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PRIMARY WEAPONS SYSTEMS MK112 GAS PISTON 5.56 PERSONAL DEFENSE WEAPON BUILD

PRIMARY WEAPONS SYSTEMS MK112 GAS PISTON 5.56 PERSONAL DEFENSE WEAPON BUILD

Mission:

    Our goal for this build was to set up a PDW (personal defense weapon) package to be used for Personal Security Details. A PDW is a weapon similar in size to a sub-machine gun that fires rifle rounds for more knockdown power and a greater effective range. A weapon designed to be used for Personal Security Details has to be small enough to allow the user to get in and out of vehicles with the weapon at the ready, while maintaining an effective range capable of engaging enemy targets at rifle ranges. High magazine capacity, light weight, high rate of fire, and knock down power are the desired attributes. This build turned out to be an excellent example of a modern PDW.


Upper:
    They say only the weapons change and that the warrior does not.  Well one thing is for sure, weapons are continuously changing. Smaller, lighter, faster, more powerful, extended range, higher magazine capacity... These are all manifestations of the arms race toward more effective firearms. But there's another saying that goes, "if it ain't broke don't fix it"... That was our outlook on piston driven firearms readily accessible to civilians until we built this PDW in 2011. Many early piston system manufacturers had problems with carrier tilt, pre-mature cam pin wear, and improperly tuned gas systems. Primary Weapon Systems ( www.primaryweapons.com ) long stroke piston design, in our experience, does not suffer these ill-fated issues.
​    PWS originally designed their piston system for use in the Mk107 "Diablo" SBR package. Although the PWS Diablo was an incredible design and functioned very well it only has a seven inch barrel, and we prefer not to employ 5.56 firearms with a barrel length of less than twelve inches. The reason for this is the inherent light weight of the 5.56 projectile and subsequent loss of velocity out of a dramatically shortened barrel make the already small round a much less effective man stopper.
    The PWS long stroke piston design is a step above other short-stroke piston operated systems. Some people will tell you that a piston driven AR-15 is more reliable than a DI (direct impingement) gun, but this is simply not true. A clean, well built, and properly maintained DI AR-15 is as reliable as any piston AR on the market today. Direct impingement AR-15's were finely tuned, battle proven fighting machines before the world started trying to turn an AR into an AK. However, DI guns get dirty fairly quickly since the spent gas from the fired cartridges is vented directly onto the bolt carrier group via the auto-loading operation process. DI guns are also plagued with malfunctions when they are overly dirty (a maintenance issue) due in part to the tight tolerances of the AR platform design. These same tight tolerances also lend the AR its great accuracy. Piston driven AR-15's seek to build a bridge between accuracy and cleaner operating firearms for a best of both worlds, reliable and accurate, scenario.
    We have had our eye on PWS products since their debut of the Diablo, and when they introduced the Mk112 version with a 12" barrel we took this opportunity to set up a piston AR platform. The Primary Weapons Systems Mk112 complete upper itself fit the bill for what you would expect from a high end weapons company. Fit and finish was excellent. The long stroke piston system is easy to dissemble and clean. You don't have to remove any pieces of the quad rail as you do on some piston guns, and the field stripping process is very similar to a DI firearm. The long stroke piston system also eliminates the metal on metal contact of other piston systems (another way they reduce or eliminate carrier tilt and wobble). Mass has been added to the rear of the bolt carrier to increase dwell time, delay unlock, decrease chamber pressure, and further reduce felt recoil.  The bolt carrier group is precision machined from tool steel and Nickel Teflon coated. The coating greatly reduces friction and carbon build up while making cleaning easier similar to hard chrome.
     A fully adjustable gas system allows the PWS to fire a variety of ammo with or without a suppressor flawlessly. We had some minor malfunctions when testing this rifle due to poor quality ammo, but none related to the firearm itself. The gas system is easy to adjust and is found at the 12' O'clock position on the fore-end at a gap in the rail. The 12.75" barrel is button rifled, 1:8 twist, Isonite QPQ treated, and truly free floating within the rail system. What this means to you is that it's tough, very accurate, and propels rounds at an average of 2800 feet per second. 
​    The Mk 112 is chambered in .223 Wylde which gives this weapon the ability to function properly with .223 Remington or 5.56 ammo. The chamber in .223 Wylde is not as tight as .223 Remington which allows 5.56 brass to cycle without affecting accuracy. The rail is picatinny along the top with two inch sections at the front on either side. It is lined in the middle with keymod cutouts to allow a variety of attachments and has QD sling attachment points at the front and rear. The upper weighed in 3lbs, 9oz when we received it and came fitted with a nice upgraded charging handle from BCM.  

The Finished Build:
    I must say in the interest of full disclosure that I didn't use a PWS lower for this build. The only insight I have into PWS lower receivers comes via a combat disabled Green Beret friend of mine who had a terrible run with PWS. He dropped the rifle from his wheel chair to the ground and the lower receiver shattered. As this is the only registered part of the firearm it's basically a worst case scenario. PWS was also very difficult for him to deal with over this issue to the point that he will no longer do business with them, and I agreed to share his experience in my article.
    We mated the PWS Mk112 upper with a Colt HBAR match grade lower. Colt has been making high quality ARs for decade's and we knew we could trust this lower to be made to spec and without any flaws or issues that could negatively effect the build.
    We used an Aimpoint micro red dot optic ( www.aimpoint.com ) in a LaRue mount for light weight, durability, quick target acquisition, and long battery life. Aimpoint makes some of the best red dot optics on the market and just about everything that comes from LaRue in Texas is very well machined and high quality. Our shooters have trained themselves to be able to provide accurate fire from their red dot equipped carbines whether the optic is on or off. Muscle memory and thousands of rounds at the range allow us to guage where our shots will go simply by using the tube of the optic. This provides extremely useful in cases of battery failure. We added a TangoDown rubber cover for the Aimpoint Micro in flat dark earth that for a little extra durability, lens protection when not in use, and overall aesthetics of the build.
    We used Magpul BUIS (back-up sights) just in case the optic should be lost or destroyed and set them up to co-witness the red dot. We chose a Hogue overmolded grip ( www.hogueinc.com ) mainly for the color and texture. The grip was relatively inexpensive, fits the hand  very well, and maintains a good amount of "tack" even with gloves. For a weapon light we chose the Inforce WML ( www.inforce-mil.com ) in sand color, the inforce light has since taken on water during training and no longer works.
​    The PWS CQB muzzle brake looks cool and helps project gas and noise away from the user while minimizing muzzle flash. This PDW has been an SBR and a pistol in various stages of it's life and testing. This mostly depended on what lower it was attached to and whether it could legally be an SBR at the time. Aside from their revolutionary long stroke piston system, PWS has also designed their own "enhanced buffer tube". This buffer tube does not use a castle nut to secure, instead it uses indexing screws to create torsion that holds it in place. It is very easy to install and works quite well when properly adjusted. Another big feature of the buffer tube is the extended lip which 'holds" the bolt carrier and eliminates carrier tilt (carrier tilt can be a major issue with piston AR's). The enhanced buffer tube also features a QD (quick detach) sling attachment point on either side. As set up here the rifle balances well and we found it was a pleasure to run through drills. Function is smooth, reliable, and efficient. This is an excellent choice for a piston operated firearm.

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