The civilian .50 BMG shooting world is primarily about bolt-action precision rifles built for long distances. Three years ago, Kerry O’Day and Roger Hunziker at MG Arms in Spring, Texas, decided to attempt to build the most accurate semi-auto .50 BMG rifle in the world. Enter the Behemoth.

MG Arms is best known in hunting circles for its high-end, ultra-light rifles. The company’s jump into the .50 BMG world was driven by customer requests. As development progressed on a rifle to meet the expectations of long-range shooters, there were also serious inquiries from law enforcement agencies who wanted an anti-materiel weapon with sniper-grade accuracy and semi-auto firepower.

The Behemoth’s design has a long history. Robert Pauza, a close friend of O’Day, designed its progenitor, the P-50, in the 1980s based on the Soviet Tokarev SVT-40 of World War II. Pauza built around 100 rifles in the early 1990s before Pauza Specialties closed its doors in the economic downturn. Each P-50 was essentially hand-fitted without the kind of parts interchangeability you would expect today, but O’Day appreciated its great potential. The Tokarev tilting bolt was simple, easy to machine and locked up strongly. Its adjustable gas system allowed for the use of ammunition of varying power and helped to dissipate recoil. Most important for accuracy, the barrel was screwed solidly into the receiver.

With Pauza’s blessing, O’Day updated his basic design to reach its maximum accuracy potential while improving safety and practicality, and achieving a level of consistency in manufacturing through CNC machining. that allows parts interchangeability. Should a part wear or break, the shooter can order a replacement and install it themselves instead of sending the rifle back to the shop for repair. For quality-control purposes, MG Arms does all of the Behemoth’s machining and heat-treating in-house with the exception of the barrels.

The MG Arms Behemoth is a massive rifle at 35 pounds unloaded and 61 inches long with a 31-inch barrel. To save weight, the receiver cover, trigger housing,trigger/hammer support frame, buttstock and handguard are made from 7075-T6 aluminum.

True to its Tokarev rifle origins, the Behemoth is simple to field-strip. It takes less than 10 minutes and a 3/16-inch Allen key to break the gun down. Once you get the aluminum receiver cover off (eight screws) and carefully depress the heavy bolt spring for removal, you can slide the bolt carrier and bolt to the back of the receiver, tilt it to the left and lift it from the receiver. The bolt slides out of the carrier, exposing the inertial firing pin held in place by a roll pin, which can be pushed out by hand to remove the pin and spring for cleaning. For those unfamiliar with the tilting bolt design, while in battery, the rear of the bolt falls into a channel in the bottom of the receiver and is held down in place by the spring-loaded bolt carrier body above it.

The bolt has left and right lugs on the back, too, but these aren’t to lock it in battery—in fact, it’s just the opposite. The lugs match up with corresponding cuts in the bolt carrier. When the rifle fires, the bolt carrier is knocked backward a short distance by a gas-driven piston. On its way back, the carrier notches grab the bolt lugs and lift the rear of the bolt up, unlocking it. Once unlocked, the action becomes in essence a blowback system, and the rearward inertia of the cartridge case and the remaining gas pushes the bolt and carrier backward, past a removable ejector slotted into the lower receiver.

The lower receiver completely encloses the bolt and carrier. From a shooter safety standpoint, that is hard to top. In the event of some kind of catastrophic bolt failure, it should not end up in your head. The receiver is made from 17-4 stainless steel that has been heat-treated to a Rockwell hardness of 40. The carrier is also made of heat-treated 17-4 stainless steel. The bolt is made from 4130 carbon steel that has been heat-treated to 45 Rockwell. All of the major parts of this rifle are machined from billets.

A Closer Look

The Behemoth’s match-grade barrel is made by Pac-Nor and available in 26-, 29- or 31-inch lengths. My test rifle came with a 31-inch barrel with three lands and grooves in a 1-in-15-inch twist rate. MG Arms chose Pac-Nor because of the company’s reputation for making competition-winning barrels for 1,000-meter benchrest shooters. The barrel is free-floating inside the aluminum handguard, which features KeyMod slots on the sides and Picatinny rails on the top and bottom. The bottom rail is perfect for mounting a bipod, and for now, MG Arms equips the Behemoth with a specially adapted military M240 machine gun bipod, though a GG&G large-caliber bipod will be available soon.

The barrel is equipped with MG Arms’ own Super Eliminator muzzle brake, which reduces noise and blast by about 25 percent and felt recoil by 20 percent. Of course, like most brakes on a .50 BMG, it has some blast to it. On the range, the Behemoth’s muzzle blast knocked light items off the tables of shooters adjacent to us.

The Behemoth’s tappet-style gas system is much simpler than the Tokarev rifle’s and requires no tools to adjust. It has three settings. To change the amount of gas being tapped off the barrel, the shooter pulls back the bolt half an inch, depresses the knurled knob sticking out in front of the handguard, turns it, and releases it into one of three fixed positions signified by one, two and three drilled holes in the knob. The knob locks positively. To break the system down for cleaning, you pull two pins out of the locking block that surrounds the barrel and slide everything forward and out. It can be done in less than one minute. No gas is ever vented back into the receiver toward the shooter. The outer tube of the gas system has vent holes, but they are inside the handguard, and vented gas cannot harm the shooter. Changing the gas system setting from position two to one also reduces felt recoil and the distance the spent cases fly.

The trigger, hammer and safeties are a modular assembly inside a skeletonized aluminum frame, making it easy to inspect and clean. To access it, you remove the four Allen-head bolts that hold the aluminum trigger housing to the bottom of the receiver. This takes about one minute. The hammer is driven by an exceptionally heavy spring. The trigger is not user adjustable but comes factory tuned in the 5- to 6-pound range.

The Behemoth is finished in either PTFE or Cerakote in any color scheme the buyer wants. My test gun had a charcoal glaze Cerakote finish that resembled a faint tiger striping. As you would expect, the finish wore a bit on the bearing surfaces and sharp edges, but it seemed to hold up everywhere else.

The rifle can be single-loaded or fed from a five-round, heavy-sheet-metal magazine. The large magazine release is in front of the triggerguard and is activated by pushing it forward. The rifle doesn’t have a bolt-hold-open feature and the springs are heavy, so care must be taken while single-loading rounds in the chamber. A slip could result in an injured finger. The safety slides laterally across the bottom of the trigger housing, behind the pistol grip. This position doesn’t allow it to be actuated by the shooting hand without breaking your grip. The safety can be reversed if the shooter desires. When set to “fire,” a red ring is exposed.

The buttstock is removable for transport and attaches with a single pin that locks it solidly onto a massive dovetail on the rear of the receiver. The adjustable stock uses ball-detent-
positioned wheels to change the length of pull and comb height, and the soft MDT foam recoil pad is adjustable left and right as well as up and down. I noticed that after a bit of shooting the screws that hold the pad to its backing plate got loose and had to be retightened. Some blue Loctite would surely address this minor deficiency.

Harnessing The .50

On the range, I immediately noticed that the Behemoth had less overall felt recoil than a Barrett semi-auto. The Behemoth is comfortable to shoot, with recoil similar to a bolt-action .308 Winchester rifle. This is due to its hefty weight and the adjustable three-position gas system. I noticed that opening the gas system to the one-hole setting (the largest) noticeably reduced recoil and increased the distance spent cases were thrown. Don’t expect to be reloading all your once-fired brass. Even with the gas system opened up, I still found some bending of the case rims by the extractor, which I would expect to cause feeding and headspace problems if not corrected in the case preparation process. On the two-hole setting, the extractor damaged the case rims beyond salvage, and the ejector gave them a deep chop mark on the back to boot. If you can afford a $12,200 rifle, chances are you can afford to leave your brass on the ground.

In testing, the magazine twice demonstrated a failure to feed the sharply pointed Hornady A-MAX match ammunition, but only on the third round. Rounds one through two and four through five fed fine on these two occurrences. I suspect some minor adjustment in the magazine’s internals may be in order. The angle of the third round put the point right into the lower flat of the barrel chamber. Care must also be taken to install the magazine correctly. It tilts in like an AK-47’s rather than sliding in like an AR-15’s.

The test rifle was equipped with a superb Nightforce NXS 3.5-15X scope mounted solidly to the Picatinny rail machined into the receiver cover. Though the cover is screwed onto the receiver, I didn’t detect any negative effect on accuracy. Unfortunately, the front hood of the scope covered the two screws that attach the front of the cover to the receiver. Because of this, I had to dismount the scope to field-strip the rifle. When I reattached it, I was within 5 inches of the original zero. A scope with a shorter hood or the addition of a quick-detachable mount between the rail and scope could solve that problem.

My test rifle’s accuracy was quite good. At 15X magnification, the scope reticle perfectly framed the target bullseye at 200 yards, so I fired my test groups from that range without bothering to readjust the scope’s zero. Firing from a stable bench (the ones at Knob Creek Gun Range in West Point, Kentucky, are made of steel and bolted to the concrete floor) and using sandbags as a rest, I was as steady as I could hope to be. Hornady’s 750-grain A-MAX ammunition lived up to its reputation and shot a very impressive best group of 2 inches. It gave me 2.25-inch groups on average. Using the one-hole gas setting, the average velocity was 2,608 fps, a bit less than the 2,820 fps at the muzzle for which the ammunition is advertised.

The MG Arms Behemoth is a beast to carry around, but it is easy and fun to shoot, and quick to field-strip. With its fixed barrel, it is also the most accurate .50 BMG semi-auto rifle I have tested—it’s on par with many bolt-action guns. Where its weight is not an issue, its accuracy and firepower could make it very useful as a countersniper/anti-materiel rifle.

For more information on the MG Arms Behemoth, visit or call 281-821-8282.


  • Caliber: .50 BMG
  • Barrel: 31 inches
  • OA Length: 61 inches
  • Weight: 35 pounds (empty)
  • Stock: Adjustable, skeletonized
  • Sights: None
  • Action: Semi-auto
  • Finish: Cerakote
  • Capacity: 5+1
  • MSRP: $12,200

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