Rock River Arms has developed a carbine and pistol chambered for 5.56mm using their proprietary Piston-Driven System (PDS). Several items make the PDS unique, such as the matte black, hard polymer handguards and the full-length Mil-Std-1913 rail running from where the gas block traditionally would be back to the rear of the upper receiver. It is underneath this rail that the operating system is housed.

Gas piston versus direct gas-impingement is a topic currently dominating the AR world. Some are embracing the new piston guns while others view it as an answer to an unasked question. Whichever side you take, the fact remains that there are high-quality rifles in both camps.

The majority of AR platform manufacturers have taken the “op rod/piston” route replacing the gas tube assembly and carrier key on the bolt carrier. These carbine-sized guns on the outside are virtually identical to their gas-impingement counterparts. The lower receivers are unmodified with the same buffer tube assembly being used.

When Rock River Arms (RRA) decided to throw their hat in the gas piston ring they didn’t want to simply copy something that was already out there. They wanted their piston system to be unique. I believe they have accomplished that mission.

The absence of the traditional operating system means that no buffer assembly is required, allowing the addition of a side-folding stock on the carbine.

Piston-Driven System
Two new firearms chambered for 5.56mm have been released with RRA’s Piston-Driven System (PDS) — a carbine and pistol. Several items make the PDS unique; the first thing that most shooters will notice are the handguards. These matte black, hard polymer grips are specially designed to mate with the new operating system.

The next item of note is the full-length Mil-Std-1913 Picatinny rail running from where the gas block traditionally would be back to the rear of the upper receiver. It is underneath this rail that the operating system is housed.

To work the bolt you will use an ambidextrous charging handle located out in front, above the forward portion of the handguard. The charging handle folds against the side of the gun when not in use and is “non-reciprocating.” It is not affixed to the operating rod so it does not run back and forth when the gun cycles. Out front on the right side of the gun is a two-stage, adjustable gas regulator. You can adjust it with the tip of a round and a coin or flat-head screwdriver.

Inside the rail you’ll find the operating rod and recoil spring. The bolt and bolt carrier are not interchangeable with a standard AR. The absence of the tradition operating system means that no buffer assembly is required. For the PDS pistol this mean a clean, pistol-like profile without that odd looking, naked buffer tube sticking out the back. On the carbine this allows the addition of a side-folding stock.

The PDS Carbine’s two-position adjustable regulator is found on the right side of the regulator housing and features a Picatinny rail for attaching any number of accessories.

Of course there are many similarities to the modern AR rifle. The lower receiver accepts all M16/AR-15 magazines. The controls: manual safety lever, bolt catch, magazine release button and trigger are all stock AR. Speaking of triggers, RRA has installed their excellent two-stage trigger, one of the finest factory versions you can get.

The retractable stock on the carbine model is a normal M4 variety. For pistol grips, RRA has installed the comfortable Hogue finger groove model. Out front at the business end you’ll find the familiar A2 flash suppressor. Both the pistol and carbine have takedown and pivot pins identical to your standard AR, connecting the upper and lower receivers.

Carbine Details

The PDS Carbine’s matte black, hard polymer handguards are specially designed to mate with the new operating system and ribbed for a comfortable grip.

Let’s take a second to address the specifics of the carbine version. First of all RRA has installed a 16-inch chrome moly barrel with a 1-in-9-inch twist. The overall length of the gun with the stock folded is only 26 inches. with the stock extended it is 37.5 inches.