Primary Weapons System (PWS) MK 114 Long-Stroke AR: Continued Evolution of the Long Stroke Piston AR
By Todd Burgreen

It is nice to witness a product’s evolution. This is especially true when one has experience with early models to better appreciate the modifications made. The Primary Weapons Systems (PWS) Mk 114 MOD 1 AR rifle is a great case in point. PWS has taken its concept of the AR rifle another step forward with its new generation Mk 1 (M16/AR-15) and Mk 2 (SR-25/AR-10) series of long stroke piston ARs. PWS literature clearly lays out the thought process behind the development of its ARs. The PWS Mk 1 series stemmed from appreciation of AK47 simplicity and reliability combined with deep-rooted experience with M4/M16 ergonomics. PWS is confident that the Mk 1 long stroke piston operated AR is an improvement over not only the original gas impingement design, especially when incorporated into short barrel platforms, but also other piston AR designs featuring short stroke pistons.

Significant differences are present in the Mk 1 PWS compared to both the direct impingement operating system and other piston-driven operating systems. PWS product improvements address reliability and performance issues as well as reduced carrier tilt, cam pin wear and carrier bounce. Proponents of piston-driven ARs point to greater reliability in adverse conditions and less reliance on routine maintenance compared to direct impingement operating method. The increasing use of suppressors for civilians, law enforcement, and military highlight another advantage of the piston-driven ARs – the ability to adjust the amount of gas siphoned to operate the action. Suppressor use increases operating pressures a considerable amount and ability to quickly turn a knob turning down the gas vented to the operating rod is a big advantage. Currently, piston driven ARs also enjoy a perception of better operating reliability in harsh environments, especially as barrel lengths shrink. Instinctually, the gas-piston ARs appeal to many by the very fact that hot gases and powder residue is not dumped into the action a la the original direct impingement (DI) design. Anecdotal evidence of bolt carriers being handled soon after long strings of fire and merely wiped down compared to their DI cousins reinforces this. Do not take this as lampooning of the gas impingement ARs. The DI rifles are more robust than most give them credit for and sub-14 inch rifles are possible with attention to detail by the manufacturer in terms of gas port sizes and timing of the ejection process. Within the piston-driven AR world there is a less well known debate happening between long-stroke and short-stroke piston-driven designs.

The Primary Weapons Systems PWS Mk 1 series represents cutting edge long-stroke AR design. This is different than the original Stoner designed gas impingement design as well as the growing numbers of short stroke piston ARs arriving on the market. A look at a PWS Mk 1 disassembled with the AK-like operating rod connected to the charging handle and bolt carrier leaves little doubt of the AK inspired PWS design. The PWS long-stroke piston evolved out of early PWS short-stroke conversions by eyeing simplicity, fewer and hardier parts. PWS Mk 1 ARs require no tools needed to disassemble with maintenance requirements even less than other piston systems. The PWS MK 1 gas-piston system can be disassembled for maintenance without requiring the rifle’s handguards to be removed, unlike some of the short stroke ARs on the market. Cleaning methods for the PWS Mk 1 AR are familiar to anyone with experience with the AR-15. Proponents of the long-stroke AR method argue that it does not have multiple parts banging into each other with the operating rod and bolt carrier group moving together under the impulse of a fired round; This contributes to minimizing carrier tilt and carrier bounce typically found on short-stroke AR piston designs. Overall, the PWS long-stroke is simpler in operation with fewer parts involved than short-stroke AR; on top of this the parts that are used are larger and thus more than likely robust in the long term.

With all of this stated, PWS continues to evolve its design. The rifle helping us to explore the latest PWS has to offer is their Mk 114 MOD 1 tested herein. The Mk 114 features a .223 Wylde chamber (more on this later) and weighs in at 6.5 pounds. A 14.5 inch stainless Isonite treated barrel with permanently attached PWS FSC 556 flash hider allows the Mk 114 to comply with BATF regulations requiring 16 inch barrel length; thus not requiring NFA SBR paperwork. Overall length of the rifle is 31.4 inches. The PWS long stroke piston used with the Mk 114 is a mid length variant, which helps to smooth out recoil impulse especially when combined with other PWS design features such as the large dwell chamber in front of the operating piston’s head. Further along this design motif is the forward canted gas tap directing gas from the barrel for the piston in a forward orientation versus the more typical horizontal or rearward angle. The forward cant also helps to minimize carbon build up on the operating rod.

A new feature on the Mk 114 MOD 1 is the VLTOR Key Mod 15-inch rail system that permits the barrel to be free floated. PWS uses a forged M4 style upper receiver and equips the Mk 114 with Magpul MOE pistol grip and buttstock. PWS has eliminated the typical AR castle nut method of attaching the carrier tube to the lower receiver via indexing screws instead. PWS has designed an enhanced buffer tube with an extended lip to eliminate carrier tilt adversely wearing on a buffer tube. Magpul flip up BUIS iron sights are fitted to the rifle’s top rail. The Magpul accessories are topped off with the 30round PMag that arrives with the rifle. An extended BCM charging handle offers the user plenty of purchase to operate the Mk 114’s bolt group. Lastly, an ALG Defense QMS trigger is fitted into the forged lower receiver.

Speaking with PWS’s Stacey Nagy indicates the changes to the Mk 1 and Mk 2 series indicated in the MOD 1 designation have grown from PWS’s place in the market. Suppressor use is increasingly popular both in the civilian and military/LE realm. As a result, PWS wanted to increase the Mk 1 and especially the Mk 2’s user friendliness in regards to suppressor use. While the original Mk 1 was able to be suppressed by simply changing the buffer weight, this was hardly conducive to on-the-fly mission adaptability. The need for simple adjustment at the gas block was obvious with PWS responding.

The Mk 2 on the other hand cannot benefit from a change in buffer weight to counteract the bolt carrier speed increase caused by a suppressor. Adding more weight behind the bolt group in the form of a heavy buffer will not slow the Mk 2’s bolt down as it will in an AR-15 platform. This is due to the heavy mass of a .308 bolt group. As a result, suppressing a .308 AR-10 type rifle is difficult. There is simply too much gas volume and pressure in an AR-10 type rifle to allow for proper function without some kind of gas adjustment to account for the suppressor. This reasoning is born out by the fact that nearly every piston driven .308 semi-auto rifle available now has an adjustable gas block. PWS’s desire to be more suppressor friendly led them to adopting an adjustable gas block as well. Since PWS was implementing it on their Mk 2 .308 models, it was an easy decision to make it standard throughout the Mk 1 product lines also.

The PWS gas adjustment is very simple. There is a tool in the grip panel of the MOE grip. This tool inserts into the adjustment holes in the gas knob and you simply rotate it to the next position/detent until the desired setting is selected for your specific suppressor set up. An adverse condition setting is not needed as the PWS system is designed to operate with enough back pressure at all times no matter how dirty or fouled – that is something to be appreciated in a fighting rifle. There is a standard setting indicated by an O. Then there are three suppressed settings each distinctively marked as well and a take down setting marked X. The PWS knob can be turned in either direction. There is no “closed/single shot” setting. The setting used depends on the suppressor and the ammo type being used.

The MOD 1 Mk 114 tested arrived with PWS’s new lighter, free-float hand guard that helped trim 4 to 7 ounces from all of our upper halves depending on the barrel length. This new hand guard features the Key Mod interface designed by VLTOR and now open sourced to the industry. PWS along with Noveske were a couple of the first companies allowed access to the Key Mod design. The Key Mod design is going to bring about a new generation of AR accessory mounts and furniture that connects directly to the hand guard instead of needing a rail surface to clamp onto. All of this keeps the overall rifle package lighter and more compact.

A quick word about the Mk 114’s .223 Wylde chambering is in order. The .223 Wylde was designed as a match chambering for semi-automatic rifles. It will accommodate both .223 Rem and 5.56mm ammunition. It is relieved in the case body to aid in extraction and features a shorter throat for improved accuracy. As most know, the 5.56mm is not synonymous with the .223 Rem chambering. Generally, it is advised as acceptable to fire .223 Rem in a 5.56mm, but not the other way around. The .223 Wylde was meant to give a slightly tighter body spec than 5.56mm, but a bit looser than .223 Rem, while retaining a longer lead/throat. In effect, the .223 Wylde is a hybrid splitting the difference between the 5.56mm and .223 Rem while extending benefits in terms of accuracy and reliability.

A Trijicon model TA02 4x32 ACOG was mounted to the PWS Mk 114 MOD 1. This ACOG is the new Trijicon variant offering an LED illuminated reticle in lieu of the traditional tritium reticle. The concern for operating the magnified Trijicon ACOG in a CQB environment is quelled once one becomes familiar with the Bindon aiming concept. Human vision is based upon a binocular (two eyes) presentation of visual evidence to the brain. Trijicon has invested into vision research to understand the optically aided weapon aiming process. The simple substitution of a bright red dot for the usual cross-hairs makes it very easy to keep both eyes open. The brain merges the two images. During dynamic movement, the scene through the telescope blurs because the image moves more rapidly due to magnification. The one eye sees the bright dot against the blurred target scene, so the brain picks the scene from the unaided eye. The shooter swings the weapon towards the target while perceiving the dot indicating where the weapon is pointed. As soon as the weapon begins to become steady in the target area, the brain switches to the magnified view. As many shooters can attest, the single focus plane with ACOG is easier to shoot accurately than coordinating front and rear sights.

Ammunition tested with the PWS Mk 114 MOD 1 was a combination of Black Hills Ammunition loads, multiple Hornady TAP loads and Winchester 55gr FMJ. The M 400 kept all loads tested at 1.5 inches at 100 yards with multiple Black Hills loads – 50gr TSX and 77gr Match – producing near MOA groups out to 200 yards. This accuracy level justifies mounting a magnified optic such as a Trijicon ACOG or Accupoint 1-4x scope. Of course, red dot optics such as the Trijicon SRS or Reflex are viable options as well. Accuracy test protocol consisted of three five-shot groups with each ammunition type; group sizes were averaged. Velocity figures ranged from 2,700 fps to 3,000 fps over a RCBS chronograph. Very little velocity is sacrificed with the 14.5 inch barrel while gaining a certain amount of graceful handling compared to 16 inch or longer barreled ARs.

The PWS Mk 114 MOD 1 was evaluated at Echo Valley Training Center (EVTC). The rifle was evaluated not only within the 100 yard bays at EVTC firing from barricades and engaging multiple targets, but also back at the prepared firing position line with targets placed out to 300 yards. Numerous drills were run involving magazine changes and moving between barricades simulating cover. A High Speed Gear (HSG) patrol belt configured with their “Taco” magazine pouches was used. The HSG magazine pouches do not compromise retention for the sake of convenience. TacStrike steel silhouettes were situated randomly from 20 yards to 110 yards. The PWS Mk 114 MOD 1 made short work of striking various targets in quick fashion. The rifle was then fired supported via bipod or pack and proved just as efficient against a fluid drained automobile located at EVTC with TacStrike steel popper targets located inside the cab. The Black Hills Ammunition bonded bullet loading for the 5.56mm is a top consideration for this type of engagement. The bonded rounds are more reliable performers for barrier penetration compared to Match or other high accuracy bullet types. Firing the PWS Mk 114 MOD 1 from the prone position supported via a pack produced consistent hits out to 250-300 yards on TacStrike 1/4 Scale steel targets. The Trijicon 4x32 ACOG’s crisp image aided in spotting the targets in the vehicle’s interior with accurate fire then placed.

The PWS decision to improve its gas piston operating system along with other tweaks including a sleeker rail system is sound as found in the Mk 114 MOD 1. While the PWS is not going to be issued to the bulk of our military services, SOCOM operators have found their way to it. Law enforcement agencies could easily consider using the rifle. (Not to mention private contractors, and security conscience civilians.) With budgets constantly under pressure, the Mk 114 MOD 1 could be the answer to fulfilling a multitude of roles – DMR rifle, general-purpose patrol rifle, or entry weapon. The PWS Mk 114 MOD 1 could perform any of these tasks with equal aplomb. This is a positive reflection on the rifle’s accuracy, reliability, handling and ergonomics.


Primary Weapons Systems


Black Hills Ammunition

Echo Valley Training Center

Trijicon, Inc.

This article first appeared in SmallArmsReview.com on May 31, 2013


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