A High-Tech .45-70

A High-Tech .45-70

By Ralph M. Lermayer

Rock River’s .458 SOCOM has the thump of a .45-70, but feeds in AR-15 platforms.

There are several ways new cartridges come into the world. One is the military. They put out a request for a set of specs they want, and arsenals and commercial manufacturers go into high gear until the round is created. The .308, .223, 7mm Mauser and venerable .30-06 were all created this way.

Then there are the guys in lab jackets at the commercial plants who try to bring out new factory cartridges with a little more oomph and, hopefully, sales appeal. The recent rush of short and super-short cases that followed the .300 RUM are a perfect example of this, as are the .444 Marlin, .338 Federal and many others.

Then, of course, you have the wildcatters who take everything out there and neck it up and down just to see what happens. Occasionally they hit a home run and the likes of the .22-250 Rem, .257 Roberts, 6.5-284 Norma or .224 TTH are born.

This cartridge, however, the .458 SOCOM (.458 Special Operations Command), was reportedly conceived over a barbecue and some cold brew. It was at an informal gathering of Special Ops personnel, specifically Task Force Ranger, when the subject of stopping power arose. One point of discussion was the multiple hits required to take out the opposition  in Mogadishu, Somalia. The consensus was a one-shot stop would sure be nice. Marty ter Weeme, founder of a company called Teppo Jutsu LLC, went to work, and in 2000, the .458 SOCOM was born. The sledgehammer cartridge launches .45-caliber bullets from 250 to 600 grains from a standard-size AR-15 with a proper barrel and chamber.

A Bit of AR History

Back in the late ’50s, the military decided to replace the venerable M1 Garand. Most of the companies bidding for the new gun contract assumed another .30 caliber was the way to go, and the early designs submitted were in .30-06 or .308. Armalite’s entry, the AR-10, was a semiauto of simple design (with a full-auto option) chambered in .308. As is often the case with government requests, the higher brass decided about halfway through the trials that the emerging dust-up in Asia would be a close-quarters affair better suited for a smaller, lighter and faster round than the .30 caliber. That sent competitors bidding for the lucrative new rifle contract back to the drawing board.

Armalite engineers simply took their .308 AR-10, made it smaller, chambered and timed it for a new, hot .22 based on a blown-out .222 called the .22 Special, later named the .223, and resubmitted as the AR-15. The rest is history. The government bought it, Colt bought the design rights from Armalite, and AR-15s wearing the Colt logo tromped the rice paddies of Vietnam for years, going on to become the primary military firearm to this day.

Sounds Simple, But . . .

What does all this have to do with the .458 SOCOM? It’s a size and timing thing. Because of the small size of the AR-15 action and receiver area, and the timing of the bolt cycle rate, you can’t use cartridges much longer than .223 length. You can make a fatter case work, but it can’t be much longer. The pressure and burn rate must also be close to that of the .223 or the timing goes to pot. You tear things up, and reliability suffers.

These were the design limitations Marty ter Weeme had to work around to give the little AR-15 real muscle, but it didn’t stop there. The cartridge had to be suppressor-friendly, meaning it had to be capable of subsonic muzzle velocities (below 1,000 fps), and still reliably cycle the action in semi- or full auto. That’s a tall order.

Short, Fat & Odd

The end result was a bit of an odd-looking case, but one that accomplished all the goals. It’s a short, fat case (1.575 inches long) with a very small rebated rim (.473 inch), minimal taper and a slight shoulder. The case headspaces on that minimal shoulder. It’s a peculiar set of dimensions, but an extremely clever design for its purpose.

A High-Tech .45-70The Why of It

In the world of bolt actions, levers, single shots and even handguns, hunters have an abundance of big-bore .45-caliber options to choose from, ranging from the .454 Casull to the .458 Winchester. Big bullets moving at moderate velocities are proven game stoppers when a fast kill is the main objective.

The rising popularity of the AR-15 and its rapid move to the dark timber has generated a need for that kind of muscle in the AR. Other cartridges like the .450 Bushmaster and .50 Beowulf were created to fill this void, and they are fine cartridges that get the job done. But both require that the rifle be modified. Parts need to be changed to make them function. Buffer springs, unique magazines, different bolts, etc. are required.

The real cleverness in the .458 SOCOM design is that nothing need to be changed except the upper — same magazine as the .223, same buffer spring in the buttstock, same everything. Just get a .458 upper available from Rock River Arms, Teppo Justsu or  other custom builder, get loaded ammo from SBR Ammunition, Corbon or Reeds, and go. Your .223 AR has just morphed from a varmint getter to a full-fledged deer, bear and elk stopper by simply changing the upper. No new FFL transfer required.

In the Field

Much of the research on the capabilities of big-bore AR-15 rounds like the .458 SOCOM is on subsonic (1,050 fps and lower velocities) and suppressed loads. That’s the where and why of the super-heavy 450-, 500- and 600-grain bullets. I would imagine folks hiking, fishing or living in big bear country might also opt for the heavyweights — the Coast Guard uses them to put big holes  in boats — but for hunters, bullets in the 250- to 350-grain range will be most useful.

A well-constructed 300- grain bullet moving about 2,000 fps is bad medicine for just about anything, including bear and moose, out to 200 yards. The .458 is certainly big enough for elk, and there’s not a deer alive that will walk away from a solid hit.

The energy of such a combination is 2,400 foot-pounds at the muzzle, and within 100 yards, it doesn’t weaken much. That puts it squarely in the territory of the modern .45-70 or .458 Marlin, and it will do anything those rounds will do. Even the mighty .458 Winchester Magnum only bumps 2,100 fps with the 300-grainers.

Of course, all of these other cases soon outperform the .458 SOCOM as bullet weights get heavier, since they have the case capacity to push the 400-, 500- and even 600-grain heavyweights much faster. With those weights, you don’t have enough powder capacity in the .458. Velocity drops fast, and they slow way down and go subsonic, precisely as the design required.

A High-Tech .45-70Rolling Your Own

Dies are available from Hornady, Lee and CH, and Redding plans to offer them as well Bullet choices are vast with Nosler, Hornady, Barnes, Speer, Woodleigh, Swift, A-Frame, Sierra and Lost River all offering choices from 250 to 600 grains. Both Remington and Winchester produce bulk bullets in .458. Whatever your preference, bullets won’t be a problem, nor will the large pistol primers this case uses.

A wide diversity of powders have also proven accurate in the SOCOM, from Hodgdon’s Lil Gun to 4198. It’s a very easy case to load as long as your dies don’t push the minimal shoulder back.

The one component currently in short supply is cases. Made by Starline, they’re available from that company or through Midway or Graf & Sons. However, with a fast-rising demand and the general shortages we’ve experienced since the last election, brass has been hard to come by, and purchases are going on backorder. Both Starline and Cor-Bon assure me that supplies should increase by the time this goes to press.

At the Bench

My test rifle was a Rock River A4 upper and lower, with a flat top receiver and a 3-9x scope mounted on medium rings. In my opinion, 200 yards a narrow pressure and velocity range practical hunting range. Good shooters using high-BC bullets will no doubt stretch that, but hunting is way different than hitting steel gongs.

After the accuracy testing,  I’ll replace the more precise high-magnification scope with a 3x or 4x ACOG or a red dot sight for faster target acquisition. The rifle is fitted with a Rock River match trigger, but has no other modifications and is pretty much stock as delivered.

Rock River offers either complete rifles or uppers in .458. As of now, they only offer a 16-inch chrome barrel with a flash hider (good to protect the crown, even if you’re not concerned about flash), and no front sight. I wish an 18- or 20-inch barrel was available to quiet things down quite a bit. It probably would add soome velocity as well.

Rock River is known for quality, and this rifle is no exception, but a couple of things I would like to see are an elevated optical platform to get the scope higher, and a real recoil pad.

Despite the plethora of  aftermarket accessories available for ARs, no one offers an A2-style buttstock with a good  recoil pad. The standard hard plate, more than adequate for the .223, is a real meat grinder with the .458. There is an ACE skeletonized buttstock that comes with a ?- or 1-inch rubber pad, and my order went in to Brownells immediately after doing the testing. I have it on good authority that DPMS will be offering an A2 buttstock with a real recoil pad in the near future. Stay tuned.

Cor-Bon’s factory cartridge with what appears to be a 300-grain Sierra flat-base hollowpoint was the only load I had. Dies ordered from Hornady have not yet arrived. Pulling the bullet showed a powder charge of 29.2 grains of a flake powder that filled the case to about two-thirds capacity. A lot of powder room was left in that case. I’m anxious to work up reloads, but you can’t hot-rod AR loads. They must be loaded to a narrow pressure and velocity range, or timing goes to pot and reliability suffers. Hot loads are more trouble than they’re worth. Over the chronograph, velocity 15 feet from the muzzle measured 1,927 fps with a deviation of only 18 fps. That’s very consistent performance, and it showed on the target.

A bit on the results before I continue. If you’re like me, you soon get pretty suspicious of gun writers who seem to get sub-1-inch, 100-yard groups from everything they shoot.

Maybe these guys have a magic trigger finger. Because that’s not the way things are in the real world.

This factory load from this unmodified rifle never exceeded 2 inches at 100 yards, with most groups holding at 1? inches and several sub-1-inch clusters in the mix. That is extraordinary accuracy from an unmodified semiauto rifle. I am not surprised. Short cases moving heavy projectiles at moderate velocities are usually capable of excellent accuracy, but this is exceptional.

I fired about 50 rounds downrange, all fed from standard 10-round .223 magazines. The .223 magazine only holds three .458 rounds, making it legal for hunting where gun capacity is limited to five rounds or less. There was never a jam,  failure to feed or any other problem. This AR design likes this load. Other loads for reloaders are available from the Teppo Jutsu website, and a lively reloading discussion is always underway at the .458 SOCOM Forum.

Big Medicine

Thoughts of AR-15s automatically lean toward its most familiar load, the versatile .223. And the expansion to .204 Ruger, 6.8 Rem SPC, .30 Remington AR and the like have given it solid deer and antelope hunting credentials.

No one will argue the effectiveness of those cartridges on whitetails, but for bear, elk and moose, the job had to be left to its bigger, heavier brother, the AR-10, which can handle the mega rounds. No more.

With the introduction of the .458 SOCOM, the trim AR-15 is more than tough enough for the big guys.

.458 SOCOM Sources


.458 Rifle or Upper


Padded Buttstock
(and instructions on how to build one)

This article was published in the November 2009 edition of Buckmasters GunHunter Magazine. Subscribe today to have GunHunter delivered to your home.

Copyright 2021 by Buckmasters, Ltd.

Copyright 2020 by Buckmasters, Ltd