.458 SOCOM

The .458 SOCOM is a relatively large round designed for a specialized upper receiver that can be mounted on any AR-15 pattern rifle. The 300 gr round offers a muzzle velocity 1900 ft/s and 2405 ft.lbf.

History
Inspired by the lack of power offered by the 5.56 NATO cartridge used in the M4 and the M16, the .458 SOCOM came about from informal discussion of 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. Marty ter Weeme designed the cartridge in 2000 and Tony Rumore of Tromix was contracted to build the first .458 SOCOM rifle in February 2001. Kody Horne fired the very first round of .458 SOCOM ammunition in the Tromix shop located in Broken Arrow, Oklahoma on February 16th, 2001, at 8:30pm.

The project sponsor set forth a number of specific requirements that led to the ultimate design as it stands right now: The cartridge had to fit in the M-4 platform and magazines and be capable of firing heavy-for-caliber projectiles at subsonic velocity using suppressors. During the developments 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, 9×39mm 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 because in an interview with industry press, the developer of said cartridge 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 in 2000 only two bullets were offered in .501 diameter, both developed as pistol bullets for the .50 AE and not heavy enough for the subsonic suppressed role. Research had 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 subsonically, but not ideally suited for its role due to the size and weight of the platform.  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, as the .473" diameter rim is arguably the most common rim size globally (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.

As noted above, the cartridge was designed to be compatible with the M4 platform. This included the stock magazine and magazine well. In .223 caliber, cartridges stack in a staggered fashion. With the much larger .458 SOCOM, however, rounds "single stack" without any modification to the .223/5.56 standard magazines or feed lips.