Meet Rock River Arms’ .458 SOCOM big-bore–the hunter’s X factor to dropping big game in its tracks! Stick around and we’ll share some handloads to feed the beast, too.
Marty ter Weeme’s .458 SOCOM brain child has been with us for more than two decades and its power and compatibility with the AR15-platform has created sizable following among shooters. To feed the need for more rifles chambered in that caliber, Rock River Arms added the caliber to their X-1 lineup.
The .458 SOCOM is a relatively large round designed for a specialized upper receiver that can be mounted on any AR15 lower receiver. Inspired by the lack of power offered by the 5.56 NATO cartridge used in the M4 carbine and the M16 rifle, the .458 SOCOM was conceived during informal discussions between members of the special operations community, specifically Task Force Ranger’s experience that multiple shots were required to neutralize members of the opposing force in Mogadishu during Operation Gothic Serpent.
.458 SOCOM Rises to the Top
Marty ter Weeme, owner of Teppo Jutsu, designed the cartridge in 2000 and Tony Rumore of Tromix was contracted to build the first .458 SOCOM rifle in February 2001. The project sponsor set forth a number of specific requirements that led to the ultimate design: The cartridge had to fit in the M-4 carbine platform and magazines and be capable of firing heavy-for-caliber projectiles at subsonic velocity using suppressors. During the development phase, various other cartridges were considered and proposed to the project sponsor, but rejected as not meeting all the requirements. The cartridges considered were 7.62×39mm M43, 9x39mm Grom, .45 Professional (which has since become the .450 Bushmaster), and .50 Action Express. At the time, the .499 LWR cartridge was still in development phase and had not been chambered commercially.
The .45 Professional was ruled out owing to an admission to industry press the developer of said cartridge (Izzy Anzuldua) stated that steel proprietary to General Motors was used in the bolts and extensions to withstand the high operating pressures. The .50 AE and .499 LWR were ruled out because at the time only two bullets were offered in .501 diameter, both developed as pistol bullets for the .50 AE and were not heavy enough for the subsonic suppressed role.
Predecessor used in Vietnam
Research indicated that a short belted cartridge called the .458 × 1.5″ Barnes had been adopted for use in suppressed bolt-action rifles for use in SE Asia during the Vietnam War. It was shown as effective in terms of ballistics, firing a 500-grain bullet at subsonic velocities. But, it was not ideally suited for its role due to the size and weight of the weapon. Combined with the wide selection of bullets available in .458 diameter, this cemented the choice of caliber.
The cartridge case design was finalized based on discussions with Tony Rumore at Tromix suggesting a lengthened .50 AE case would work well in the magazines as well as be the largest diameter case to be able to feed through the barrel extension. The rim size was chosen for compatibility with other platforms, primarily bolt-action rifles. The .473-inch rim diameter is the most common rim size globally, since all bolt-actions chambered in 7.92×57mm Mauser, .30-06 Springfield or .308 Winchester share this rim size. The case length was chosen to be compatible with the Barnes 300-grain X Spitzer bullet. In 2009, Barnes developed a new bullet specifically for use in the .458 SOCOM, the 300 grain Tipped Triple Shock X, also known as the TTSX or TAC-X.
Compatible with the M4
As noted above, the cartridge was designed to be 100 percent compatible with the M4 platform. This included the buffer, buffer spring, GI magazines, many aftermarket .223/5.56 magazines, and magazine well. In .223/5.56 caliber, cartridges stack in a staggered (double stack) fashion. However, with the much larger .458 SOCOM, rounds “single stack” without any modification to the standard GI magazine feed lips or follower, making it the most versatile and cost effective big bore offering in an AR-15 platform. A standard 20-round 5.56mm NATO magazine can hold seven .458 SOCOM rounds and a standard 30-round 5.56mm NATO magazine can hold nine .458 SOCOM rounds.
Today, .458 SOCOM rifles and complete upper halves are available from Rock River Arms, Wilson Combat and others. Barrels and bolts can be purchased from Southern Ballistic Research. Brass for hand loading is exclusively produced by Starline. Loaded ammunition is also manufactured by Southern Ballistic Research, Wilson Combat and Cor-Bon, in addition to several smaller boutique loaders. The hot 300-grain round offers a muzzle velocity of 1,900 fps and 2,405 foot-pounds of energy.
Gun Specs
The Rock River Arms’ .458 SOCOM upper and lower receivers are forged, a common feature of the company’s X-Series rifles. To get a handle on this heavy hitter, a Hogue rubber pistol grip is a good match for RRA’s Operator A2 stock. One of my favorite features of a RRA AR is their Star Safety. It allows more user-friendly flip of the switch. Once you flip the switch, RRA’s 2-stage trigger provides a crisp, smooth break. Housing the trigger is their Winter trigger guard.
The Rock River Arms’ .458 SOCOM wears an 18-inch fluted stainless steel barrel. It is rifled with a 1:14-inch twist, cryogenically treated and bead blasted. At the muzzle, a RRA Hunter muzzle brake helps tame recoil and keep follow-up shots on target. The muzzle is threaded 5/8-32 threads per inch. To drive the action, low profile gas block diverts the necessary energy from a wide range of factory and handloads.
To give the carbine functionality and balance a RRA TRO-XL extended free-flat handguard provides a full-length top Picatinny rail. With the A2 stock option, the carbine weighs 8.7 pounds and measures 39.5 inches.
Range Time
The care and feeding of a .458 SOCOM takes special consideration. One of the first things to keep in mind is that this rifle cartridge takes pistol primers. Although I’ve never had problems while using rifle primers, they present a small risk of producing a “run away” when they slam fire upon feeding. Another aspect of brass care that can make a difference is neck tension when seating. This has a direct impact on consistent bullet release and subsequent accuracy. Cases should be trimmed so that they are equal in length, and annealing would provide yet another measure of consistency.
To put the RRA X-1 .458 SOCOM through its paces and create a benchmark for handloads, I tested several factory loads from Southern Ballistic Research (SBR). These factory loads, as well as the handloads, were fired through an Oehler 35P chronograph to record velocity data. Accuracy testing for factory loads consisted of 5-shot groups at 100 yards, while handloads were recorded at 50 yards. A Leupold Mark IV 2-8X scope was mounted for range work.
Heavyweight Bullets
The first factory load from SBR was their 300-grain jacketed hollow-point. The average velocity, recorded at 15 feet from the muzzle, was 1,632 fps, and the standard deviation 18 fps. The smallest group measured 1.660 inches.
The 405-grain SBR load averaged 1,448 fps, while producing a standard deviation of 15 fps. This rifle didn’t like this load, but it did produce a glimmer of hope. The smallest 5-shot group measured 6 inches, but it would typically put three shots into a cluster under 1 ½ inches.
SBR’s 350-grain full-metal jacket loads flew down range at an average of 1,546 fps, and produced a consistent 7 fps standard deviation. At 100 yards, the smallest group recorded was 2.774 inches.
The 300-grain solid brass projectile load from SBR produced an excellent 1.246-inch 5-shot group at 100 yards. Surprisingly, it produced an average spread of 53 fps, 22 fps standard deviation and 1,487 fps velocity average.
Now that I had a baseline for factory loads, I tried a variety of powders and bullets to come up with a solid handload recipe. I got some surprises with some starting loads, with bullets not even making it to the target at 50 yards.
Handloads
I recorded several attributes of each load that may prove beneficial to others crafting their own loads. The first load was 26.2 grains of H110 driving a 344-grain Lee cast bullet. This bullet is designed to use a gas check, but I left the gas check off of some bullets to compare results. It averaged 1,342 fps, while producing a rather high 56 fps standard deviation. Primer inspection of fired rounds revealed no signs of excessive pressure.
The 344-grain Lee produced a crisp recoil impulse and ejected cases at 2:00 of the bore axis. The fired cases were blackened, indicating powder gas leakage back into the chamber. The rounds prepared without gas checks produced 5-shot groups at 50 yards that hovered around 5 inches, with the smallest measuring 4.860. The bullets with gas checks installed during sizing fared much better, with the smallest group measuring 1.682 inches.
Subsonic Loads
The next cast bullet load was an attempt at finding a subsonic recipe. I used a 425-grain gas-checked Lee bullet. A 26.6-grain charge of IMR 4198 averaged 1,035 fps and a standard deviation of 10 fps. Its extreme spread ranged fro 1,020 to 1,050 fps, which stayed subsonic. It produced amazing accuracy, with several bullet holes connection among 5-shot groups. The smallest measured 0.981-inch center-to-center. Driving this bullet faster with 26.3 grains of H110 produced negative results. It achieved 1,364 fps, but groups opened up to 3 ½ inches.
Another subsonic load that needs further development was created with a 405-grain plain-base RCBS mould, which is the same profile as the standard .45-70 Govt. bullet. A 16.0-grain load of H110 only achieved an average of 784 fps, but it did produce a 1.433-inch group.
Swapping over to a 500-grain Hornady round nose driven by 18 grains of H110 produced 919 fps. It produced a single ragged hole with a flyer, which measured 1.464 inches. The overall loaded length was 2.316 inches, which was too long for magazine feeding.
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