Primary Weapons Systems Diablo – a Devil in Disguise!

A small package with effective results.

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Is bigger always better? When it comes to designing a small, lightweight carbine with the ability to quickly change barrel lengths or configurations, the AR15/M16 series rifle is an ideal base platform that allows modification according to current need. Primary Weapons Systems, based in Boise, Idaho is known for manufacturing effective piston conversions, compensators, and complete AR-15/M-16 series upper receivers. Primary Weapons Systems recently released the Diablo, a seven-inch piston upper for the AR15/M16 series rifle. Without getting into the debate of direct impingement versus gas impingement systems in the AR15/M16 platform, the bottom line is that the PWS Diablo works, and it works well.
The genesis of the Diablo was the desire to make the AR15/M16 series rifle as short as possible without affecting reliability, while at the same time maximizing accuracy and effectiveness. Rather than simply taking commercially available parts, bolting together a new product, and calling it revolutionary, PWS designed a system from the ground up to work specifically with the extreme pressures of the 7-inch barrels.


The commitment to quality and reliability is demonstrated through the use of components used to build the Diablo. At the center of the upper receiver is a Vltor Modular Upper Receiver. Dean Sylvester of PWS explained that any upper receiver could have been used, but Vltor offers a high quality product that could not be found in other commercially available products. In addition, the Vltor MUR receiver features a thicker wall to provide increased accuracy and reliability while also extending the life of the bolt carrier group and barrel assembly.

Accuracy from a small package was of foremost importance. Dean explained: “Accuracy and reliability was the paramount concern when I designed the system. I ultimately decided on a seven-inch stainless barrel, polygonal bore, with a 1:8 twist. The 1:8 twist was used to give the ability to shoot both light and heavy loads without sacrificing accuracy. In determining the barrel length, I wanted to use a pistol length piston system – using a seven inch barrel allowed 1/2" of barrel to extend beyond the quad rail hand guard. This half inch of exposed barrel length allowed the barrel to be threaded and CQB compensator mounted flush to the quad rail.”

Dean added that the polygonal bore was used to increase bullet velocity. “When we tested barrel systems, we learned that barrels with polygonal bores exhibited faster bullet velocities when compared to button rifled barrels of the same length. We also completed a great deal of testing with non-chrome lined barrels, chrome lined barrels, and stainless steel barrels. We also saw the greatest accuracy with stainless steel barrels. In addition, the stainless barrels exhibited an uncanny ability to eat any ammo we fed through it. It was a non-issue. Lothar Walther manufactures our barrels, and is able to deliver polygonal bore barrels from Germany to give us that perfect combination.”

A match grade bolt carrier assembly is mated to a PWS designed long stroke piston system. Designed by PWS, the long stroke system was chosen for increased reliability, additional dwell time within the recoil process, and smoother recoil. Low profile Mil-Std-1913 Picatinny rails provide ample real estate for any mounted optics or accessories. Magpul XTM rail covers are included for installation when the rails are not in use.

Rounding out the system is the CQB compensator. While discussing the system with Tuttle, it was disclosed that the CQB compensator was not an afterthought – it was designed as part of the Diablo system. The CQB compensator is unusual looking, and bears a resemblance to a Bulgarian AK74 barrel-type flash hider. When asked, Todd Tuttle of PWS explained that the effectiveness of the CQB compensator is due in part to the two piece system. “The original design of the CQB comp was classified as a sound suppressor by ATF. We changed the design of the CQB compensator to mimic the Bulgarian four piece flash suppressor to avoid classification as a sound suppressor.” The redesigned CQB compensator was resubmitted to ATF for classification, with the final determination that the CQB compensator is not a sound suppressor. The CQB compensator is fully ATF-compliant, and very effective.

Additional kit
Specifications are good, but how does the system run? In reviewing the Diablo, additional kit was added to the upper receiver. First and foremost, the upper receiver is delivered without optics. The upper was tested with two separate Leupold optic systems. The first test was conducted with the Leupold Mark 4 1-3x14mm CQ/T with an ever dependable LaRue Tactical mount. The second test utilized the Leupold Mark 4 1.5-5x20mm MR/T optic. Again, LaRue Tactical mounts were used to secure the optics to the upper receiver.

The Diablo is delivered as an upper receiver – it must be mated to an AR15 or M16 lower receiver. While conducting tests of the Diablo system, two lower receivers were utilized. A Colt Model 6933 M4LE semiautomatic lower receiver was used to test functionality when fired from an unmodified semiautomatic platform. A restricted Colt M4 lower receiver was also used to test functionality when fired fully automatic. Finally, out of necessity, a Knight’s Armament forward grip was used to hang on to the beast while being fired.

Off to the range… CQB applicability
Once at the range, the Diablo was remarkably tame when fired as a semi-automatic rifle. Designed for CQB engagements, the Diablo was tested under realistic conditions. Upon arriving at a private firing range, the Diablo was carried (unloaded) within the passenger compartment of the vehicle to determine whether the rifle could be maneuvered from vehicle to firing position. The Diablo’s compact design proved easily maneuverable in and out of the vehicle. Once at the firing position, three rounds were fired from a kneeling unsupported position with a Leupold Mark 4 1-3x14mm CQ/T scope at a 25-yard target. Given that most CQB engagements occur at ranges less than 25 yards, the accuracy was deemed more than adequate.

100% Full Auto Reliability
Satisfied with the function in semiautomatic mode, the Diablo was taken to the Spring 2009 Knob Creek shoot in West Point, Kentucky. The Diablo was fitted to a restricted Colt M4 lower receiver and tested for full-auto function. Several types of 30-round magazines were used during the full auto testing – a Magpul P-Mag, a Mil-Spec Okay Industries magazine, and a Fusil USA magazine. In each case, the Diablo gobbled up the 30-round magazines flawlessly. Not to be deterred, a 100-round Beta drum magazine was tested – again, all 100 rounds were fired without any malfunction. Clearly, the Diablo was hungry for more, and the 100-round Beta mag was reloaded. In what can only be described as abuse, 190 rounds (representing a loaded Beta mag, and three 30-round magazines) were fired in four long continuous bursts The Diablo ate every round and continued to ask for more until all 190 rounds of ammunition were gone - without jamming, short stroking, or other failure to fire. Not to be outdone, the Diablo was fired in this manner multiple times over the course of the weekend and consumed more than 2,000 rounds in full automatic mode without a single jam or other failure to fire.

To those that only fire semi-automatic rifles, this may seem second nature - after all, why wouldn’t an upper receiver function reliably? To those familiar with short barreled gas impingement AR15 and M16 systems, the Diablo results are nothing short of amazing. The M16 was designed as a system, with a specific barrel length, bolt weight, and recoil spring tension to make the system run reliably. When any single variable within the system is changed, the system frequently ceases to operate reliably. Shortening the barrel on a gas impingement system is often a recipe for disaster – at a minimum, the gas port diameter within the gas block often needs to be modified. Cutting the barrel on a gas impingement M16 to seven-inches is asking for trouble – the gun is very unlikely to run reliably without major experimentation and modification. Unlike its gas impingement cousins, the Diablo functioned flawlessly, with no issue of reliability. The only complaint that arose was related to heat when an excessive amount of rounds were fired through the system in a very short period of time. After firing more than 100 rounds in full-automatic mode, the forward section of the upper receiver became unbearably hot, to the point that it was impossible to hold the gun. The solution was to add a Knight’s Armament forward grip to the lower Picatinny rail. Once the forward grip was in place, shooting continued without further issue.

One of the main highlights of the Knob Creek shoot is the night shoot, where machine guns of all types are fired in near complete darkness. These conditions provided an excellent opportunity to test the effectiveness of the CQB compensator. The CQB compensator eliminated all visible flash during daylight hours, but how would the compensator function at night? To great surprise, the CQB compensator was very effective, often emitting sparks rather than a fireball at the muzzle. On several occasions a muzzle flash was observed, however the size and density of the fireball was much smaller than expected, given the short barrel length of the system.

Finally, a chronograph was used to measure bullet velocity. Three types of ammunition were tested – 55 grain, Winchester Q3131 (the commercial equivalent of M193 ball ammunition), 62 grain, Mil-spec M855 ammunition, and 55 grain, frangible ammunition from Engel Ballistic Research. Ten rounds of each type of ammunition were fired, with the chronograph situated 15 feet from the muzzle. While the complete results may be compared in the table below, velocities were not surprising, given the short barrel. Average velocities ranged from 2104 fps with the EBR frangible, to 2430 fps with the M855 ball. The difference in velocity can be attributed to the M855 being loaded to NATO specifications, while the Q3131 and EBR ammunition was loaded to slightly lower SAAMI specifications. With the right choice of ammunition, the Diablo could prove to be a highly effective weapon system.

Long Range Accuracy?
Long range testing was not considered due to the short barrel length and intended use as a CQB weapon. While discussing the system, Todd reported that he personally used a Diablo during the 2009 3-day long MTM Ironman 3-gun match hosted in Parma, Idaho. “Although the weapon has a seven-inch barrel, the design provides 1 MOA accuracy. During Ironman, I was able to consistently hit steel targets at 440 meters with the Diablo. I was also successful when shooting at 650 meter targets, but at that distance, the wind plays a much larger role in accuracy.” While the veracity of the claims was not tested, video footage of 330 meter hits with a Diablo were provided to support the claim.

Armed with the report that the Diablo was capable of 1 MOA accuracy, a trip to the range was arranged to conduct accuracy testing of the Diablo system. Firing mil-spec Lake City M855 ammunition, the Diablo lived up to its claim, and fired a three round group slightly smaller than 1-inch at 100 meters.
For those that need a dependable and compact rifle, the Diablo may be the system for you. At 23 ½ inches long, the system is almost 4 inches shorter than a Heckler and Koch MP5, yet provides a rifle caliber response. For law enforcement with a need for a small rifle caliber package for CQB operations, the Diablo is a serious contender. The Diablo is available direct from Primary Weapon Systems, at a suggested retail price of $1650. Additional information may be found online at

Engel Ballistic Research
 55gr frangible
M855 62 gr. Ball
Q3131 55gr Ball
2104 fps
2436 fps
2207 fps
2139 fps
2465 fps
2306 fps
2078 fps
2369 fps
2146 fps
Standard Deviation
19 fps
27 fps
42 fps

Primary Weapon Systems Upper Receiver Specifications:

Caliber: 5.56x45
Barrel: Seven inches
OA Length: 23.5”
Sights: None supplied. As tested, Leupold Mark 4 1-5x14mm CQ/T and Leupold Mark 4 1.5-5x20mm MR/T optics were tested with LaRue Tactical optic mounts.
Stock: None provided with the system. As tested, an M4 collapsible stock
Action: Long Stroke piston impingement
Finish: Mil-spec hard coat anodized
Capacity: Accepts all standard AR15/M16 series magazines
Price: $1650

Performance: As tested, a Colt Model 6933 M4LE semiautomatic lower receiver and a restricted Colt M4 full automatic capable lower receiver were used.

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