POWER OF GOOD DESIGN
DECONSTRUCTING A PISTON-DRIVEN AR PLATFORM
Deconstructing a piston-driven AR platform.
If you’ve paid any attention to the tactical rifle industry in the last decade, you may be familiar with the continuing debates over which features provide an optimal, all-purpose platform that excels in ever scenario: self-defense, competition, hunting or just having fun at the range.
Industry professionals and weekend warriors alike deliberate ad nauseam on things like ideal cartridge, barrel length, twist ratio, and other specs that may or may not translate into varying degrees of reliability, accuracy, recoil control and versatility. You know that what they say about opinions: everyone has one.
Probably the most hotly contested debates that have risen to the top of the list are direct impingement AR versus a gas piston AK. Many readers may have already heard both sides and settled on a preferred platform, but what if I were to tell you that you didn’t have to choose one or the other and that you can enjoy the benefits of both systems in one platform? In response to this question, the piston-driven AR platform was born. A piston-driven gas operating action in an AR upper is not exactly a new concept. Several big players in the tactical rifle market have utilized this design to great success, including LWRC, Barrett, as well as Heckler and Koch. It is my hope that I can provide a comprehensive understanding of what the obstacles are in creating a piston-driven AR and to analyze the unique approach Primary Weapon Systems adopted when creating the PWS MK2MOD1 .308 Match.
This information should clarify the process of choosing a piston-driven AR platform. Before getting into the details, however, it’s important to first understand what fundamentally separates direct impingement platforms from their piston driven cousins.
DIRECT IMPINGEMENT VERSUS GAS PISTON AR
I hope you all can tread water, because we are about to get neck deep into what engineers may call “small talk” at a party, so just bear with me! Eugene Stoner, father of the modern sporting rifle, popularized the direct impingement (DI) system in the mid-1950s with his original Armalite military rifles. These rifles eventually permeated the civilian firearms market and still continue to dominate the AR market.
Throwing a rock down range begins with the pull of the trigger, which releases the hammer, igniting the cartridge, discharging high-pressure gas, and pushing the bullet through the barrel. About three quarters of the way down the barrel, the bullet will pass the gas port, where a portion of the high-pressure gas is redirected into the gas tube. This gas expels its force directly onto the bolt carrier key on the bolt carrier group (BCG) located in the upper receiver.
It is at this point an argument has been made that having these gases expelled directly into the upper receiver will result in carbon buildup of the BCG and other internal components. The constant exposure to carbon requires periodic cleaning and therefore reliability issues. The force the gas exerts on the bolt carrier key pushes
“THE SUCCESS OF THE MK2 MOD1 RESIDES IN THE DETAILS THE ENGINEERS UTIALIZED TO INCORPORATE ADVANTAGES OF BOTH THE AK-47 AND THE AR-15 …”
the BCG rearward, ejecting the spent casing and redirecting itself forward utilizing the force generated by the buffer spring; this movement also chambers the next round. A key detail to remember about the DI system is that all the components driving the operation of the rifle are traveling in one straight line, reducing recoil and resulting in improved accuracy.
The template used for the pistondriven AR was grounded off the highly-respected AK-47 designed by Mikhail Kalashnikov. Manufacturers have taken different approaches incorporating the piston driven system within the upper receiver of an AR, but the overall operation is very similar. The high-pressure gas expelled from the cartridge is directed down the barrel until it goes through the gas port into the piston chamber where the gas expands, driving the piston rearward until it impacts the carrier block located on the topside of the BCG. The advantage here is that the carbon containing gas is not expelled directly into the upper receiver; therefore, it requires less cleaning and maintenance.
Once the piston drives the BCG rearward, the case is ejected and the bolt is driven forward by the buffer spring chambering the next round. Unlike the DI system, the forces driving the operation are not on the same horizontal axis, due to the location of the piston system being above the barrel. As a consequence, a phenomenon known as carrier tilt occurs, increasing recoil and wear. It takes place when the BCG is driven rearward and downward caused by the impact of the piston on the top side of the bolt carrier. This adverse movement will increase wear on the bottom of the carrier, the upper receiver and the buffer tube.
With the fundamentals out of the way, check the sidebar for the advantages and disadvantages of each system.
UNDER THE CHASSIS
The PWS MK2MOD1 is available in a variety of configurations, available in barrel lengths of 14, 16 and 20 inches. The barrel has a 1:10 twist with caliber options of .308 Match or 6.5 Creedmoor. They are manufactured in-house through a button rifling process. An isonite QPQ treatment enhances corrosion resistance and increases hardness. The MOD FSC muzzle device is affixed to the barrel to aid in muzzle control and flash suppression. A four-position adjustable gas block gives the operator the ability to match the performance of the rifle to the ammunition and configuration of choice. The positions are pre-set for standard ammunition, hotter loads,
suppressed using standard ammunition and suppressed using hotter loads. The MK2MOD1 is outfitted with a free-floating handguard incorporating a full-length Picatinny top rail machined from aircraft grade 6061 aluminum. A Keymod interface reduces overall weight, while providing the real estate for any attachment your heart desires.
The upper and lower receiver are Cnc-machined from milspec 7075 aluminum forgings and then hardcoat anodized type III for corrosion and wear resistance. To round out the package, the MK2 comes ready to go with a BCM Gunfighter charging handle.
The bolt carrier group is machined from tool steel, and then treated with nickel-teflon to reduce friction and increase durability. The bolt carrier was designed to minimize contact points by creating gaps and channels between the skid pads to decrease any metal on metal wear that could be caused by normal operation and carrier tilt. This is yet another subtle, but very important, feature that directly addresses the issues of wear caused by carrier tilt as seen on other piston ARS.
In concert with the modified BCG, PWS has introduced a carrier tilt feature onto the buffer to negate the effects of carrier tilt. The buffer tube was modified by the addition of a small lip extending into the lower receiver to support the downward forces created by BCG during operation.
FIRING ON ALL PISTONS
The success of the MK2MOD1 resides in the details the engineers utilized to incorporate advantages of both the AK47 and the AR-15, innovating a design of their own. The engineering ingenuity and the backbone of the MK2 platform centers on the long stroke piston design. This was no easy feat to tackle.
Dean Sylvester, president of PWS, expressed that the biggest challenge his team faced was fitting the dual head piston and op rod, which is attached to the carrier into the space normally used for a gas tube in a direct impingement rifle. Always keeping the end user in mind, they did not see raising the sighting platform over a quarter inch like others in the market to be an acceptable solution to the problem. Furthermore, he adds that designing around the space allowed has definitely been one of the factors that has taken the most engineering effort.
What features does PWS bring to the table that are not found in other piston driven ARS? Who better to answer this question than Sylvester? He said other designs on the market use a system where the op rod impacts the carrier block to provide the energy to cycle the weapon.
“Key difference between the PWS
system and others is that the op rod is attached to the carrier and the piston is attached to the op rod,” he continued. “This provided a gas to metal interaction much the way a direct impingement rifle operates, giving the weapon a more controllable feel and less impact wear to the key parts over time.”
To add to Sylvester’s statement, the recoil felt from a one-piece design is smoother due to its pushing action versus the snappy movement generated by a high-speed metal on metal impact traditionally seen in a two-piece design where the piston slams against the BCG. Because the piston is a two-piece design, it allows the MK2MOD1 to accept a charging handle. There is a hole located on the muzzle end of the charging handle where the operating rod is passed through and then connected to the dual head piston forming the long stroke piston.
I was excited to take my new girlfriend out on our first date at the range. The 16-inch PWS MK2 was my model of choice, equipped with U.S. Optics MR 10MIL 1.8-10x scope with ADM Recon 20 MOA scope mount. The scope added some noticeable weight to the rifle, but the rifle itself was relatively light and very maneuverable. The quality of the U.S. Optics MR was superb, as you would expect, the glass was crystal clear, quick to zero and felt very solid. Besides being a fan of an illuminated reticle, I was very fond of the EREK elevation adjustment drum that allows the user to make quick, large adjustments shortening the time spent zeroing the scope.
I could instantly tell that the rifle is made with a lot of care and precision. Everything is crisp and clean, with seamless transitions between the upper and lower receiver. Breaking down the rifle for maintenance is very simple and intuitive. Disconnecting the op rod and the dual head piston is a cinch, but it takes a little getting used to as one might expect. I shot from the seated position on a bench rest, using shooting bags for stabilization. The ammo used for testing was Federal Premium 168-grain Sierra Matchking BTHP, Winchester Match 168-grain and Gorilla 175-grain Sierra Matchking, which was provided by the manufacturers.
When push comes to shove, this rifle is very accurate. My three-shot groupings at 100 yards resulted in a .76-inch group using Winchester, 1.062 inches using Federal and 1.125 inches using Gorilla.
I achieved some sub MOA groups with all brands of ammo during my time on the range and was consistently able to shoot around 1-inch groups. The MOD FSC muzzle device performed well, but
it is noticeably louder than others in its class. Recoil was very manageable when performing rapid-fire shots; I was able to get back onto target quickly with follow up shots and put everything on a 12x13-inch target at 100 yards with very desirable groupings.
The trigger is one of the smoothest, most crisp triggers I have shot and has little to no creep. Trigger pull was tested using the Lyman electronic trigger pull gauge, averaging around 7 pounds, 7 ounces.
You may feel like you deserve some college credits by the end of this article, but these are questions and answers being discussed in forums by other likeminded enthusiasts in the quest of choosing the right AR piston platform.
You can say that I am fan of the PWS MK216. I am a believer in the design concept and feel that it was well executed with a special attention to detail that can only be expected coming from PWS. After hundreds of rounds sent down range, it performed without a single hiccup and delivered consistent, precise shots.
My one critique is that for a rifle in this price range I would expect PWS to incorporate a completely ambidextrous receiver, including an ambi charging handle. With the MSRP of $2,200, you are getting a best in class rifle. Don’t get me wrong, two grand is not light on the wallet, but in my eyes, it is money well spent considering the variety of applications this rifle will afford you.
If any of you have caught the piston AR bug, like I have, do your research and choose wisely. A good design makes all the difference. TW