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Big Kids on the Block: Rock River Arms' LAR-458 vs. CMMG's XBE

Big Kids on the Block: Rock River Arms' LAR-458 vs. CMMG's XBE


A lot of readers are fans of the .45-70. It's a big cartridge that's been around for a long time, and some of the most popular and beautiful rifles are chambered in it. It is a great choice for hunting just about anything in North America.

That same level of ballistic performance is also available in AR-pattern rifles thanks to the .458 SOCOM. It's unlikely that .45-70 lovers think much of the AR, but multiple generations of American riflemen have carried it to war and the shooting range for over 50 years and have become attached to the little rifle's many strengths.

lar458_cmmgxbe_46Giving all these riflemen a chance to hunt anything in North America with their most familiar rifle seemed like a good enough idea that both Rock River Arms and CMMG now offer models chambered in .458 SOCOM. I managed to get my hands on Rock River's LAR-458 and CMMG's MkW-15 XBE Anvil to give them a workout and to see which one is the better rifle.

My instructions were very clear: All test criteria had to be data based to eliminate personal opinion and to prevent any whining about unfair test practices after the fact.

In pursuit of this goal, I shot every round G&A sent me (after pilfering company stock and calling every manufacturer that loaded .458 SOCOM). Before the dust settled, I had a bruised shoulder and a slight headache, but the results surprised me.

I didn't expect that level of performance, and I thought the guns would beat me up worse than they did. I also have a newfound appreciation for and budding romance with the .458 SOCOM, even though she slapped me around a little bit.

The Rock River LAR-458 is built on AR-15-pattern upper and lower receivers.


I grew up near a machine shop that specialized in making cars fast and powerful. There was a sign inside the shop that said: "Speed costs money. How fast do you want to spend it?"

That same principle applies to these two rifles. There's a lot of machine time for each, and there are a lot of parts unique to each. Both pack a ton of power into a light and handy package. Neither company will sell thousands, because this is a niche market.

The CMMG MkW Anvil has AR-10-pattern upper and lower receivers, but the magazine well accepts AR-15 magazines.

All that being said, both rifles are mid-grade models from their respective lines; the Rock River LAR-458 is about $500 cheaper than the CMMG Anvil.

If we wanted an easy answer, a quick look at the MSRP of each would say that Rock River won that round. The problem is that even though both rifles are ARs and mid-grade models, this isn't an apples-to-apples comparison.

The CMMG bolt (left) scores with beefier lugs on either side of the extractor.

The LAR-458 is based on an AR-15 (.223 Rem.) and the Anvil is based on an AR-10 (.308 Win.). They weigh almost the same, but the Anvil has a much larger bolt and bolt carrier. Since the .458 SOCOM has a .473-inch case head size (just like the .308 Win.), the additional mass found on the Anvil's bolt face is a significant advantage when considering long-term durability.

Put enough rounds through any rifle and something will break. When AR-15s break, it is almost always the bolt. Usually, lugs on either side of the extractor will snap off because they are only supported on one side.

The standard-size LAR-458 receivers are forged, just like most AR-15s.

The LAR-458 bolt starts out the same as a standard AR-15 bolt, but the bolt face gets expanded to accommodate the .473-inch case head.

This process removes a big chunk of the already limited bolt lug material. There is only a thin ring of steel supporting the forwardmost one-third of the lugs. Hot ammunition or high round counts will be hard on the LAR-458 bolt.

The CMMG receivers are machined from billet aluminum to account for the AR-15-sized magazine well in an AR-10-sized lower.

Contrast that with the AR-10-sized lugs of the Anvil. It's rare to break an AR-10-size lug, even in a .308, because they are much thicker and extremely strong. The Anvil's lugs are the same size as those found in a .308 because the case heads are the same size.

However, before we can condemn the LAR-458 for the skinny bolt lugs, it's also important to remember that the .458 SOCOM operates at a very low chamber pressure. The maximum chamber pressure for the .458 SOCOM is 35,000 pounds per square inch (psi). This level of pressure is normally associated with pistol cartridges.

The CMMG adjustable gas block allows the Anvil to work reliably with any .458 bullet weight in any temperature.

Modern rifle cartridges usually have a maximum pressure of 58,000 to 62,000 psi. The AR was designed around rifle pressures, so how much of a liability do the skinny bolt lugs of the LAR-458 really pose?

Chamber pressure is directly correlated to gas port pressure, and gas port pressure is the largest determining factor in bolt speed for AR-pattern rifles. Bolt speed is what breaks bolts. Slow bolts last a long time, and 35,000 psi makes for a slow bolt.

While the skinny lugs are a little disconcerting, I wouldn't worry too much if you're not intending to shoot hot loads or a lot of rounds through the rifle. Most hunting rifles will never see 300 rounds. If that's the case, the LAR-458 definitely wins the price category because 500 bucks is 500 bucks.

If you like to handload and/or like to shoot your rifle, the Anvil wins the price category. AR-10s are usually $1,000 more than comparably equipped AR-15s from the same manufacturer, so the fact that CMMG offers the larger frame for only $500 more than the LAR-458 is a pretty big value.

Gas Systems

Gas systems are a big deal on AR-pattern rifles. Gas must flow through a small port in the top of the barrel, through the gas block and gas tube, and back into the bolt carrier to push it rearward to cycle the action. The system is a closed loop that needs the correct amount of gas to work properly.

The .458 SOCOM loads into a standard double-stack 5.56 NATO 30-round mag, but it only holds nine rounds of .458. SOCOM.

One of the most important dimensions on any AR is the size of the gas port. The small hole in the top of the barrel is what controls how much gas enters the system. If there's too much, a rifle can have extraction and feeding problems and will experience shortened bolt life due to excessive bolt velocity. If there isn't enough gas, the bolt will fail to lock back on an empty magazine or it might not even cycle.

Things get tricky when we get into cartridges other than the two for which Eugene Stoner designed the AR-pattern rifle (the .308 Win. and the .223 Rem.).

Problems start when we use ammunition with a broad range of bullet weights that are loaded to minimum and maximum pressures in a rifle that fails to account for the new set of variables.

The ammunition used in testing the LAR-458 and Anvil had bullet weights of 300 to 350 grains in six loads, with one sample that had a bullet weight of 140 grains. The LAR-458 handled all of the 300- to 350-grain loads without so much as a hiccup.

However, the only malfunctions experienced during testing occurred when I tried to shoot the 140-grain Polycase ammunition out of the LAR-458. The round would fire, and half the time the empty case didn't have enough oomph to get out of the upper receiver. The bolt coming forward would pin it like a stovepipe malfunction on a pistol.

The rest of the rounds had a fired case that cleared the ejection port, but the bolt failed to pick up a fresh round from the magazine, so the bolt went forward on an empty chamber.

Those malfunctions happened because the rifle couldn't get a big enough gulp of gas to cycle completely. The LAR-458 gas block is non-adjustable, so there is nothing I could do to correct the issue except to manually cycle the action while testing the round's accuracy.

While that's just one load, the non-adjustable gas block is a slightly limiting factor for the LAR-458. This rifle will also likely struggle with 500- to 600-grain subsonic loads, just as it did with the lighter and faster Polycase.

The Rock River Two-Stage Match trigger measured two pounds lighter than CMMG's standard trigger.

The Anvil has an adjustable gas block, so the shooter can make the gas port bigger or smaller as necessary to cycle any load that gets stuffed in the chamber.

The screw at the front of the gas block rotates between 15 settings, allowing the port to be properly sized for any type of ammunition. The adjustable gas block is also beneficial if the owner plans on using a suppressor or shooting in very cold weather.

While the Anvil has an adjustable gas block, I didn't adjust it to get the 140-grain Polycase load to cycle. This is because the rifle ships with the gas block turned all the way to position 15, so the gas port was huge and the rifle was over-gassed for the 300- to 350-grain ammunition I tested.

I was under strict orders to not do anything to one rifle that I didn't do to the other, so I left the adjustable gas block alone during testing. The Anvil may have been over-gassed, but it had no malfunctions of any kind.

If it were mine, I'd adjust the Anvil's gas system down to match the ammunition I intended to shoot.

This is a simple process where the shooter loads one round in the magazine and fires the rifle. If the bolt locks back, adjust the gas block to a smaller gas port and try again. When the bolt fails to lock back on an empty magazine, open the port up one or two clicks and you're set.

If you intend to only shoot 300- to 350-grain loads out of a .458 SOCOM, the simplicity of the non-adjustable LAR-458 gas block makes it a good choice.

For those shooting bullets from 140-grain speedsters to the 600-grain subsonics, or those shooting suppressed and hunting in very cold weather, the Anvil with its adjustable gas block is the right choice.

Triggers and Accuracy

Upon opening the boxes the rifles shipped in, I removed each model and tested trigger pull weights using a digital Lyman trigger pull gauge. I tested each trigger five times and have included a table of the results.

The Rock River muzzlebrake is more effective than the CMMG brake, but it does direct some of the concussive muzzle blast back onto the shooter.

The LAR-458 ships with Rock River's excellent two-stage trigger that was 2 pounds lighter than the standard AR trigger that comes in the Anvil. More important than the lighter pull weight is the almost non-existent creep of the Rock River trigger.

The trigger on the LAR-458 made it significantly easier to accuracy test the rifle compared to the standard trigger found in the Anvil. (For an additional $200, CMMG offers a Geissele SSA trigger upgrade.)

Accuracy testing the LAR-458 and Anvil comprised the bulk of effort invested in this article. I had seven loads to test, and I fired five, three-shot groups at 100 yards for each load out of each rifle.

I did all accuracy testing in one range session over the course of approximately five hours, and I felt and looked like 50 miles of bad road when I finished. I did alternate rifles during testing to allow each one to cool before moving to the next load.

Screen Shot 2017-05-19 at 11.46.41 AMAccuracy testing a .458 SOCOM is like accuracy testing a .45-70. It's necessary during load development, but not something I'd get really focused on.

This round should not be fired past 200 yards, maximum, because the bullet drops rapidly past that distance. As long as it holds a 2-inch group or so at 100 yards, I'd be more than comfortable heading into the field with it.

What I found at the range was that both rifles are capable of exceptional accuracy. This is a testament to the extremely high quality machine work done on both rifles and helps illustrate why they don't sell for $500.

Also, keep in mind that all accuracy testing was done with hunting ammunition and not match ammunition. Match ammunition doesn't exist for this cartridge because accuracy shouldn't be the primary consideration for its use.

The best group of the day came from the LAR-458 in the third hour of testing. The load was Ventura Tactical's featuring the 300-grain Controlled Fracturing projectile. Three shots went into a .41-inch group at 100 yards.

The Anvil's best group wasn't far behind it. Underwood Ammunition's 300-grain Xtreme Penetrator load put three rounds into a .62-inch group at 100 yards.

Screen Shot 2017-05-19 at 11.46.32 AMAfter crunching the results of all that shooting, the average group size for the LAR-458 was 1.45 inches (1.29 inches without the Polycase data) and the Anvil's average was 1.72 inches (1.4 inches if we discount the Polycase results).

Polycase ammunition shot just fine through both rifles and has one of the best bullet designs tested, but the polymer-based bullets needed a few rounds to season the dirty barrel before groups tightened up to normal.

I didn't have enough ammunition to re-shoot the test, but would recommend cleaning the barrel if you've been shooting copper-jacketed bullets before shooting Polycase for accuracy testing.

Range Observations

There are a lot of rumors out there about the punishing amount of recoil the .458 SOCOM generates in a relatively light AR. I didn't find that to be the case. Both rifles I tested had muzzlebrakes, which come with pros and cons.

Screen Shot 2017-05-19 at 11.46.55 AM

Both brakes reduced recoil, but the Rock River brake was much easier on the shoulder. It did, however, literally blow my limited amount of hair back each time I pulled the trigger. That's the first time I've had a brake do that.

Screen Shot 2017-05-19 at 11.47.02 AM

The brakes reduced recoil but it still took a toll. I enjoy a little of the rough stuff as much as the next guy, but I wasn't having a good time after about 100 rounds.

While the recoil generated from the cartridge isn't much different from a 7- to 8-pound rifle chambered in .300 Winchester Magnum, it isn't much fun to shoot for a couple hundred rounds from the prone position. Less than 100 rounds isn't overly punishing.

The chronograph data is the exact same for the two rifles because there was only enough ammo left to get muzzle velocities from one rifle. All muzzle velocities come from the new CMMG MkW Anvil.

Both rifles come with standard 30-round AR-15 magazines. I used my own Lancer 20-round magazine for both rifles to better accommodate the prone position. I had no issues from the standard Lancer 20-rounder. CMMG states that their rifle ships with a modified Lancer magazine that is optimized for .458 SOCOM use.

The cartridge was designed to work in a regular magazine so I closely inspected CMMG's Lancer that came with the rifle. The only modification I detected was ".458 SOCOM" laser-etched to the side.

Taking a moment to get off the beaten path of traditional AR chamberings has been well worth the effort. The .458 SOCOM offers a lot of versatility, accuracy and big-bore performance in a very popular rifle that's usually limited to a couple of cartridge choices.

The LAR-458 and Anvil are both excellent rifles that should appeal to slightly different shooting demographics. Rock River's LAR-458 is the top pick for hunters because of its value, excellent trigger and superb accuracy. The CMMG MkW-15 XBE Anvil gets the top pick for enthusiasts that handload or shoot a lot.

The highly accurate rifle has a larger bolt that will last much longer that of the LAR-458, and the adjustable gas system can be tailored to work with any load.


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