Last year CMMG surprised the shooting community with the innovative Mk47 Mutant, a gas-impingement operated semi-automatic AR pattern rifle built around the 7.62x39 mm cartridge (which is commonly associated with the AK-47). Encouraged by the success of the Mutant, CMMG has taken what they learned and applied it to the new big-bore MkW Anvil chambered in .458 SOCOM.
To better understand the thought process behind the design of the MkW Anvil, it's helpful to take a closer look at the .458 SOCOM cartridge. Rumor has it that the .458 SOCOM evolved out of conversations amongst members of the Special Forces community. They were looking for a cartridge that would provide more immediate close-range stopping power than the 5.56 NATO. The cartridge would need to be compatible with existing M4/AR-15 style carbines, fit into standard-issue AR magazines and operate at sub-sonic speeds for use with sound suppressors.
It was Marty ter Weeme of Teppo Jutsu who developed the .458 SOCOM in 2000 with Tony Rumore of Tromix building and firing the first rifle chambered for it in February of 2001. As the cartridge name implies, the bullet diameter was increased from 0.224” (5.56 NATO/.223 Rem.) to 0.458” but the overall cartridge length remained the same. As a result, the .458 SOCOM fits into 5.56 magazines.
When topped with lighter bullets ranging from 140 to 400-gr. in weight, .458 SOCOM achieves velocities of 1600 to 3000 feet per second (fps) with solid accuracy and effective levels of stopping power out to around 150 to 200 yards. For suppressed fire, the cartridges can be fitted with heavy 500- to 600-gr. bullets launched at around 980 to 1300 fps. Not only does the .458 successfully fill its intended military roles, it's also a great choice for close-range big-game hunting.
But modifying a typical AR-15 built for 5.56 NATO to fire .458 SOCOM is a lot like converting a 1911 pistol from .45 ACP to the much hotter 10 mm cartridge. It can be done safely and the gun will operate reliably. However, the components are subjected to stress levels above and beyond their original design. As a result, they tend to wear out or break down more quickly.
To address this issue, CMMG's engineers didn't look to the AR-15 as a starting point for the MkW Anvil. Instead, they turned to its sibling, the AR-10, for inspiration. The AR-1 0 is designed to fire the longer and more powerful 7.62 NATO/.308 Win. cartridge. This rifle's medium-profile receivers and beefier bolt carrier group provide plenty of support for the .458 SOCOM.
The MkW Anvil is available in three configurations. The T model arrives with a mil-spec trigger and furniture. The XBE (used for this review) has a mil-spec trigger with a Magpul MOE grip and six-position stock. The XBE2 ships with a Geissele SSA 2-Stage trigger and Magpul MOE furniture.
The rifle's proprietary milled billet 7075-T6 aluminum receivers feature a unique profile, a matte- black hard-anodized finish and magazine well sized to accept 5.56 NATO AR-15 magazines. The trigger guard is an integral part of the lower receiver. The controls are typical AR configuration with the charging handle on top, the bolt catch and safety on the left and the magazine release button on the right. The upper receiver does have a dust cover for the ejection port but there is no forward assist. The mil-spec trigger on this rifle was better than the typical factory offerings. It was smooth and grit free with a trigger pull of 5 lbs. 15 oz.
The direct gas-impingement system features a low-profile gas block fitted with an adjustable metering screw. This allows the gas flow to be tuned to match the operator's preferred bullet weight. The 16.10" long medium-taper barrel is made of 416 stainless steel with a matte-black finish. The 5/8-32 TPI threaded muzzle sports a CMMG SV muzzle brake. The 15" long RKM15 free-float aluminum KeyMod hand guard is topped with a full length Picatinny rail for mounting optics and removable sights (no sights are provided). QD (flush cup) sling swivel ports are located on the right and left sides of the handguard and Magpul MOE shoulder stock.
The rifle arrives with one Lancer L5-AWM polymer magazine stamped “458 SOCOM” on the left side of the metallic feed lips. It has the same follower and configuration as a 30-round 5.56 NATO magazine, which allows it to hold 10 rounds of the thumb-sized .458 SOCOM cartridges. Out of curiosity, I broke out several of the AR magazines I have on hand to see how they would fit. Mil-spec 5.56 magazines of all sizes would accept the cartridge. The 20-round magazines would hold 7 rounds of .458 and the 10-rounders would hold 4 rounds. This is good to know for hunters who live in states with magazine capacity restrictions. Be sure to check your magazines before heading out into the field. I found that some commercial and aftermarket magazines would not load or feed the .458 SOCOM.
Although the .458 SOCOM functions reliably in standard AR mil-spec magazines (including those marked .458 SOCOM), it's not a perfect fit. It is possible when pressing the cartridges into the magazine to push the top round a little too far causing the rim to stick out past the rear of the feed lips by a fraction of an inch. This can hang up the feeding process if it's not corrected. Also, when I slammed the magazines home with too much vigor while the bolt was in the open position, the top round occasionally popped loose from the magazine and had to be dumped out the ejection port before the bolt could be closed. When I checked the position of the top round before reloading and pressed the magazine into place a bit more gently, these two problems went away. So, I'm going to write them off as user-induced malfunctions for this review. When I did my part, the rifle ran nicely with all of the controls and features doing exactly what they are supposed to.
With an unloaded weight of 7 lbs. 8 oz., the MkW Anvil handles nicely off the bench, like a light- barrel hunting rifle. In regard to felt recoil, this gun and ammunition combination produced a kick comparable to a defensive 12-ga. shotgun loaded with full-power buckshot loads when firing 300-gr. and heavier bullets. It's stout but not so punishing as to be painful for more experienced shooters.
Formal accuracy testing consisted of five, five-shot groups fired at 100 yards from a bench rest with a Leupold VX-R Patrol 3-9x40 Firedot optic attached to the rifle. Bullet velocity was averaged for 10 shots using a Lab Radar chronograph. Black Butterfly Ammunition, a new ammunition company, provided two loads. The practice-grade 350-gr. Copper Plated Solids produced an average group size of 1.53" at 1821 fps. The hunting grade 300-gr. Nosler Ballistic Tips averaged 1.88" at 1953 fps.
Polycase Ammunition offers a .458 SOCOM Inceptor load topped with a 140-gr. ARX bullet. Traveling at 2424 fps., this round averaged 2.06". Underwood Ammunition offers a load featuring a Lehigh 300-gr. Xtreme Penetrator solid brass bullet. Traveling at 1900 fps., this load turned in the best group average of 1.38".
CMMG is a company that thoroughly understands and appreciates the AR platform, inside and out. Their devotion to building excellent rifles is clearly reflected in the fit, finish and features of their guns. The new MkW Anvil rifle and ammunition combination offers an excellent level of reliability and stopping power for big-game hunting at close range. And should it become necessary to defend yourself from predators of either the four-legged or two-legged variety, this rifle would certainly do the trick. Like other CMMG products, this rifle ships with the company's Lifetime Quality Guarantee.