Skip to main content

New Reloading Data for the .458 and .375 SOCOM Cartridges

Shooting Times readers have asked, so expert Lane Pearce has provided reload data for the unique .458 SOCOM and .375 SOCOM cartridges.

New Reloading Data for the .458 and .375 SOCOM Cartridges
The .458 SOCOM was developed in 2000 and is based on a necked-down .50 AE case. The .375 SOCOM came along in 2013, and it is further necked down to accommodate 0.375-inch- diameter bullets.

Recently, reader Pete Swanson asked me about the .458 SOCOM cartridge. Coincidentally, reader Ed Stafford asked about the .375 SOCOM. I first encountered the .458 SOCOM cartridge several years ago on a visit to the PolyCase munitions firm then in Savannah, Georgia. The company made an injection-molded bullet that consisted of a mixture of copper powder metal and a polymer binder. Named the ARX, these unique projectiles significantly enhance the ballistic performance of conventional ammo because the much-lighter-for-caliber bullets are launched at faster velocities than lead, jacketed, or all-copper bullets. Kinetic energy is directly proportional to the bullet’s velocity squared, so the ARXs fly flatter and strike harder. While we were test-firing different ammo loaded with these fascinating copper/polymer bullets, Buddy Singleton, president of Southern Ballistic Research, joined us, and he had an AR-15 chambered in .458 SOCOM and some SBR ammunition loaded with PolyCase’s 140-grain ARX bullet. We were shooting at various reactive targets, and the extreme velocities and proprietary tip design dramatically exploded the melons and water bottles. Soon after the PolyCase event, I ordered an SBR upper chambered for the stubby, bottleneck round.

Marty ter Weeme (Teppo Jutsu LLC) and Tony Rumore (Tromix) developed the .458 SOCOM in 2000. Based on creating a smaller-caliber, bottleneck cartridge from the .50 Action Express parent case, the .458 SOCOM is adapted to a typical AR-15 platform by simply replacing the bolt assembly and the barrel. Typical performance listed is a 300-grain jacketed bullet at about 1,900 fps in a 20-inch barrel. Rumore followed up in 2013 with the .375 SOCOM. As its moniker suggests, the .375 SOCOM is simply the .458 round necked down to accommodate 0.375-inch diameter bullets. The lighter 200-grain bullets typically achieve 2,400 fps in a 20-inch barrel. SBR offers upper assemblies and complete rifles for both SOCOM rounds, and it is the primary source for factory-loaded ammo. I have an SBR AR-15 upper in .458 SOCOM and a Tromix upper for the .375 SOCOM.

Reloading Tips

reload-data-458-375-socom-02

Reloading the SOCOMs is readily accomplished with only a couple of special considerations. Along with the upper assemblies, I received a generous supply of once-fired Starline brass from SBR. Singleton recommended Redding dies and included an SBR cartridge case gauge with instructions that I carefully adjust the full-length sizer to ensure every reformed case passed muster. Although they are rifle cartridges, the typical operating pressures (30,000 psi to 34,000 psi) are similar to magnum pistol levels. The Starline brass primer pockets are sized for Large Pistol and Large Pistol Magnum primers, so a Large Rifle primer will protrude. I’ve never tried it, but this would surely cause a headspace problem and possibly a slam-fire if you attempted to chamber the round. I always trim my brass after resizing so that I can apply a uniform but moderate crimp, ensuring the bullet is securely retained in place after seating. Depending on the bullet design, you may need to apply a cannelure to assure an effective crimp. The Redding seater die is easily adjusted to provide a roll crimp, and Lee Precision makes a factory crimp die for the .458 SOCOM. Since I prefer to apply the crimp in a separate step after seating the bullet, I usually use the Lee die.

The SOCOMs have been adapted to many diverse applications, including close quarter battle and suppressed fire. It’s also suitable for hunting deer and hogs, so I loaded and test-fired several different bullet weights. The details are shown in the accompanying chart. Note that the Hornady JHP bullet was designed for modern .45-70 handloads, but Barnes developed the TTSX (TAC-TX) specifically for the .458 SOCOM. SBR provided 250-grain plated bullets, and I included a Sierra Spitzer boattail from my component inventory for reloading the .375 SOCOM. Sources for load data include an earlier edition of the Lyman AR Reloading Handbook and ter Weeme’s website. The website provided Quick Load-generated propellant recipes for several bullets and also referenced two previous articles written years ago by Shooting Times contributors. Barnes published a data sheet for the 300-grain TTSX bullet.

Range Results

reload-data-458-375-socom-03

After sighting-in, I fired five-shot groups with the factory loads at 100 yards, and accuracy averaged about 2 MOA. Then I built and fired almost 250 handloads. But since reliable load data is scarce, not all of my handloads generated muzzle energies similar to the factory loads I tested. One must start at lower charges and work up cautiously! Norma 200 propellant worked well with the Hornady 300-grain bullet in the .458 SOCOM, and Lil’Gun performed best with the Barnes TTSX bullet. In the .375 SOCOM, old standby IMR 3031 and Vihtavouri N135 yielded admirable results. I shot a couple of three-shot groups at 200 yards with the best Barnes .458 SOCOM handload, and it measured roughly 2 MOA. However, compared to the 100-yard target, the bullets dropped another foot. That’s just like lobbing a modern .45-70 slug, so don’t assume any downrange points of impact. You must fire your best loads at various distances to fully understand the trajectories.




GET THE NEWSLETTER Join the List and Never Miss a Thing.

Recommended Articles

Recent Videos

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Guns

Browning X-Bolt Mountain Pro

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Guns

Taurus TX 22 Competition

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Gear

Federal FireStick Precharged Loads

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Gear

Remington Core-Lokt Tipped

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Guns

Walther PDP

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Learn

Hodgdon Shooting Powder

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
News

A World Record Attempt: Practice Round and Media Day

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Learn

How to Aim with Iron Sights

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
News

Interview with Israeli Defense Forces, Part 1

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Guns

Custom Mossberg 500 at the Range and Live Turkey!?

Learn the difference between revolvers and semi-auto pistols and what makes them different. Learn more here: http://bit....
Learn

SHOOT 101: Know Your Handgun Types

Shooting Times Magazine Covers Print and Tablet Versions

GET THE MAGAZINE Subscribe & Save

Digital Now Included!

SUBSCRIBE NOW

Give a Gift   |   Subscriber Services

PREVIEW THIS MONTH'S ISSUE

Buy Digital Single Issues

Magazine App Logo

Don't miss an issue.
Buy single digital issue for your phone or tablet.

Buy Single Digital Issue on the Shooting Times App

Other Magazines

See All Other Magazines

Special Interest Magazines

See All Special Interest Magazines

GET THE NEWSLETTER Join the List and Never Miss a Thing.

Get the top Shooting Times stories delivered right to your inbox.

Phone Icon

Get Digital Access.

All Shooting Times subscribers now have digital access to their magazine content. This means you have the option to read your magazine on most popular phones and tablets.

To get started, click the link below to visit mymagnow.com and learn how to access your digital magazine.

Get Digital Access

Not a Subscriber?
Subscribe Now

Enjoying What You're Reading?

Get a Full Year
of Guns & Ammo
& Digital Access.

Offer only for new subscribers.

Subscribe Now

Never Miss a Thing.

Get the Newsletter

Get the top Shooting Times stories delivered right to your inbox.

By signing up, I acknowledge that my email address is valid, and have read and accept the Terms of Use