Rock River Arms X-Series Rock River Arms has been in the AR business since 1996, which makes it ancient in the AR world. Back when Mark and Chuck Larson were building ARs in a small building on the banks of the Rock River in central Illinois, only a handful of companies produced ARs, unlike today. In the years since, they landed numerous LE contracts (including the DEA) and have acquired a strong following of loyal customers. When manufacturers sell a lot of rifles and have been doing so for a long time, it’s easy to get and stay focused on the current product line.

Several weeks ago, Eric Poole, the editor of this magazine, approached Rock River and inquired about any new models the company might have. The short answer was, “Nothing on the horizon.”

Eric thought it’d be a good idea for Rock River to incorporate the complete upper receiver from its new R3 competition rifle and offer it on a fully ambidextrous lower with fixed Operator stock.

The two rifles you see here are the result of the collaborative effort between Eric and Rock River. One rifle is chambered in .223 Wylde (a chamber that safely shoots either .223 Rem. or 5.56 NATO), and the other is in .308 Win. What makes these rifles new is the specific combination of parts, including Rock River’s forward-thinking TRO-XL handguards, stainless barrels, match triggers and muzzlebrakes.

Most unique to the Rock River .308 rifles are the FAL magazines they require. I found it an unusual choice for an AR-pattern .308, so I asked Steve Mayer (LE/military sales manager for Rock River) why the company’s rifle takes FAL mags. “We designed our rifle during the ban years, so we needed an existing supply of magazines that worked well. We liked the magazine and the ambidextrous bolt catch, so we started there and designed the lower around those components.”

Brilliant. Rock River .308 ARs take either metric or inch FAL magazines, so if you’re a dedicated FAL lover with a pile of magazines, these guns are a great way to get into a .30-caliber AR without having to buy a bunch of magazines.

According to Steve, “There are millions of FAL magazines on the market. There are also millions of bad FAL magazines on the market.” Rock River has some exhaustive quality-control measures in place to ensure that only the best ship with its rifles. “We were throwing away 40 to 60 percent of the magazines we bought,” Steve said. The fix that Rock River came up with to remedy the shortage of quality FAL mags is to start making polymer magazines. The rifle I tested came with two polymer mags that functioned flawlessly.

The next obvious question was whether Rock River was ever going to design a .308 that took SR-25-pattern magazines. “Never say never, but probably not. The KAC SR-25 magazine doesn’t transition well to our lower design. We’d have to lose the ambidextrous bolt catch in order to make the lower work.” The ambidextrous bolt catch on Rock River’s .308 is one of the best designs found on an AR because it is truly ambidextrous. It doesn’t require any aftermarket attachments, and it is identical for right- and left-handed shooters. It definitely needs to stay.

The new .308 and the 5.56mm come with 18-inch fluted stainless steel barrels. The material used to make the barrels is 416R that Rock River cryogenically treats once it is received. The barrels come from Wilson Arms, not to be confused with Wilson Combat. Wilson Arms is a Connecticut-based company that has been making barrels for almost 60 years and creates high-quality button-rifled barrels. Wilson makes the barrel, flutes it and cuts the chamber, then Rock River hits the chamber with a finish reamer and cryogenically treats the whole barrel.

The cryogenic process is necessary to stress-relieve the barrel. Much like heat treating, the exact details of the cryogenic process are a closely guarded secret. Steve did tell me that the barrels see temperatures as low as 300 degrees below zero. The barrels are cooled in stages, where they spend a precise amount of minutes at a certain temperature before dropping to the next temp. Once they make it to and spend their allotted time at 300 below, the barrels are brought, in steps, all the way up to 200 degrees above zero. The process is time-consuming and a bit tedious, but it’s something Rock River deems necessary for optimal accuracy.

The barrels for both new rifles are 18 inches long, have medium contours with flutes and feature mid-length gas systems. I asked Steve why the mid-length gas system when rifle-length systems work well with 18-inch barrels. “We like the higher pressure of the mid-length system and feel it offers an additional measure of reliability with the direct-impingement system, especially with the .308. The .308 has a lot of variation in port pressures due to a wide range of bullet weights, so it needs the higher port pressure to ensure it functions reliably with everything.”

The 18-inch barrels offer an additional bump in velocity over their 16-inch counterparts, and, with the flutes, the 18-inchers are just as light and handy. The new .308 is a great choice for the competitive shooter, hunter or tactical shooter who needs an accurate rifle and plans on shooting more than just from the bench. When combined with the light and comfortable handguard, the barrel makes this .308 ideal for positional and/or offhand shooting. It’d be an easy rifle to carry on an extended walk, too.

The barrel found on the 5.56 gun is the same barrel Rock River puts on its new R3, just blasted to a nonreflective finish. The .308 barrel, however, is entirely new. Rock River paid attention to the chambers selected for each of these rifles and put the .223 Wylde chamber in the 5.56 model and a slightly altered .308 chamber in the bigger rifle.

The .223 Wylde is popular because it combines the tight .224 throat of the .223 with a longer freebore and the shallower leade of the 5.56x45mm. We get our best accuracy when we have .0005 inches of clearance on each side of the bullet when it’s chambered. The area of the bore just forward of the cartridge case (when chambered) is called the throat, and it needs to be tight to prevent the bullet from yawing before it contacts the rifling. Next, we need enough freebore (smooth area between the case mouth and rifling) to let the cartridge chamber without the bullet getting pushed back into the case, creating dangerous pressure spikes. Last, there needs to be a bevel on the leading edge of the rifling to prevent another pressure spike that would occur if the bullet just slammed into rifling that is usually .004-inch high. A bevel on the rifling also helps keep the bullet centered in the bore, which is good for accuracy.

The .223 Wylde has that .0005-inch clearance, so it has a nice, tight throat and the gentle bevel on the lead edge of the rifling (the leade) of the 5.56 NATO. The tight throat helps keep things accurate, and the gentle leade controls pressure of even hotly loaded rounds. The .223 Wylde is one of my favorite chamberings for 5.56 guns.

Steve wouldn’t get super-specific about the chamber drawing of the .308 (I don’t blame him, as that information is the company’s intellectual property), but he said it was something that was developed in-house. He went on to further elaborate, stating that the .308 chamber was generous around the cartridge body but had a standard throat and a shorter, steeper leade. The shorter leade works well for the .308/7.62x51mm because the 7.62 NATO loadings are loaded to a lower pressure than the .308.

The handguard on the .308 is the newest product from Rock River. It obviously has some styling and design cues taken from the R3’s TRO-XL line, as it has the same octagon shape and machined openings that cut weight and allow the barrel to cool. The handguard is a bit larger than most of the round-ish handguards available from other manufacturers. While the slightly larger diameter won’t make it feel quite so svelte, it also does a better job of keeping our hand away from a hot barrel.

Aluminum conducts heat much more efficiently than steel, so any heat coming off the barrel gets radiated directly into the aluminum handguard that surrounds the barrel. A small handguard that tightly surrounds the barrel is going to get uncomfortably hot under sustained fire, especially in .308, where each trigger pull burns twice the powder of its 5.56 little brother. The Rock River handguard puts a little more space between hot barrels and their handguard, which saves our hand from some of that heat.

The handguard is threaded to accept a 2-inch section of Picatinny rail at the 3, 6 and 9 o’clock positions at the muzzle end of the forend. If you want to mount a light, sling swivel stud or bipod, it has to go at the end of the handguard.

The muzzlebrakes are also new products from Rock River. The brake found on the 5.56mm rifle made its debut with the R3 and has been out for several months now. It has a very slender cylindrical profile and is a multiport tuned compensator that effectively controls recoil by venting the gas back and away from the rifle through several small holes in the top and sides of the device. The brake works so well that it pushes the muzzle down slightly each time the rifle fires.

The .308 brake is entirely new and has a more unique appearance than the 5.56 model. The small teeth on the muzzle end of the comp are aesthetic, but the four elongated chambers along the side and the ports in the top most certainly are not. They handily manage the .308’s recoil and do a good job of it. Shooting the .308 for a lengthy range session was never uncomfortable or punishing.

I asked Steve why Rock River is putting so much emphasis on muzzlebrakes and doing them all in-house when there are plenty on the market already. “Nobody likes to get beat up shooting their rifle, and a lot of our customers participate in shooting sports that fire a lot of rounds. We want them to have fun and not feel like they got beat up the next day.” Also, by producing the brakes itself, Rock River avoids production delays associated with outsourcing.

The 5.56 rifle that I tested had the same upper receiver group as the R3, the competition rifle debuted in 2013, except the barrel had a dull, sandblasted finish instead of the polished stainless of the R3. The mid-length gas system and the TRO-XL handguard make the rifle very comfortable to carry and shoot for extended periods of time.

The lower receiver has the additional features of an ambidextrous safety selector and the Norgon ambidextrous magazine release. The Rock River safety selector is unique in that it has a tactile nipple that protrudes from the end of the lever away from the rifle. It affords easy purchase and makes rotating the selector a snap. The selector lever mirrors itself on both sides of the rifle.

The Norgon ambi magazine release is low-profile and does an excellent job of allowing a left-handed shooter to drop his magazine with his index finger. The mag-release lever is located just beneath the bolt release on the left side of the rifle. The release works very well and stays out of the way when not in use. It is one of the better ambi magazine releases on the market.

Settling behind the rifles, I appreciated the fixed Operator A2 stock with the angled comb. The stock rides comfortably up against the face with no seams to tug on facial hair (as it applies to some of you). The toe is angled but has enough of a flat spot to be manageable with a rear bag. The two-stage match trigger is clean and light and does well when shooting for groups.

I had three loads to test with each rifle, but I was only able to test two of them on the .308. The inner dimensions of the polymer magazine are snug, and the 155-grain Lapua load dragged the noses of the bullets down the inside of the magazine. The Black Hills 168-grain A-MAX worked well, as did the Hornady 178-grain BTHP. The best five-shot group at 100 yards from the 168-grain A-MAX was .93 inch. Groups averaged 1.14 inches. The Hornady had a best group of 1.42 inches and an average of 1.61 inches.

Accuracy for the .308 prototype rifle was OK, but not great. Seeing as it is a newly designed barrel and these were the first rounds ever fired through one, I’d reserve judgment on the barrel until it has had a couple hundred rounds through it. It’s a light, fluted barrel, so we should remember that the rifle is made to carry and shoot, not just shoot from a bench. If, and when, these rifles go into production, Rock River Arms will offer them with a 1-MOA guarantee.

The 5.56mm rifle was a shooter. It did extremely well across the three loads, with each of them averaging less than 1 MOA. The Black Hills 52-grain Match load had a best group of .43 inch and an average of .71 inch. The 75-grain Hornady Match had a best group of .58 inch and an average of .75 inch. The Black Hills 77-grain had a best group of .83 inch and an average of .87 inch — very consistent across each of the groups.

While the 5.56 rifle was the better performer of the two, both models would come with accuracy guarantees if Rock River decides to put them into production.

Eric was able to convince the company to build these two models as special-order items, but it’s going to be up to the public to convince Rock River to make them permanent offerings. If you like what you see, let Rock River know. The success of Project “X” depends on your input.

Rock River Arms X-Series

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