[Leland Pate]

Mine is a 10.5" barrel.

I don't know where to find some clear ballistics comparison charts, but here is some detail on the cartridge. I'm told it's similar to the 45-70.

.458 SOCOM

The .458 SOCOM cartridge was developed in order to provide superior stopping power for AR-15 based weapon systems, in view of the questionable effectiveness of the 5.56 mm round in recent conflicts. A new cartridge in .458 calibre was selected as this not only offers considerable stopping power but also offers the user a vast choice of commercially available bullets, ranging from 300 gr. (19.4g) HP and Spitzers, 405 gr. (26.2g) FN, up to 500 gr. (32.4g) Solids and Tungsten Core RN. A case of the same diameter as the .50AE that would completely fill the AR-15 magazine seemed the right choice, but the rim needed further reducing to avoid much work to the bolt lugs. The use of an even more drastically rebated rim seemed the solution, and the 0.473” (12.0 mm) diameter rim employed by the .308 and .30-06 based cartridges was chosen, as this would allow easy retrofit of bolt action rifles in this calibre.

The .425 Westley Richards cartridge was identified as the ideal basis as it had all the proper measurements, but it was too costly. So Starline Brass agreed to a pre-production run of .50AE brass, but unformed, untrimmed and with the rim rebated to 0.473”. It is headstamped: * 458 * SOCOM. Case length is 39.9 mm, rim diameter 11.9 mm, body diameter 13.6 mm, neck diameter 12.3 mm.

Initial loads, and the overall case, were designed around the Barnes 300 gr. X Spitzer, as this appeared to be the lightest spitzer bullet available. This determined the case length of 40 mm to ensure that the cartridge was short enough to still function through the AR magazine. The .458 SOCOM uses standard AR-15 magazines with no modification. The 20-rd magazines will hold 7 rounds, the 30’s will hold 10 rounds, and the 40-rd .223 magazines will hold 15 rounds of .458 SOCOM ammunition. The AK-47 can also be chambered in this calibre.

The case is a pistol case so operates at relatively low pressure, and thus imparts less stress on parts of the rifle. The following results are obtained from a 16” (406 mm) barrel:

300 gr (19.4g) JHP at 2,000 fps (610 m/s): (CorBon factory load equivalent)

400 gr (25.9g) Barnes RN Solid at 1,700 fps (518 m/s): (CorBon factory load equivalent)

500 gr (32.4g) Hornady RN at 980 fps (300 m/s)

500 gr (32.4g) Speer AGS-T at 1,300 fps (396 m/s) 600 gr (38.9g) Barnes Original at 900 ft/sec (274 m/s): (CorBon factory load equivalent. Subsonic; complete combustion in 10.5 inch barrel).