PWS Long-Stroke Piston Driven AR Tandem

MK107 Diablo & MK112.

Newly-improved piston-driven ARs — in lieu of Stoner’s original gas-impingement design — seem to be dominating the AR market. Proponents of piston-driven ARs point to greater reliability in adverse conditions and less reliance on routine maintenance, compared to the direct-impingement operating method. Urban fighting in Iraq has led to an increased use of suppressors, which points out another advantage of the piston-driven ARs: better operating reliability is associated with piston-driven ARs with sub-14″ barrel. Within the piston-driven AR world, there is a lesser-known debate happening between long-stroke and short-stroke piston-driven designs. A long-stroke piston is exemplified on the AK-47 (with its op-rod connected to the bolt carrier), where as short-stroke consists of a more complex arrangement of tapped gas imparting force to an op-rod, which then impacts another part of the mechanism and acts on the bolt carrier.

Instinctually, the gas-piston ARs appeal to many for the very fact that the hot gases and powder residue are 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. The ability of the gas piston ARs to operate with barrel lengths below the generally accepted 14″ barrel threshold of DI ARs reliability adds further legitimacy to the piston ARs. Don’t take this as lampooning of the gas-impingement ARs. The DI rifles are more robust than most give them credit for, and sub-14″ rifles are possible with attention to detail by the manufacturer in terms of gas port sizes and timing of the ejection process.

A 7″ PWS Diablo (center) shown between two 16″-barrelled AR variants. The
compact size of the Diablo combined with reliable operations makes it a
unique and valid contender for CQB application.

PWS Mk1 Series

The Primary Weapons Systems (PWS) Mk1 series represents cutting-edge long-stroke AR design. This is different than Stoner’s original gas-impingement design, as well as the growing numbers of short-stroke piston ARs arriving on the market. A look at a PWS Mk1 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 targeting simplicity and fewer and hardier parts.

Proponents of the long-stroke AR method contend that it doesn’t have parts banging into each other with the entire 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 in the short-stroke AR. These parts are larger and thus more robust in the long term. For example, there is no gas adjustment on the PWS Mk1. The system is designed to operate with enough back pressure at all times, no matter how dirty or fouled; this is something to be appreciated in a fighting rifle.

PWS 7″-barrel Diablo upper ready to install on properly registered
AR lower. Various ammunition and web gear was accessed in this gun’s T&E.

Mk107 Diablo

I was fortunate enough to evaluate two separate PWS SBR uppers mounted on registered lowers, including select-fire variants. The first PWS upper represents one of the shortest operational AR barrel lengths available — 7″ barrel Mk107 Diablo. The Mk107 Diablo is designed for CQB/Direct Action and personal security detail operations. It is best to view the 7″-barreled Diablo through the lens of a PDW or SMG weapon versus a rifle.

The 7″ barrel produces approximately 2,100 to 2,300 fps velocities with the 5.56mm round while maintaining enough rail space for solid purchase and accessories as mission dictates. An innovative PWS handguard system utilizes a proprietary non-free-float system, allowing the end user to replace individual rails as needed without removing the barrel, barrel nut or even other rails. Imagine if you will the PWS gas/piston tube surrounded by rails that don’t rely on being attached to the tube; rather, the rails are attached at the pinned gas block in the front and the receiver at the rear. The design facilitates better airflow around both the gas tube and barrel for more efficient cooling during long strings of fire. The Mk107 Diablo’s user can attach numerous devices such as sights, lights, vertical forward grip and laser aiming devices. PWS points to the size similarity of the Mk107 compared to the MP5 SMG, which was the gold standard for military and LE entry teams for many years. The Mk107 firing the 5.56mm rifle round, compared to the MP5 chambered in 9mm pistol round, provides an all-around ballistic advantage; even at the reduced 2,100 fps velocity, the Mk107 will be effective out to 200 yards in terms of penetration and terminal ballistics.

The PWS Mk107 installed on a lower weighs 5.5 pounds and has an overall length of 24.5″ with the adjustable stock collapsed and approximately 28″ extended. The Mk107’s stainless steel, button-rifled, Isonite-treated 7″ barrel features 1:8″ twist. A CQB Comp muzzle device comes fitted to the Diablo’s barrel. PWS designed the CQB Comp specifically to address the notorious muzzle flash associated with short-barreled rifles. Its 2-part design eases cleaning and, more importantly, serves to starve off oxygen at the muzzle area, limiting the means for unburned to ignite, thus producing spectacular fire balls so associated with short-barreled weapons.

The key with the PWS Mk107 is having a 7″-barreled AR that works, and works at the same reliability level as longer-barreled ARs. This is where the PWS long-stroke piston comes into play. The PWS proprietary system utilizes an operating rod that is attached to the carrier and a floating-head piston that is attached to the operating rod. Bravo

Company’s BCMGUNFIGHTER Charging Handle is used with the PWS uppers. The extra purchase gained from the large BCM latch allows for better weapon manipulation during reloads and any malfunctions encountered. The Mk1 series is built to be a true combat carbine. With no unnecessary gas adjustments and only one moving assembly, proponents point to the Mk1 as the most rugged, simple and versatile piston operating system available. The Mk1 series is available as complete carbine, rifle or as a “drop-on” upper receiver for use on the lower receivers of existing M4/M16 style lowers, which is the path chosen for this review.

PWS Mk107 Diablo bolt carrier group after 700 rounds of hard use.


The second PWS upper utilized for evaluation was the Mk112 featuring a 12.5″ barrel. The 12.5″ barrel represents the optimum crossbreeding of handling-to-cartridge performance. The Mk112 PWS produces approximately 2,800-plus fps velocity with the 55-grain/62-grain 5.56mm round. The PWS Mk112 is installed on a lower combine to weigh 6.5 pounds. The Mk112’s stainless steel, button-rifled, Isonite-treated 12.5″ barrel also features 1:8″ twist. The Mk112 featured the same rail design as found on the Mk107, only longer. A PWS Triad flash suppressor was fitted on the Mk112’s barrel.

PWS Mk1 ARs need no tools to break down, and have even less maintenance requirements than other piston systems. The PWS Mk1 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 Mk1 are familiar to anyone with experience with the AR-15. While the PWS long-stroke gas system serves to isolate the action from heat and fouling, especially when compared to the typical AR direct gas impingement method of dumping gas directly into the action, this doesn’t mean the fore-end doesn’t get hot. This is a natural result of the gas tube location and typical for long-stroke designs that utilize the cartridge ignition gasses to operate the piston. Gloves, small vertical forward grip and/or rail panels provided with the PWS uppers are necessary if any volume of fire is expected. AR short-stroke pistons using a “tappet” system to impart force to the piston minimize this heating up in the handguards, confining it more towards the muzzle end compared to along the full length. As with most methods, there are pluses/minuses to each approach, and the long-stroke heat in the handguards is no different and not detrimental to operational use in any way.

PWS Mk112 upper with Leupold VX-R Patrol optic mounted with a
Leupold base. The PWS Mk112 represents the “sweet spot” of
sub-16″ handling while maintaining some semblance of rifle
ballistics and terminal performance.

A Variety Of Accessories

I decided to take full advantage of the flattop uppers on both rifles by mounting a variety of optics — including the newly-introduced Leupold VX-R Patrol with illuminated reticle. The VX-R Patrol is available in both a 1.25-4x20mm model with the FireDot Special Purpose Reticle (used in this article) and 3-9x40mm with TMR FireDot reticle. The Leupold VX-R with FireDot SPR reticle is designed for instinctive, close-range/low-magnification situations, yet allows shooters to engage targets with greater precision at longer ranges than generally possible with non-magnified red-optic optics or other reticle style low powered scope. The Leupold VX-R 1.25-4x is an optic candidate-worthy of being left on the PWS Mk112 full time with its longer barrel length. It’s able to handle multiple roles as a user decides, albeit close range engagements or further. A Trijicon RX30 Reflex and Trijicon ACOG were also mounted along with a Leupold Mk4 CQ/T 1-3x. A Leupold Mk4 2.5-8x was employed to ascertain the two PWS upper’s accuracy potential with its 8X magnification. The optic types intended to be left on the PWS Mk107 after accuracy testing would be the Trijicon RX30 Reflex or Leupold Mk4 CQ/T. The Trijicon ACOG and Leupold Mk4 CQ/T sight offer the capability to engage multiple targets in rapid sequence compared to open sights, while at same time provide adequate accuracy out to a couple hundred yards. The 5.56’s flat trajectory aids in making hits out to 200 yards without having to compensate excessively for bullet drop, even with the short barrels associated with the Mk107 and Mk112.

Ammunition tested was a combination of Black Hills Ammunition loads, Federal Premium Law Enforcement 55-grain Triple Shok and 62-grain Bonded SP loads and Winchester 55-grain FMJ. The two PWS uppers kept all loads tested under 2.25″ at 100 yards with the Federal Premium 55-grain Triple Shok and Black Hills 69-grain Match producing near-MOA groups out to 200 yards. Overall, the accuracy level was more than acceptable considering the barrel profiles. Accuracy test protocol consisted of five 3-shot groups with each ammunition type; group sizes were averaged. Velocity figures ranged from 2,800 to 2,900 fps over a RCBS chronograph with the Mk112 and 2,100 to 2,300 fps with Mk107.

Range testing commenced with a functionality test consisting of dumping multiple magazines at several targets in rapid fashion once sight zeros were verified. Any fighting rifle must be reliable to be worthy of further consideration. The importance of quality magazines can’t be overstated in terms of impacting a weapon’s reliability. The majority of malfunctions in a select fire, or even semi-auto rifle, can be traced to a faulty magazine. I primarily use Brownells’ AR magazines for this reason, though random magazines were mixed into testing at times. The PWS long-stroke, gas-piston operating rod system proved its worth in keeping the bolt carrier group cooler and cleaner compared to the direct-impingement AR rifles. While not trying to be unrealistic or overly harsh, both PWS uppers were exposed to multiple magazine dumps at various times of the T&E. The method was not mindless ammunition wasting, but rather wanting to confirm beyond a doubt that the PWS Mk1 action was reliable under the most extreme circumstances.

One of the lowers used with the PWS uppers contained a patented Wilson Combat Tactical Trigger Unit (TTU). It is simple to install by anyone with rudimentary AR maintenance skills and instantly enhances shooter-to-rifle interface. Wilson Combat designed the TTU not as a match trigger, which can be less than hardy under robust conditions, but rather as single stage 3.5- to 4-pound trigger with a short reset suitable for all conditions of use. The Wilson Combat TTU requires no field adjustment and doesn’t have any parts or screws that seem to come loose at the worst time. The TTU hammer geometry was designed with military/surplus primers in mind, allowing for reliable ignition of all ammunition. The Wilson Combat TTU meets or exceeds military specifications for drop safety — something “match” triggers usually can’t do. All in all, the Wilson Combat TTU allows for multiple rounds to be sent downrange quickly/accurately and for precise longer-range shot placement better than your standard trigger unit; yet it retains reliability lacking from some of the other trigger units on the market.

The Trijicon ACOG and Leupold Mk4 CQ/T sight offer the capability to
engage multiple targets in rapid sequence compared to open sights,
while at same time providing adequate accuracy out to a couple hundred yards.

Which Is Right For You?

By doing Team Tactic drills, I got a good sense of the way a weapon handles reliably when it expends that much ammo in a short time frame. PWS’s attention to detail by including the Bravo BCMGUNFIGHTER Charging Handle paid big dividends during the repeated magazine changes and induced malfunction clearance drills. I took advantage of the range time and put the Diablo through the same vigorous evaluation. Both rifles experienced no malfunctions during multiple range trips and over a thousand rounds fired — a true credit to the soundness of the PWS design.

Some may question the utility of employing a rifle with a barrel measuring 12.5″. Is the 3.5″ worth the non-NFA 16″ version? Along these same lines some may question the viability of the 7″ PWS Diablo giving up so much in terms of velocity performance. These are questions each individual will have to answer for himself. The fact is that most engagements happen well within the 100-yard range, especially in law enforcement or civilian settings, with handling and reliability playing more of a factor in quick, reactive engagements than muzzle velocity. Premium ammunition such as the Black Hills 50-grain TSX and Federal 55-grain Triple Shok and 62-grain Bonded loads elevate 5.56mm terminal ballistic performance and barrier penetration across the board, especially in short barreled rifles such as the PWS offerings. The BATF regulations of what constitutes a rifle compared to a NFA SBR weapon as pertaining to barrel length serves as an artificial threshold for barrel length on civilian rifles. As can be imagined, most consumers are content to go with the 16″ barrel to avoid onerous paperwork and tax stamp fees associated with a less than 16″ SBR rifle even if a shorter barrel is better suited for the task intended. The PWS Mk1 SBR uppers are worthy of consideration for those willing to go the extra mile in acquiring or not confined by having to comply with NFA mandates overseas.
By Todd Burgreen

For More Info:

Primary Weapons Systems
(208) 344-5217

Black Hills Ammunition
(605) 348-5150

Echo Valley Training Center
(540) 450-7998

Federal Premium
(800) 831-0850

(800) 338-3220

(800) 538-7653

(248) 960-7700

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