When it comes to AR upper receivers, there are a ton to choose from. You can get dirt cheap, or super hi-end. There’s also different ways for the firearm to cycle those upper receivers. One of those is the long stroke gas piston. Which leads us to Primary Weapon Systems (PWS for short). Primary Weapon System has been making long stroke gas piston AR uppers for some time, as well as complete rifles. PWS is well known for their long stroke gas piston AR’s and upper receivers.

Primary Weapon Systems sent me one of their MK116 Pro Upper receivers. Which version they sent isn’t as important as how the upper operates. As all of the PWS uppers use the long stroke gas piston for operation. The long stroke gas piston operates differently than your standard AR’s direct impingement system. Instead of using the gas to cycle the Bolt Carrier rearward, the PWS long stroke gas piston uses a piston that is, essentially, directly hooked to the BCG through the piston rod. When gas comes into the gas block, it directly pushes on the piston to cycle the action. This gives a few advantages over the normal direct impingement system. For one, the gun runs cleaner because all the gas isn’t being directed directly into the action. It also allows the action to run cooler, since the hot gas isn’t being directly blown into the action. A big plus if you are running an upper on a full auto gun. Lastly, the action can tend to run smoother because all the operating parts are fixed together. There are also a few downsides to a long stroke piston upper. They can weigh a little more. The PWS upper is only a few ounces heavier than my comparable DI upper, so it’s not a lot of extra weight. The only other notable thing is that it comes apart differently than a DI upper. Not a big deal at all, just different.

The PWS MK116 Pro upper has a lot of great features. First off, I’ll mention that I received the 16” barreled version. This is important to note because it uses a mid length gas system for the piston. It also has a 15” M-Lok handguard. The handguard is a little different then what most people are used to. This is because it is missing a small chunk of the top picatinny rail, or basically one lug of the rail. This lug is missing so that you can easily adjust the 3 position gas block. The MK116 also runs a slick side upper, so no forward assist. There is a small shell deflector on it to keep the ejected brass from hitting you, but it will hit your camera man in the face. This particular upper came with an A2 birdcage. If you want flash suppression, the A2 birdcage works very well. Lastly, the PWS MK116 comes with a Radian Raptor charging handle. I really like the Raptor charginging handles, PWS made a good choice in using them.

Staking on Carrier bolts is different due to long stroke piston setup

PWS gave the long stroke gas piston uppers, and firearms, a 3 position gas block. As mentioned earlier, this is easily adjusted through the handguard. One can even use the tip of a bullet to adjust it. The 3 positions are labeled 1,2,3, and “X”. All 3 positions have a detent and click to them. The X position is for disassembly and cleaning, it’s not a click adjustment. Position 1 is for unsuppressed use, or normal everyday use for those without a suppressor. Position 2 is for suppressed light loads or low back pressure (flow through) suppressors. Position 3 is for suppressed heavier loads or higher back pressure suppressors. I did try out all 3 positions. Position 1 works exactly as advertised. Position 2 would eject rounds without a suppressor, most of the time, Position 3 would not cycle without a suppressor. With my 556 suppressor, position 3 was spot on. Positions 1 and 2 had a bit of force behind the ejection pattern. I really like how PWS gives you a simple, yet effective, way to adjust the upper for suppressed and unsuppressed use. Through all my testing on the PWS upper, I had zero malfunctions, not counting the ones I caused by using the wrong gas position (I had to test them). I also used a variety of ammunition types and brands for testing, both suppressed and unsuppressed.

Being that the Primary Weapon Systems MK116 was just a complete upper receiver, I needed something worthy of mounting it on. I ended up using an Aero Precision lower. This lower has a few options that make it a great companion to the PWS upper. The lower has a Blackout Defense Zero trigger installed in it, a VZ stipple grip, and an ambi safety selector. The trigger is what really makes such a great combo with the PWS long stroke piston. You can shoot this rifle super fast if you want to. Also, I needed to have some way to aim the rifle. I ended up using a 1x prism sight. Well, I did have a 1-6x on it at first, but I took it off in favor of the prism. The last thing I did to enhance the PWS upper was to install some NILE grip panels and a sling. The NILE panels use a M-Lok attachment, so they mounted up super easy to the PWS M-Lok rail. I had to use a M-Lok QD attachment for the sling. This particular PWS upper doesn’t have QD mounting points built into the handguard, that’s not a big deal for me, but it would be nice. I was really happy with how the PWS upper worked in conjunction with my lower receiver. The fit to the lower receiver was a bit tighter than what I would call “normal”, but not tight enough that it was hard to install on the lower. The PWS upper, when mated to my lower, made a really nice rifle.

Anyone looking for a great way to upgrade an AR rifle, needs to look into the Primary Weapon Systems long stroke gas piston uppers. The PWS system works very well. The action is super smooth. The piston system is robust and will keep your rifle cleaner than the original gas system does. If you are using a suppressor, it also keeps the blowback gasses out of your face. So if you are searching for a top tier upper receiver, do yourself a favor and go check out what Primary Weapon Systems has to offer.

Firearms Insider Reviews - Key Points

Claim to Fame:

Long Stroke Gas Piston Upper

Target Market:

Those wanting a super reliable gas piston rifle

Features & Benefits of this product:

Operating System: PWS Long Stroke Piston System, 3 setting Adjustable Gas Regulator
Caliber: .223 Wylde
Barrel Length: 16.1”
Gas System Length: Mid-Length
Barrel Twist Rate: 1:8
Muzzle Device: A2 Flash Hider
Upper Receiver: PWS Proprietary Piston Upper, 6061 extrusion, Anodized
Handguard: PWS M-Lok™ 15” Handguard (1.7” width, 2.25” height)
Charging Handle: Radian Raptor™ Ambidextrous Charging Handle