.458 SOCOM – The Best Big-Bore AR-15 Cartridge?

Big-bore AR-15s first hit the scene in 2001 with the launch of Alexander Arms’ .50 Beowulf. Suddenly hunters and shooters had the option of stepping up from the common intermediates of the day to a hard-hitting big-bore cartridge. Let’s face, small-bore high-velocity cartridges like the .223 Rem appeal to some, but others prefer throwing huge chunks of lead at moderate velocities. If you wanted to blow great big holes in things, create all kinds of carnage, and stop man or beast dead in their tracks the .50 Beowulf was your ticket to ride. Basically, the .50 Beowulf neatly shoehorned the performance of a traditional large-caliber lever-gun into a compact lightweight AR carbine. It quickly caught the attention of many who loved the performance of rifles like Marlin’s M1895 .45-70 Government.

While the .50 Beowulf got the glory, another big-bore cartridge developed expressly for use in the AR-15 was launched around the same time. Dubbed the .458 SOCOM, this rebated rim bottleneck design is the child of Maarten ("Marty") P. ter Weeme of Teppo Jutsu LLC. Rather than being developed as a sporting cartridge, like the .50 Beowulf, this interesting round came about through an informal discussion with a member of the U.S. Special Operations community. This occurred after the Task Force Ranger debacle in Mogadishu, Somalia as described in the book "Blackhawk Down". During this informal beer and BBQ discussion, they focused on the possibility of developing a new cartridge capable of providing superior stopping power for the AR-15 weapons system.

Marty became intrigued with the idea and decided to try to develop such a cartridge. He eventually decided on using common .458-inch diameter projectiles. Doing so would give the user a wide choice of commercially available projectiles, ranging from 250-grain JHPs to 600-grain JSPs. In hindsight, this was a very wise decision on his part. Keep in mind, there were few .50 caliber projectile offerings available for the .50 Beowulf 20 years ago. Using a large .458-inch diameter would also enhance terminal performance of subsonic ammunition.

The next question was what to use for a cartridge case? After receiving input from Tony Rumore of Tromix, Marty settled on unformed preproduction .50 Action Express cases provided by Starline Brass. A case this size will completely fill an AR-15 magazine, allowing it to function as a single-stack design. Case length was shortened to 40mm to allow a Barnes 300-grain X Spitzer to be seated to a depth which would still function through an AR mag. The case rim was also further rebated to 0.473 inch (.308 Win dimensions) both to leave more material on an AR-15 bolt and allow easy retrofit into a bolt-action rifle. Since it's based on a pistol case, it operates at relatively low pressure, and thus imparts less stress to the rifle. When his cartridge design was finally completed Marty dubbed it the .458 SOCOM, as the initial impetus came from a discussion with a member of US Special Operations.

What I like best is how the .458 SOCOM fits neatly into a relatively stock AR-15. It's hard to believe that a system specifically designed for the 5.56x45mm can be so readily converted to throw such heavy slugs. A new barrel, proper bolt, enlarged ejection port and that’s about it. Rounds feed from unmodified 5.56mm magazines, although dedicated mags are now offered for this caliber. Three of the fat rounds will fit in a standard 5.56mm 5-round magazine, seven will fit in a 20-round magazine and 10 will fit a 30-round magazine.

Now, what about performance, which after all is the name of the game, right? Well, how about a 300-grain .458-inch bullet at 1,900 fps from your AR-15 carbine? Want to go bigger? How about a 400-grain slug at 1,600 fps? Plus you can run 500 and 600-grain bullets at subsonic velocities. When the cartridge was first released ammunition was a bit scarce, but today a number of companies load it including Buffalo Bore, Steinel Ammunition, SBR Ammunition, and Underwood Ammunition. Common bullet weights today are 250, 300, 350, 400, 450 and 500 grains. You can select projectile designs which will perform well on lighter framed medium-size game, like whitetail, or heavier and tougher animals like moose and bear.

Ammunition is not cheap though, even during normal times. That is one downside to all the big bores. On the plus side, the .458 SOCOM is easy to reload. Dies, reloading data and components are all available. .458-inch projectiles are common. For powder, my favorites are Lil Gun, H110 and IMR 4198. Lil Gun works extremely well behind bullets running in weight from 300 to 405 grains. Accuracy is typically very good. My favorite handload consists of a Starline case, Lil Gun and a Remington 405-grain JFP seated to give an OAL of 2.08-inches. This load averages 1,607 fps from a 14.25-inch barrel and averages 1.3-inch 5-shot groups at 100 yards. This load develops 2,322 ft-lbs of energy at the muzzle and retains 1,740 ft-lbs at 100 yards.

On the range, my 14.25-inch Tromix .458 SOCOM AR carbine is a lot of fun to shoot. It belches flame and fire and fast as you can pull the trigger. It hits hard, on both ends, but with a good stance control is surprisingly good. It's relatively easy to place multiple shots on target in rapid succession up close and will put lead on target at 200 yards. At just 34.75 inches long and barely 7 pounds it's also very maneuverable and quick to the shoulder. To be blunt, it's a great deal of fun to shoot. In the hunting field, the .458 SOCOM has taken all manner of game from pigs to moose to big bear. If you do your part it will pole-ax a whitetail or elk.

What do I think of the .458 SOCOM? It's a very interesting cartridge which adds even more versatility to the AR platform. Short range performance is impressive, even with just 14.25 inches of rifled barrel. It will work nicely on any big game animal one should meet in the US. As a plinker fun factor is extremely high, other than ammo cost. Subsonic loads are a great deal of fun, especially its on-target performance compared to say, 300 AAC Blackout subsonic.

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How does the .458 SOCOM compare to the other popular AR big-bores? Without a doubt, both the .50 Beowulf and the .450 Bushmaster are popular and have their own following. Both are good cartridges. The .50 Beowulf will always have its followers simply due to it being a .50 caliber. But, as the US Government and American hunters found out in the 19^th Century, the .458-inch diameter does have certain advantages over the big .50. The .458-inch bullets can provide better exterior ballistics, better Sectional Density and deeper penetration, especially on big and tough game animals. The .450 Bushmaster was designed to use .452-inch projectiles. Due to this it doesn’t have as wide a variety of projectiles to choose from. Plus, it typically runs much lighter 220 to 265-grain projectiles. With all this in mind, I prefer the .458 SOCOM over its peers.

Is the .458 SOCOM perfect? No, it's mortal like the rest of us. My one negative comment is I wish the rim had been rebated down to 0.447-inch (7.62x39mm dimensions) rather than 0.473 inch. This would have left a bit more strength in the bolt. Plus, as stated previously factory ammunition is very expensive. My only other remark is to enjoy the cartridge for what it is and don't try to 'magnumize' it into something it’s not. Accurate, hard-hitting and reliable, the .458 SOCOM is definitely one to consider if you are considering a big-bore AR-15.

If you have any thoughts or comments on this article, we’d love to hear them. Email us at FirearmsNews@Outdoorsg.com.

About the author:

David M. Fortier has been covering firearms, ammunition and optics for 23 years. He is a recipient of the Carl Zeiss Outdoor Writer of the Year award and his writing has been recognized by the Civil Rights organization JPFO. In 2007 he covered the war in Iraq as an embedded journalist.

.458 SOCOM Accuracy Chart

Load: Cor-Bon 300 grain JHP
Velocity: 1,839 fps
100 yards: 1.6 inches

Load: Cor-Bon 400 grain Solid
Velocity: 1,511 fps
100 yards: 1.8 inches

Groups are an average of four 5 shot groups fired from a rest at 100 yards. Velocity readings are an average of 10 rounds recorded from a 14.25-inch barrel at 15 feet from the muzzle with an Oehler 35P chronograph at 100 feet above Sea Level at an ambient temperature of 31 degrees F.