CMMG’s “Thumper”

The .458 SOCOM Anvil Brings
To The AR Platform

By Holt Bodinson

Packing the equivalent of a .45-70 round in a mid-sized AR platform is routine at CMMG, which, this year, offers three refined models of their hog-slaying “Anvil” in .458 SOCOM caliber. It’s truly the mouse gun that roars, made possible by some creative engineering and the successful commercialization of the wildcat .458 SOCOM cartridge.

I asked CMMG’s Product Design Engineer, Von Davis, what led to the development of the Anvil. He observed the owners of the company were enthusiastic hog hunters and wanted a rifle with the ballistic energy of the .458 SOCOM in a compact, strong, durable platform not only able to handle the .458 SOCOM, but allowing the cartridge to be pushed to its limits.

The .458 SOCOM cartridge itself is creating quite a stir. As an original design, it’s a short, fat, capacious, round with a rebated rim, generous bottleneck, capable of handling bullets weighing from 140 to 600 grains and designed to fit into and feed from existing AR-15 magazines and M4 lowers. It was developed and nurtured by Marty ter Weeme, founder of Teppo Jutsu LLC in 2000. Named after the joint services, Special Operations Command, the .458 SOCOM was designed by ter Weeme to put real horsepower and lethality into the M4 platform as well as offering smack-down performance at subsonic velocities from a suppressed rifle.

CMMG’s new .458 SOCOM Anvil series consists of three models differing only in the triggers and furniture they’re fitted with at the factory. The Anvil features a new mid-size AR receiver developed by CMMG based on a shortened and lightened AR-10 receiver and is powered by a beefed up bolt, called no less, the Powerbolt.

CMMG’s new .458 SOCOM Anvil is a compact hunting rifle weighing 7-1/2 pounds.
Based on their mid-size platform, the rifle features a beefed up upper with
CMMG’s Powerbolt. KeyMod handguard slots enhance flexibility in mounting
accessories while the SV compensator really tames the SOCOM.

Meat on the table! Holt took this nice javelina with one shot at
30 yards with the SBR 140-grain loading in the CMMG .458 SOCOM.

As Von Davis explained it, CMMG moved to a more robust action and larger Powerbolt when chambering the SOCOM because it is still a wildcat cartridge without SAAMI pressure standards, and being a large diameter case, it generates considerably more bolt thrust than the 5.56mm round. Bolt thrust is the internal area of a cartridge case times maximum chamber pressure. It’s a force expressed in pounds. The thrust on standard 5.56 on the AR-15 bolt head measures roughly 3,887 pounds using standardized loads while the thrust on a .458 SOCOM bolt head measures approximately 5,853 pounds, or roughly 2,000 pounds more. The 5.56mm proof load measures roughly 5,183 pounds while the .458 SOCOM proof load generates 7,804 pounds, or over 2,500 pounds more. Pictures are better than words in this case. Notice the two bolt heads pictured. Both are machined to accept the rim of the 7.62×39 cartridge. CMMG’s beefy Powerbolt is on the left. An AR-15 bolt is on the right. The Powerbolt offers a more robust margin-of-safety, a larger extractor and ejector and extended service life.

All three Anvil models sport 416 stainless steel, 16-inch barrels, considered the optimal length for the .458 SOCOM cartridge, delivering both excellent ballistics and balanced handling in a carbine-length package. CMMG chose to use a 1:14-inch rate-of-twist in its barrels which enables the Anvil to handle bullet weights ranging from a 140-grain speed king to a nasty 600-grain thumper.

The Anvil barrels have a medium weight taper and are threaded 5/8-32 for CMMG’s very effective SV compensator or an aftermarket silencer. The barrel is further fitted with an SLR adjustable gas block enabling the owner to fine tune the Anvil for loads using this wide range of bullet weights.

All three models of the Anvil are furnished with a full-length CMMG KeyMod handguard and optics rail. Average weight is 7.5 pounds unloaded. Ranging in price from $1,850 to $2,150, the only differences are the brands of stocks they’re fitted with and, in the case of the most expensive model, a Geissele SSA 2-stage match trigger. Our test model was the MkW Anvil XBE selling for $1,950 with a Magpul MOE pistol grip, 6-position, CTR buttstock and a single stage, mil-spec trigger.

While AR-15 mags (above, foreground) will accept .458 SOCOM rounds,
CMMG modifies the feeds lips of Lancer mags to insure optimum performance.
Imagine .45-70 Gov’t ballistics packed into the AR-friendly, short, fat
.458 SOCOM round (below).

Mag Capacity

While the .458 SOCOM cartridge may well feed perfectly well through a standard AR-15/5.56 magazine, CMMG has taken the extra step to optimize the feed lips on the Lancer magazines they supply with their rifles to ease loading and accommodate the widest variations in bullet forms. The .458 SOCOM is a big, little round, and it single stacks in a 5.56 magazine. I measured the SOCOM capacity of several 5.56 magazines on hand. The results were: 10 round mag = 3 rounds of SOCOM; 20 round mag = 7 rounds; 30 round mag = 10 rounds.

While the .458 SOCOM ammunition and components are still a wildcat, the .458 SOCOM cartridge is loaded by a number of excellent companies (listed at the end of this article). Brass is available from Starline. There’s an endless selection of .458 diameter bullets. Loading dies are available from Lee, Hornady and Redding. Reloading data can be found from Barnes, Hornady and Nosler.

Check out the performance standards for this wildcat. Average spans of velocities from a 16-inch SOCOM barrel were recorded by Marty ter Weeme as: 300-grain bullet: 1,800 to 2,000 fps; 400-grain bullet: 1,600 to 1,800 fps; 500-grain bullet: 980 to 1,300 fps; and 600-grain bullet: 1,000 fps.

Along with the CMMG test rifle came three very different, very intriguing, .458 SOCOM loadings.

SBR Ammunition, which currently offers the greatest variety of SOCOM offerings, provided a 140-grain PolyCase ARX loading. The ARX bullet is an injection-molded bullet formed from a heated mixture of powdered copper and an epoxy/polymer resin. The bullet nose exhibits three, deep, spiral flutes that laterally churn and displace soft tissue and fluids upon penetration. While it’s a non-expanding bullet, it will break apart if it impacts solid materials to minimize collateral damage. Ruger is offering the ARX bullet in its new line of handgun ammunition.

The second loading provided by PolyCase featured Lehigh Defense’s 300-grain Controlled Fracturing HP bullet. It’s machined from solid brass, and the HP cavity is pre-stressed, forming petals that break off as secondary projectiles during penetration. The full diameter, solid shank of the bullet then continues to penetrate.

Lehigh Defense X Penetrator bullets punch distinctively-shaped
holes in the paper targets.

The test loads consisted of some of the most high-tech-lethal
ammunition ever marketed.

The third loading was provided by Underwood Ammo. It featured a 300-grain, Lehigh Defense, Extreme Penetrator bullet. Machined from brass, the nose of the X Penetrator features four, deep, straight flutes, giving it the name of “Fluid Transfer Monolithic.” It leaves a very distinctive cutting pattern through a paper target.

All three loadings are simply high-tech lethal. Today’s innovative bullet designs are truly remarkable.

Mounting CMMG’s Anvil with my favorite Leupold Mark 6 1-6x20mm tactical scope, I liked the way the Anvil balanced just in front of the magazine well. I was curious about the performance of the rather exotic selection of ammunition and associated recoil. As it turned out, even with the 300-grain loadings, recoil was not a factor in the gas gun. I would compare felt recoil to that generated by a .308 Winchester.

Handloads listed on the web based around Barnes TTSX/TAC-TX and Hornady’s FTX bullets and Hodgdon’s Lil’ Gun powder appear to give MOA or better groups, but think of the .458 SOCOM as a 100- to 150-yard cartridge, much like the .45-70. The accuracy results I experienced with factory ammunition are very acceptable.

With the Anvil fully loaded with the SBR 140-grain ARX ammunition, it was off to the field for the last days of Arizona javelina season. In the closing hours, I found the herd I had been searching for and took the shot at about 30 yards. The copper/polymer ARX bullet entered the near shoulder of the javelina, cut a large wound channel and exited the off-shoulder without breaking up. You could not have asked for better ballistic performance.

CMMG’s Anvil in .458 SOCOM is an intriguing rifle and a lot of fun to shoot. The Anvil is just one of CMMG’s diversified AR products that include pistols, rifles, uppers, lowers, barrels, parts, .22 LR conversion units and their Mk47 Mutant—CMMG’s super accurate AR in 7.62x39mm.

1311 Industry Rd.
Sturgis, SD 57785
(800) 626-7266

Black Butterfly
3620 Pelham Rd.
Locked Box 206
reenville, SC 29615

Lehigh Defense
130 Penn Am Dr., Ste. D-1
Quakertown, PA 18951
(215) 536-4100

PolyCase Ammunition
41 Artley Rd.
Savannah, GA 31408
(912) 335-5101

140 Indigo Dr.
Brunnswick, GA 31525
(912) 264-5822

Underwood Ammunition
5730 Bottom Rd.
Sparta, IL 62286

Ventura Munitions
5725 S. Valley View Blvd., Ste. 12
Las Vegas, NV 89118
(702) 998-7727

MkW Anvil
MAKER; CMMG, P.O. Box 68
Boonville, MO 65233
(660) 248-2293

ACTION: Semi-auto direct gas impingement, adjustable gas block
OVERALL LENGTH: Collapsed: 33.5 inches, extended: 36.75
WEIGHT: 7.5 pounds
FINISH: Matt black
SIGHTS: Full-length, M1913 Picatinny rail
STOCK: Magpul MOE pistol grip, CTR 6-position buttstock
Price: $1,949.95

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