top Picatinny rail readily mounts a rear sight, carry handle, scope, electronic red dot, laser, or a combination of all of the above.

Three Steps to a Quick Transformation

At first, officers checking out the Hydra were a bit ho-hum upon inspecting what seemed to be just another AR-15 variant. Upon closer examination, officers familiar with the AR-15 platform noticed the bail and cover mounted to the underside of the quad rail forearm.  This mechanism is the first step in the Hydra’s three part transformation process and the most noticeable disparity between the Hydra and every other AR on the market. 

Lifting the bail allows the cover to slide forward and expose two locking levers.  Swinging the levers outward 90 degrees to each side releases the barrel, which is then simply pulled forward and out from the upper frame. This trick alone nets some “ohhs and ahhs” from jaded officers. With the barrel removed, cleaning it has never been simpler. Installing another barrel in a different length or caliber is just as fast and easy.

The two take-down pins, common to the AR-15 design, are pushed to the right and permit the upper receiver to be removed from the lower receiver. The Hydra’s bolt can then be withdrawn from the upper, along with its charging handle. The Hydra’s bolt can then be exchanged for one in different caliber to complete the second step of the rifle’s transformation.

The third step in the process of converting the Hydra to another caliber is the most remarkable. The lower receiver may seem the same as all the other ARs, but it is actually far different. The small pin at the front of the trigger guard needs to be pressed with a paperclip or thin punch, which will cause the bottom of the guard to rotate downward. 

Then the magazine release button is pressed halfway, which allows the entire front magazine well section of the lower receiver to slide upward on dovetailed rails as it separates from the grip area.

I performed this step at an outdoor range and a Vietnam Veteran range officer asked me to do it again so he could watch what happened again. The gentleman had an engineering degree and was fascinated with the multi-caliber ability of the Hydra system. He stated bluntly, “My M16 sure never did that! I’m impressed!”

A different caliber magwell slides right back into place in front of the trigger area.  Reassembling the rifle from its field stripped condition is all it takes to complete an entire caliber changeover of the MGI Hydra including barrel, bolt and magazine well.

The Many Calibers of the Multi-Headed Hydra

The test rifle came in a Plano rectangular hard case. It was surprising that a full-size AR-15 rifle could be broken down into such a compact size. The ability to remove the barrel so readily allows the system to be stowed in a very small space. Inside were two layers of foam to protect the gun. 

The first layer held the 5.56mm NATO platform and the second layer underneath contained the barrel, magwell, 20-round magazine, and bolt required to fire 9mm pistol ammunition. 

The MGI Hydra will also handle any “upper” conversion commonly available for mil-spec AR-15 rifles. This would include upper calibers such as the .204 Ruger, .300 Whisper, 6.5 Grendel, 6.8 SPC, .50 Beowulf, and the multitude of other choices. The Hydra system’s incredible interchangeable magazine well and quick change barrel system exponentially expand the calibers capable of being fired through the rifle. 

The 9mm, 40 S&W, 10mm, and 45 ACP pistol ammo can also feed the Hydra. A very desirable attribute of the Hydra system for law enforcement is the gun utilizes Glock magazines. The magazines on your duty belt can be inserted and shot in the Hydra rifle.  Extended, super high-capacity (30+) magazines function great too.

MGI offers interchangeable magazine wells that match up to barrel/bolt combinations in 7.62x39 Russian, 5.45 Russian, 45 ACP with Glock mags, 45 ACP with M3 Grease Gun mags, 10mm with Glock mags, 9mm with Glock mags, 9mm with Colt AR-15 mags, and 5.56mm NATO. An AR-15 looks quite striking with a curved 30-round AK-47 magazine protruding from in front of its trigger.

The test rifle’s 9mm magwell accommodated the Colt, law enforcement style, straight magazine. Flipping the bail, sliding the cover, unlocking the levers, yanking the 5.56 barrel, and substituting the 9mm barrel completes the straightforward barrel swap. 

Exchanging the rifle bolt for the handgun caliber bolt takes only seconds. The magwell trade is just as quick and easy. The railed fit of the magwell to the lower frame is solid and wiggle free.    

The 9mm barrel did not come with a front sight so I used a scope and an electronic red dot when test firing the Hydra. Recoil in 9mm was nil. The bolt obviously works much slower with 9mm handgun rounds than with 5.56 rifle rounds. But it did work… flawlessly. It’s rather astounding the factory recoil buffer spring will function with either caliber. 

The secret is MGI’s rate and recoil reducing buffer that is self adjusting. The more recoil reducing spring power required, the more the buffer cancels out the rearward energy of the carrier. The MGI system adapts to virtually any caliber.

Shooting the Hydra in Two Calibers

The 5.56 version of the Hydra was zeroed using a Leupold 3-9x scope. It was secured to the Hydra’s top Picatinny rail via a Burris P.E.P.R. mount. The trigger on the Hydra broke at a measured 6.5 pounds with some take-up and some creep. Once I had a five shot group that hit the center X-ring at 50 yards off a sandbag rest, I switched the Hydra over to fire 9mm pistol rounds.  

The 115-grain 9mm rounds weigh more than twice as much as the 55-grain 5.56mm rounds, and the 9mm bullets were traveling half the speed of the 5.56mm bullets.  Considering that the barrel, bolt, and magwell had all been changed, the Hydra still shot to point of aim with the slower pistol rounds. Windage was still right on. 

At 100 yards, the 9mm rounds fell 2 inches from the zero hold of the 5.56mm chambering. This is to be expected due to the weight and speed differences between the rifle and pistol calibers. At the closer distances of 50 yards or less, bullet impact was virtually the same with either cartridge.

The 20-round magazine provided perfect performance and the 9mm Hydra never jammed or misfired. The Thermold 30-round magazine that came with the Hydra rifle system did not have an anti-tilt magazine follower and it failed to feed and jammed repeatedly.  When a Magpul 30-round PMAG was inserted, the Hydra fed and fired with faultless precision.

One Rifle, Multiple Purposes

The “3-Gun” shooting competitions have enjoyed steady growth throughout the United States and monthly shooting matches have become common in many areas. Three-gun competitors fire a rifle, a shotgun, and a handgun at steel and paper targets. Courses of fire are set up to include movement, cover, magazine changes, and both short and long range shooting. 

The draw of 3-Gun is the thrill of the contest created by mastering the transition between three different weapons platforms. While many outdoor ranges and some indoor ranges can accommodate rifle rounds such as the 5.56mm NATO, still others require rifles chambered only for handgun rounds.

The Hydra’s ability to fire 9mm pistol rounds makes it an ideal choice for meeting the requirements of some firearms ranges. A box of 50 rounds of 9mm ammo is also less expensive than the same box of 5.56mm cartridges. 

It is to the Hydra’s advantage that the rifle’s overall operation, ergonomics, and trigger pull remain the same, despite various caliber substitutions. The 3-Gun competition is definitely applicable to law enforcement and can provide excellent supplemental training for officers who enjoy shooting competitions.

Unlimited Caliber Choices

If one of the mythological Hydra monster’s heads was chopped off, it would grow two more in its place. With the simple removal of the MARCK-15 rifle’s quick change barrel and magwell system, MGI’s Hydra has continued to add new calibers and magazine options with no end in sight. 

The Hydra’s innovative quick change barrel system, swappable bolts, and detachable magazine well has exponentially expanded the already modular AR-15 style rifle to new heights of transformation. A single serial numbered Hydra can convert from a 16-inch barrel patrol rifle to a 10-inch entry carbine. Caliber conversions can be ordered and delivered directly to an officer.

The MGI Hydra is not your everyday, run-of-the-mill AR-15. It can fire 9mm rounds like an HK MP-5, .45 acp rounds like a WWII Thompson 1927A1, or 7.62x39 rounds like an AK-47. The Hydra can transform from a patrol rifle to an entry carbine in seconds and its ability to use Glock magazines means MGI’s creation can fit any need law enforcement desires.

Steve Tracy is a 22-year police veteran with 20 years of experience as a firearms instructor. He is also an instructor for tactical rifles, use of force, less-than-lethal force and scenario-based training. He can be reached at

Published in Tactical Response, Jan/Feb 2013

Rating : 5.0

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