CMMG MkW-15 ANVIL -fFormer U.S. President Theodore Roosevelt’s famous “Big Stick Diplomacy” has sculpted much of America’s political and cultural ideologies over the years. The expression, “Speak softly and carry a big stick—you’ll go far,” may have even earned Roosevelt a Nobel Peace prize for his soft-spoken negotiation tactics to end the Russo-Japanese war in 1905. However, there are times when speaking softly isn’t an option and the big stick needs to do the negotiating.

Echoing the latter part of Roosevelt’s ideology is CMMG’s new MkW-15 Anvil rifle, chambered for the beastly .458 SOCOM cartridge. As many know, the .458 SOCOM was designed to give U.S. military personnel greater close-range stopping power than the .223 Rem., while maintaining compatibility with most mil-spec M16 parts. Naturally, the civilian firearms market adopted the .458 SOCOM for the AR-15 rifle. The ability to fire 300- to 550-grain bullets with the simple swap of an upper receiver is an appealing feat for hunting, home defense or vaporizing watermelons.

Like anything, there’s always a trade-off, and firing the .458 SOCOM from a standard AR-15 platform isn’t immune to problems. The bruised shoulders of many shooters provide vivid illustrations of the increased felt recoil of the cartridge. In fact, the .458 SOCOM introduces standard AR-15 parts to physical stresses that they were not originally designed to withstand. Additional forces produced by the cartridge are known to cause mechanical and cycling problems with standard AR-15s such as unreliable feeding, over-gassing, and metallurgical stress on moving parts. Cycling issues are also known to destroy brass, which is inconvenient given the cost and availability of .458 SOCOM brass cases.

CMMG MkW-15 ANVIL 1
The .458 SOCOM is a potent round, and the MkW-15 ANVIL is purpose-built to handle it.

A Bigger Stick
Enter the CMMG MkW-15 Anvil rifle, which was specifically engineered to achieve optimal performance and longevity with the .458 SOCOM. From a glance, the Anvil looks like any other AR-15. Pick it up, and it even has the same ergonomics and controls as your favorite .22-caliber carbine. But pull the trigger and you’ll know immediately that you just delivered a bit more lead than a 55-grain XM-193 downrange.

While it looks, wields and operates like a standard AR-15, look a bit closer and you’ll find the direct impingement, gas-operated semi-automatic CMMG Anvil was designed more like a Hummer H1 than a Jeep Wrangler.

CMMG MkW-15 ANVIL Bolt carrier
The bolt face on the standard AR-15 bolt carrier looks small in comparison to that featured on the MkW-15.

Compared to other Mil-Spec ARs, the Anvil’s parts are beefed up all-around to give it greater longevity. Its receivers and bolt carrier group are significantly reinforced with extra aluminum and steel for added rigidity. In fact, many of the Anvil’s major mechanical parts are not cross-compatible with standard AR-15 components—an intentional engineering decision by CMMG to give the Anvil maximum performance and reliability with the .458 SOCOM cartridge. CMMG utilized a similar beefed-up design with its Mk47 Mutant rifle introduced in 2015, which was built around the 7.62x39mm cartridge.

The introduction of the Anvil rifle marks the first semi-auto rifle (that I know of) that was designed from the ground-up for the .458 SOCOM cartridge. Diverting from other manufacturers who simply slap a bigger barrel and bolt onto existing AR-15 hardware, CMMG’s Anvil is actually a bigger hammer entirely. In fact, its billet aluminum upper and lower receivers will not even attach to standard AR receivers. While shooters who desire cross-compatibility may see the proprietary design as a turn-off, the Anvil’s enlarged skeleton enables the rifle to run more reliably, reduces recoil and adds longevity through high round counts. Though its major mechanical components are scaled up to handle the .458, the Anvil maintains compatibility with standard AR-15 magazines and some common parts such as the fire control group, pistol grip and buttstock.

CMMG MkW-15 ANVIL
The MkW-15’s receiver is beefed up specifically for the .458 SOCOM, as can be seen here.

Given the Anvil’s compatibility considerations, do the potential risks of using proprietary parts outweigh the performance benefits of its custom-tailored design? In short, absolutely. Even with standard AR-15 rifles that have been converted to fire .458 SOCOM, the parts most prone to failure—the bolt and the barrel—are still relatively expensive and hard to find. Personally, I find greater comfort in the fact that the Anvil’s components were specifically engineered to maximize reliability by minimizing the known failure points of standard AR components at the mercy of the .458 SOCOM.

CMMG MkW-15 ANVIL 4Like a Wrecking Ball
Quite simply, if you had to pick one semi-auto rifle equally capable of stopping a car, a hog and a home intruder on the same day, look no further than the CMMG MkW-15 Anvil.

Thumping out almost 2,400 foot-pounds of muzzle energy, the Anvil is more than capable of delivering serious stopping power in the form of .45-caliber projectiles. Not only that, but the gun is capable of handling the gamut of .458 SOCOM loads on the market. Its adjustable gas block and threaded barrel are also inviting for custom-tuning the Anvil to run smoothly with your favorite loads through a suppressor.

CMMG MkW-15 -rangeRange Tested, Writer Approved
During testing, we found the factory gas block setting was a bit excessive, likely tuned for maximum reliability. Dialing the gas flow down a small amount seemed to reduce felt recoil without affecting reliability.

In terms of accuracy, the Anvil produced average groups of 2.25 inches at 100 yards with a Nikon M-223 1-4×20 scope. Greater magnification would likely increase accuracy; however, the intent of the .458 SOCOM round is not necessarily to offer pinpoint precision but rather superior stopping power at close range.

Among the variety of ammo tested, the MkW-15 took no distinct favorites, making it a very versatile firearm for anything from hunting to home defense. For domestic applications, some may argue the .458 SOCOM is overkill when in fact it is very suitable for home-defense with the right ammunition choice. Projectiles such as the Lehigh Defense Controlled Fracturing copper or the Polycase ARX are likely the best candidates for home-defense applications because they offer reduced penetration and devastating terminal effects on soft targets. When bigger is better—as is often the case with .458 SOCOM—we found the CorBon 300-grain Deep Penetrating Expanding (DPX) to function reliably through the Anvil and perform exactly as advertised.

After firing 200 rounds of various .458 SOCOM rounds through the MkW-15 Anvil, not one mechanical or cycling problem was observed. Firing that many beastly rounds was surprisingly comfortable, especially compared to standard AR-15s converted to fire the mighty .458. While the Anvil certainly doesn’t eliminate recoil, its beefed up design absolutely makes each trigger pull more enjoyable.

CMMG MkW-15 final
CMMG’s MkW-15 ANVIL is a serious option for shooters searching for a dedicated .458 SOCOM AR platform.

Parting Shots
With the Anvil, CMMG makes it harder than ever to justify not owning a rifle in .458 SOCOM. The Anvil is clearly capable of delivering 300- to 550-grain bullets with great accuracy. Based off the AR-15 platform, the MkW-15 is ergonomic and has familiar controls that make it easy to shoot well. Its reinforced design offers peace of mind that the major components can handle the .458 SOCOM cartridge. Most of all, this gun is just flat out fun to shoot and follows Roosevelt’s time-tested motto about carrying a big stick. As far as positives, it’s a beast. Accurate. Versatile. Reliable. Ergonomic. Fun to shoot. If there are negatives, it’s heavy (almost 10 pounds fully loaded with scope), and .458 SOCOM ammo is expensive (a great reason to reload!).

Editor’s Note: This article is an excerpt from the December 2016 issue of Gun Digest the Magazine.


Want to learn more about the AR-15?

Get a handle on your tactical rifle with Gunsmithing the AR-15 — The Bench Manual.

Gunsmithing the AR-15: The Bench Manual

In Gunsmithing the AR-15: The Bench Manual, author Patrick Sweeney covers every component that makes up the versatile firearm — from buttstock to muzzle brake. This is essential information, given that the sea of potential upgrades available today can swamp even veteran AR users. Learn more