Product Review: Rock River Arms PDS Piston Driven Carbine

The PDS is Rock River Arms’ entry into the piston AR market.  I’ve never been enthusiastic about this concept, but Rock River Arms (RRA) put some serious design thought into this weapon, and the results are impressive. Here are some pictures.

Starting at the rear, it has a side-folding stock, because the recoil system isn’t in a receiver extension inside it.  This is a significant advantage for transport and carry.  The folder mechanism does take a bit of getting used to.  It requires lifting the stock out of a deep detent.  It locks solidly in place open or closed, and has a 6 position stock.  As the stock tube is not needed for a recoil mechanism, it contains a storage section with a threaded cap, bit enough to take a rod, cleaning tools, and small maintenance tools and spare parts.  This is a big plus for a gun that sees real use.

The two stage proprietary trigger is crisp and reliable.  The test gun broke cleanly at about 6 lbs.  It was very consistent and comfortable.  The major difference on the controls is that the charging handle is a forward type, like the German H&K G3.  It is ambidextrous, non-reciprocating, and folds flat on closing.  It is reliable, but does take a bit of strength to work.  Smaller shooters and females had trouble, but could manage with effort.  The grip is a Hogue model, with good texture.

Takedown starts as with any AR, and once the receiver halves are open, the recoil mechanism is pulled down and out.  This removes the operating rod, bolt carrier and bolt in one unit.  It’s elegantly simple.

The receiver top is elevated slightly over standard ARs, because it contains the recoil mechanism.  This puts it at a very comfortable height for most sighting systems—a riser is not needed.  It is a monolithic rail for mounting sequenced optics.

The handguard is a bit odd looking and feeling, but very solidly mounted and useful. There is also an optional railed handguard.

The gas system has two settings, and the operator is cautioned not to use a suppressor with this weapon.  A different gas block is in production for that purpose.  Recoil is somewhat brisker than a gas impingement system due to the greater operating mass, but is not uncomfortable.  The bottom of the gas block includes a 1″ rail for accessory mounting.  The gas block is very solidly mounted.  I did not attempt to dismount it to examine it.  Generally, pins are preferred to screws, but as it does not protrude far, and does not mount a bayonet, the heavy construction and machine screws are plenty.

We tested using several brands of ammunition, with an Aimpoint sight mounted, shooting over a sandbag.  All the major commercial brands remained under 2.5″ at 100 yards, in 10 shot groups.  Best group was 1.55″.  Weather was 50 degrees F, 75% humidity, still air, at approximately 300 feet elevation.  Keep in mind, this was with combat optics and sandbag, not a mounted weapon and scope.

The only stoppage was a double feed, attributed to a bad magazine, as the same problem occurred with another gun with that magazine.  After 500 rounds, the mechanism was clean, and the bore only needed a pull through with a Boresnake for cleaning.  That’s certainly a big plus.  The receiver remained cool, and there was very little oil evaporation.

The PDS comes with a magazine, a simple to read instruction manual, and a hard case. MSRP is $1,685 as tested, which makes it only slightly pricier than a high-end standard AR. – Michael Z. Williamson, SurvivalBlog Editor at Large.

JWR Adds: It is likely that there will be a shakeout among the many new competing gas piston AR-15 upper designs. Beware that these are proprietary designs, and hence their parts do not interchange. It may take a few years before just two or three technologically superior and market-dominant designs emerge. Other designs–most likely from small makers–will just be a “flash in the pan” that will go out of production. Sadly, I’ve heard that Walt Langendorfer‘s excellent Rhino design (patented in 1981 and unquestionably “first to market”) is now out of production. As with the famous Betamax versus VHS war (where unfortunately the superior and more compact design lost out) this will leave some owners without any available repair parts. So beware. I anticipate that it will be designs from a few major makers (such as POF-USA, Ruger, and RRA) that will predominate. The HK 416 is a great design, but I suspect that few will of them ever make their way to civilian hands–at least at a reasonable price. The bottom line: If you buy a piston upper, don’t sell off your original gas-impingement AR upper. Someday you might need it. And, as Mike mentioned in a recent e-mail “One point to remember is that a gas impingement gun with a damaged gas system becomes a straight pull bolt action rifle.  A piston gun with damage to tube or piston locks up and becomes a very expensive club.  The type of gun you choose is very much a reflection of the environment you may operate in, and your intended purposes.”