Earlier this year Rock River Arms introduced the LAR-458 carbine chambered for the powerful .458 SOCOM cartridge. Firing a projectile twice the size in diameter and producing more than twice the energy of the standard 5.56 x 45mm NATO cartridge, the .458 SOCOM gives law enforcement a powerful punch without increasing the size of the standard AR platform.

Developed originally for military Special Operations, in need of greater stopping power and penetration, the .458 SOCOM was designed to work within the confines of the M-16/AR-15 weapon system.

Gun enthusiasts have heard of the impressive knockdown power of the .45-70 cartridge that our great-grandfathers used to hunt buffalo in the late 1800s. This cartridge fired a thumb-sized projectile, weighing over 300 grains, at around 1,810 feet per second. This load churned up a whopping 2,182 foot-pounds of energy. Loaded into their trusty Sharps and Trapdoor Springfield rifles, the 45-70 cartridge earned the respect of those firing it and the fear of those that stood downrange from it. The big frontal area of the bullet combined with its velocity made it a devastatingly effective round on game and human alike.

But guess what? That particular .45-70 load actually produces 10 percent less energy than the .458 SOCOM. OK, if you're up on your ballistics, you're saying, So what? That's the same energy level as the .308 Winchester or military equivalent: the 7.62 x 51 NATO. Knight's Armament, DPMS/Remington, and Armalite all offer .308 rifles built on an AR platform.

The difference here is that Rock River builds this gun without expanding the size of the carbine. In fact, the Rock River gun weighs just 7.6 pounds unloaded.

Designed for Power

The .458 SOCOM is a fat, slightly tapered round that possesses a shoulder, which it headspaces on, and a heavily rebated rim. The casehead diameter is approximately .472 inches—the same as a 7.62 NATO round—and its diameter requires that the bolt face on the LAR-458 be heavily modified to accept this round.

Fortunately, not much else needs to be done for this cartridge. The bolt carrier looks to me exactly like a standard AR carrier.

My test rifle arrived with a fixed A2 style stock in place; however, a standard collapsing buttstock can be easily retrofitted to the LAR-458. Rock River included two extra buffer springs with my carbine. The CAR spring designed for the collapsing stock has 38 coils and its end is colored red to differentiate it from the longer spring, which has 42 coils, is colored green, and designed to be used with the A2 stock. Cursory examination of the buffer revealed that it was the same as used in standard AR-15 rifles.

The LAR-458's flat top upper receiver is laser engraved ".458 SOCOM" just above the ejection port to let the operator know at a glance that this rifle is chambered for something other than the 5.56mm cartridge. Rock River opens the ejection port up slightly for the larger cartridge but managed to make the rifle work with a standard ejection port cover. The top rail of the receiver is numbered with white numerals, so that detachable optics and other accessories can be consistently indexed and can be easily seen with night vision equipment.

Its 16-inch heavy barrel measures .928 inches and is threaded at its end for an A2-style flash hider that has been modified for the larger bullet. Smith Vortex flash hiders are also available for the Rock River .458 SOCOM. To stabilize the big projectiles, Rock River uses a slow 1:14-inch twist rifling in the chrome moly barrel.

Rock River uses an aluminum free float handguard on the SOCOM. Its small diameter makes it a favorite of mine since it feels much more comfortable in my hand than the bulky M4-style handguards. A railed gas block is used on the SOCOM so that a detachable iron sight can be added if desired.

Rock River ships the rifle with one 5.56mm 30-round magazine. Accompanying the rifle was an instruction card that details how to bevel the front edge on the magazine for optimal performance. However, I used a number of other standard M-16 mags during my evaluation and did not need to modify the feed lips or follower for proper feeding. I managed to squeeze 11 of the fat cartridges into a 30-round mag and seven rounds into a 20-round mag. The rounds sit one atop of each other rather than in a staggered fashion as with the 5.56 rounds. Just like with the 5.56mm round, the magazine will lock the weapon's bolt back after the last shot.

12-Gauge Kick

Firing the weapon is a little unsettling. Anyone who has a great deal of AR experience and accumulated muscle memory will be lulled into a false sense of familiarity when he or she sits down behind the LAR-458. But touching off the rifle is a rude awakening, and I was unprepared for the punch of the 300-grain bullet being launched at 1,909 feet per second. After my first couple groups I took a towel and double folded it and put it between my shoulder and the rifle's buttstock. It did provide enough relief that I was able to finish my accuracy testing.

Still, after firing about 70 rounds from the bench I was done. Recoil reminded me of when I tested a lightweight 12-gauge shotgun for accuracy using slugs. It's not unbearable, but you don't want to fire a lot of rounds, either.

Firing the rifle offhand is not nearly as painful and actually fun. I was even able to fire some pretty respectable double taps at a target set up at about 15 yards with the rounds landing within four inches of each other.

Impressive Accuracy

I only had two rounds available to me for evaluation of the LAR-458 and both were provided by Cor-Bon. The 300-grain jacketed hollow point bullet recorded an average velocity of 1,909 feet per second from the LAR-458's 16-inch barrel. Cor-Bon's heavier bullet, the 405-grain JSP, actually churned up less energy than the 300-grain load and had a velocity of 1,617 feet per second.

I was impressed with the accuracy that I obtained with the LAR-458. I am certain if I had more time and more rounds to experiment with that I would have shot some even tighter groups. Still, how can you complain when the carbine is producing groups under an inch at 50 yards? Several of my groups were big ragged holes rather than five individual holes.

I'm sure that the excellent trigger on the LAR-458 contributed to my ability to shoot it well. Rock River's excellent two-stage match trigger is used on the LAR-458 and breaks cleanly with four pounds of pressure with almost no overtravel. Rock River's trigger is nonadjustable and that's perfect for a tactical gun since there are no screws that can back out and possibly tie up the action.

My test rifle functioned perfectly with the Cor-Bon loads: chambering, firing, extracting, and ejecting them without a burp. Note: Empty cases landed with an authoritative thump rather than the ting of the little 5.56mm cases.

Though the .458 SOCOM is an accurate round, its softball-like trajectory makes it best suited for shorter ranges—say under 150 yards. Given the intended short ranges I mounted a Trijicon Compact ACOG on the Rock River LAR-458 carbine. The scope is a compact design based on the tremendously successful Advanced Combat Optical Gunsight (ACOG) that has found a very popular following with our troops in harm's way.

My test sample features 1.5 magnification and also possesses dual illumination: in sunlight the fiber optic daylight collector illuminates the aiming point and tritium powers the aiming point when there is little or no ambient light. Its extended eye relief is especially appreciated given the recoil of the weapon—I didn't have to worry about my eye creeping up on the rear of the scope.

Overall I was very impressed with the Rock River Arms LAR-458 carbine. It functioned perfectly and possessed plenty of practical accuracy. If you feel that you can use the increased power of the .458 SOCOM but don't want the added weight of going to a .308 rifle, Rock River's LAR-458 may fit the bill. Outside of tactical use I think this would be a handy weapon to have if you patrol rural and semi-rural areas where livestock and big game occasionally wander into traffic.

Note: Rock River also sells a .458 SOCOM upper conversion kit that will transform any standard AR lower into a hard shooting carbine. These kits come with the two buffer springs and don't require any fitting or gunsmithing.

Mike Detty is an NRA-certified rifle, pistol, and shotgun instructor. A certified rangemaster and competition shooter, Detty served as an officer in the U.S. Marine Corps and holds a degree in criminal justice from the University of Arizona.