Rock River Arms LAR-PDS Folding AR-15

Rock River Arms LAR-PDS Folding AR-15

After so many years of politically driven panic-buying, the black rifle industry is currently experiencing what Wall Street types would call a “market correction.” Right now, only very inexpensive ARs are selling, or models that offer something unique. The Rock River Arms LAR-PDS is one of the latter.

Rock River Arms is an interesting company. I first became aware of it back in the 1990s. I was running a 1911 in USPSA competition then, and Rock River was known for making some very high-quality single stack 1911s. Then, long before everyone else jumped on the black rifle bandwagon, RRA decided to start making AR-15s. Its rifles didn’t garner much attention….until one of its models was adopted by the U.S. Drug Enforcement Administration. All of a sudden, the gun world sat up and took notice of Rock River Arms.

FYI, Rock River’s DEA rifle wasn’t like one of those “Navy SEAL” watches that has never been worn by a SEAL, they were actually issued to agents. I know, because one of my closest friends has been in the DEA since 1991, and he showed me his issued Rock River rifle one day when I was visiting. Actually, he was complaining about it—not that there was anything wrong with the rifle, he just thought it was too long for house clearing, and for that he preferred the short Colt 9mm SMG that he’d been running for years.

Rock River Arms LAR-PDS

All of Rock River’s AR-15 models are first labeled LARs. The PDS on this model stands for Piston Driven System. Before we go inside the rifle and look at the piston system, let’s go over the exterior.


Two carbine versions are offered of the model, one with a ribbed injection-molded handguard and one with an aluminum tri-rail handguard. I chose the tri-rail version not because railed forends are coming back into style, but because I thought the ribbed polymer forend version looked a little, um, what’s the technical term? Oh yeah, goofy. Rock River marches to the beat of its own drummer, and sometimes the stuff it puts out has a very unique appearance.


Generally, I try not to comment on whether or not I think a gun is ugly. Every article comes with photos, and each reader can judge from those photos how attractive he thinks a firearm is or isn’t. Plus, people tell me that with my taste in cars and shirts, I have no standing to judge whether or not something is ugly.


Rock River Arms LAR-PDS
The Rock River Arms PDS—Piston Driven System—is a proprietary design that eliminates carrier tilt and allows for a side-folding stock. Tarr didn’t care for the railed handguard or the weight that it added, but liked the smoothness of the action.

But with the LAR-PDS, all of the ugliness in this rifle is contained in the handguard. Both forend options offered on this rifle are ugly as hell. Rock River needs to ditch both of them and go with a KeyMod or M-LOK handguard. Not only will this lighten the rifle somewhat, it will make it more modular and modern-looking.

Both handguards are secured using big hex screws to blocks that are attached to the barrel between the piston housing and the chamber. It’s an unusual system, but didn’t seem to impede accuracy at all.

Rock River Arms LAR-PDS
The handguard mounts to the barrel blocks via Allen-head screws. It is an unusual setup, but didn’t seem to affect accuracy.

Rock River advertises that the rifle weighs 7 pounds 4 ounces empty, and the polymer forend version might. For a DI AR-15, 7.4 pounds is heavy, but it is not heavy for a gas piston AR. My rifle with aluminum tri-rail handguard tipped my scale at just over 7 pounds 11 ounces, which is quite beefy.


This rifle is what I consider an “M4-style” rifle in that it features an adjustable stock and a carbine-length barrel. The 16-inch barrel on the LAR-PDS is made from chrome moly steel. It has a 1:9 twist, 5.56 NATO chamber, A2-style flash hider, and M4 feed ramps. The barrel has a .65" overall diameter forward of the gas piston housing, and a .725" overall diameter under the handguard, which puts it square in the “medium contour” realm.

The rifle comes with a Hogue overmolded rubber grip standard, with standard selector and bolt release levers. The rifle is equipped with a two-stage Rock River fire control group. Normally, Rock River two-stage match triggers provide pulls of about 4.5 pounds, but due to the unique nature of the PDS’ bolt, they had to up the weight on a few springs. Trigger pull on my sample was 6.75 pounds, but in all honesty, it felt almost two pounds lighter than that, due to the two-stage nature of the trigger and the crisp break.

The upper and lower receivers are forged units. The lower receiver is pure stock-GI style, but if you look up you’ll see something different. There is a monolithic rail extending from the rear of the receiver all the way to the top of the gas piston housing. This extra height is to accommodate the piston design, but the end result is a rail that is .25" higher off the stock than a standard AR flattop upper receiver rail.


I don’t like the non-standard height of the receiver rail because it won’t be at the right height for standard AR flattop mounts, but the difference is not unworkable. Plus, most people don’t switch between twelve different ARs a month like the average gun writer, they buy one gun, set it up properly, and run that for years. It’s sort of like the difference between dating and marriage. So in that instance, a taller-than-usual receiver rail isn’t that big of a deal.

Rock River Arms LAR-PDS

You’ll see in some of the photos, the rifle sporting a Burris 2-7X scope in what would normally, for an AR, be rings that are set too low to see through the scope. With the higher rail on the Rock River, the scope is a hair low for my taste, but perfectly usable. On a standard AR rail, nobody would be able to see through that scope with those rings.

Let’s dive inside the piston system. The original AR-15/M16 was direct gas impingement operated. Gases from the fired cartridge were funneled back from the barrel through a gas tube, and those gases cycled the action. A lot of people didn’t like the idea of sending dirty gases back into the receiver where they could both foul and heat up the chamber.

And lo, gas piston-operated ARs were born!

In a gas piston-operated AR, the gases are vented from the barrel’s gas port, but instead of heading all the way back to the receiver, they push on the end of a piston right above the gas port. That piston is shoved backward by the force of the expanding gases, and it is the piston that cycles the action.

However, there are more kinds of gas piston AR operating systems than there are crazy people on a Kardashian TV show. Just about every company that makes a gas piston gun puts its own spin on the design of the internals. Rock River is no different—their Piston Driven System is a completely proprietary one, and totally unlike most of the other piston guns on the market. In fact, it has ten separate patents on its PDS.

With standard piston-driven systems, the gases shove the piston back, the piston smacks into the top of the carrier, and because of that hard impact, quite often you get carrier tilt—the rear of the carrier tilting down as it moves backward.

Rock River Arms LAR-PDS
Rock River’s PDS eliminates the back half of the bolt carrier and replaces it with an operating rod, which moves back and forth above the barrel. The guide rod, inside it, holds the action spring.

Unlike most piston-driven ARs, there is no possibility of carrier tilt with this design, as there is no traditional carrier. If you look at the photos, you’ll see that basically the rear half of the bolt carrier has been removed. The remaining part has been attached to an operating rod that goes forward above the barrel, inside which is the guide rod and spring. This design allows for a side-folding stock with CNC-milled aluminum mating surfaces, but more on that in a few paragraphs.

The bolt and carrier are disassembled exactly as with a standard AR-15. You’ll notice the bolt doesn’t have any gas rings, because they are unnecessary. The only trick to reassembling the bolt carrier components is remembering the cam pin has an angled face that needs to be to the left rear when installed in the carrier.

Rock River Arms LAR-PDS
The PDS has a two-position gas system, S (standard) and L (light loads).

The rifle has a two-position gas regulator. You’ll see it on the right side of the piston housing. The red dot indicates your setting, either “S,” for standard ammunition, or “L,” for light ammunition.

To take out the gas piston for cleaning (which you’ll only need to do very infrequently), you have to drive out a roll pin on the piston housing. The piston housing plug screw and the piston itself then come out the front of the housing.

Note that because of the unique operating system, you unfortunately cannot run a sound suppressor on the LAR-PDS. The pressure gets bumped too high with a suppressor attached to the barrel.

You will notice the lack of a traditional AR-style charging handle. The LAR-PDS is equipped with non-reciprocating bilateral charging handles way out front near the gas block, which will be a very familiar position to anyone who has spent time behind an HK91 or 93.

Rock River Arms LAR-PDS
The forward bilateral charging handle is a bit reminiscent of HK rifles. Tarr wishes the handles were a little longer to reduce the chance of skinning knuckles on the rail.

The charging handles are well-positioned and nicely curved, but I wish they were as long as what you find with the aforementioned HK91, especially with this tri-rail model rifle. The question isn’t “if” you’ll remove skin from your knuckles, but “when” and “how badly.” The handles are bilateral, so at least in that way the LAR-PDS is lefty-friendly.

To remove the bolt and carrier for cleaning, pop out the familiar receiver pin. There is no standard AR-15-type charging handle, but you’ll see an angled piece on the receiver, about where the charging handle would be. This is the base of the guide rod. Push down and in on the guide rod base and the guide rod and spring come out the back of the receiver in a fashion very similar to disassembling an AK or AR-180. Once the guide rod and spring are out, just work the charging handle to send the bolt/carrier to the rear where it can slide out.

The stock is a collapsible “M4” style on a six-position receiver extension. Notice, I did not use the term “buffer tube.” Due to the operating system, this rifle does not need a buffer tube or spring, and because of that, the folks at Rock River rightly thought the rifle should be equipped with a folding stock.

Rock River Arms LAR-PDS

The folding stock adapter is machined out of aluminum and folds the stock to the left. The pivot pin is steel. Because it extends a good two inches from the rear of the receiver, you’ll probably be running the stock one or two positions closer to the receiver on the extension than with a standard AR. There is a spring-loaded detent on the latch mechanism, so the stock stays folded to the side and doesn’t wave around. There are QD sling swivel sockets machined into the top and right sides of the folding latch body.

Rock River Arms LAR-PDS
The folding stock latch is machined from solid aluminum and features two QD sling swivel sockets.

Rock River states that the rifle with the stock folded is 26 inches long, but my tape measure showed the true length is 26.5 inches. When you unfold the stock but keep it collapsed, add another eight inches to that length. Extending the stock puts another 3.5 inches onto that number, for a total length of 38.5 inches with the stock all the way at the last position.

Rock River Arms LAR-PDS
Because it is not a buffer tube, but rather just a receiver extension on which to mount the stock, the inside is hollow, and Rock River made that into a compartment big enough for a spare bolt or batteries for your optics.

Because the receiver extension onto which the stock is mounted does not need to accommodate a buffer and spring, Rock River decided to turn the interior of that extension into a storage compartment. Accessed through the rear via a large aluminum plug with rubber O-ring, the interior of the compartment is more than large enough to store a spare bolt, a few batteries for your red dot, beef jerky, blood diamonds, whatever. It’s about seven inches long and over three-quarters of an inch in diameter and, with the O-ring, seems like it should be at least water resistant.

LAW Tactical sells a side-folding stock adapter for traditional AR-style rifles that works very well. However, when rifles so-equipped have their stocks folded, those rifles can only be fired once (and LAW Tactical doesn’t recommend that), as the bolt/carrier can’t cycle with the stock folded. It is solely an ease of transport upgrade to your rifle.

Rock River Arms LAR-PDS
Because the PDS does not use a standard AR-15 carrier or buffer spring, it does not need a traditional buffer tube. Not only can the stock be folded, but the rifle can be fired with the stock folded.

The Rock River LAR-PDS, on the other hand, don’t have no stinkin’ buffer, don’t need no stinkin’ buffer, don’t want no stinkin’ buffer— and as such, you can fire it with the stock folded all day long, or, as my editor described it, “Go full A-Team.” Everyone who gets that reference raise your hand.

When it came time to hit the range, I was pleasantly surprised. For a barrel with as many odd connection points as this one, the LAR-PDS was surprisingly accurate, with most loads running 2 MOA or better. It did not like the Black Hills 77-grainers, and I suspect the 1:9 twist was just not fast enough for that bullet, as this load has proven itself to be very accurate for me.

Rock River Arms LAR-PDS
Accuracy results are the averages of four, five-shot groups at 100 yards from a sandbag rest. Velocities are averages of 10 shots measured with an Oehler Model 35P, 12 feet from the muzzle.

The more I used the charging handle, the more I wished it were just a little bit bigger. Plus, I found I kept reaching back to the rear of the receiver for the charging handle, only to have to move my hand up to the front. This is a reminder that, no matter what kind of gun you’re talking about, if you plan on using it for self-defense, you need to train with it so that operating the controls becomes automatic.

Most piston guns that I’ve shot, even though they are heavier than standard direct gas impingement (DI) guns, have more felt recoil, I think due to the fact that there is more reciprocating weight moving around inside the gun with every shot. The LAR-PDS was a very soft-shooting gun, and I think this in part or whole is due to its unique chopped carrier setup. There is no more reciprocating weight in the LAR-PDS than you get in your standard DI AR-15 once you factor in the bolt, carrier, and buffer. Yes, I know the gun is heavy to start with, but I’ve shot lots of heavy ARs that had more recoil than the PDS.

The rifle ships with one 30-round magazine for those of you who live in free states. Rock River will substitute a 10-round magazine for an extra $10 for those customers who willingly live in states that have turned their backs on the most American value of all, freedom.

Wow, I almost made it through an entire article without a political rant. Almost.

The magazine provided with my rifle was aluminum with an NHMTG baseplate and a Magpul non-tilt follower. FYI, for those of you who haven’t seen my several write-ups on the subject, NHMTG means Colt, which means this magazine was actually made by Okay Industries, which sells its SureFeed magazines (identical to this one minus the Colt/NHMTG baseplate) for a lot less than what you’ll pay for a Colt-marked mag.

Running the PDS off the bench was easier on my upper body than off-hand (with a scope and loaded magazine, the rifle was pushing ten pounds), but recoil from every shooting position was soft and smooth. Using the provided magazine and two others, I put a little over 200 rounds through the rifle in the abbreviated test period I had, without any malfunctions.

Between the very ugly handguard and the excessive weight it adds, if this rifle were a girl, I’d say she has a nice personality. Don’t get me wrong, that wouldn’t be a lie—she is soft-shooting, accurate, and has a better-than-GI trigger, however, I’d still want to give her a makeover before I introduced her to my friends.

I like that Rock River basically started from scratch with this system. It didn’t imitate or modify an existing design; it came up with something new and proprietary. It just needs to tweak the handguard offerings for today’s consumer.

Because it is a proprietary design, the price of this rifle is higher than you might expect at $1,750.00. The polymer handguard version is $1,595.00. A few years ago, Rock River probably couldn’t make them fast enough, but right now I think that might be a little too high for what the current market will bear.

Rock River Arms LAR-PDS Specifications

Caliber: 5.56x45mm NATO
Action: Semi-auto, gas piston
Weight: 7 lbs., 11 oz. empty
Overall length: 26.5 inches (stock folded) 38.4 inches (stock fully extended)
Receiver: Forged 7075 T6 aluminum
Barrel: 16.0”, 1:9 twist, chrome moly
Gas block: 2-position adjustable
Muzzle device: A2 birdcage flash hider
Stock: Polymer M4 style, 6-position adjustable, folding
Pistol Grip: Hogue rubber overmolded
Forend: Rock River Tri-Rail
Trigger: Two-stage, 6.75 lbs. (as tested)
Sights: None
Accessories: 1 30-round magazine
MSRP: $1,750.00
Manufacturer: Rock River Arms www.rockriverarms.com

James Tarr is a longtime contributor to Firearms News and other firearms publications. He is also the author of  several books, including CARNIVORE, which was featured on The O’Reilly Factor. His current novel, WHORL, is available now through Amazon and Barnes & Noble.

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