Piston Driven Precision


For whatever reason mankind loves to debate amongst one another. It doesn’t matter what they’re about, opinions will differ. Who was hotter? Ginger or Mary Ann? Best truck? Chevy or Ford? Blondes, Brunettes or Redheads? Brunettes for sure… I digress. The same is true for us 2A loving patriots and gun geeks. We argue about all kinds of stuff. 9mm or .45ACP? AR or AK? If AR, piston driven or direct impingement? We will spend hours getting all worked up and wasting time online trying to prove a point on social media, but to what end? Years ago, I asked myself the AR DI vs Piston question, "Which was better?" Hell, I didn’t have any piston guns at the time so I started buying them. Over the years, I have purchased piston driven AR type rifles by LWRC, LMT, Adams Arms, a Robinson Armament XCR, and an ACR or two… I even bought a PWS DC-16. I have lots of trigger time on both DI and piston AR blasters. So which one is better? They both have pros and cons and I like them both for different reasons. There’s no end all solution that works for everyone. That being said, I do want to focus on one manufacturer who makes both piston and direct impingement AR rifles.

The last piston gun manufacturer I mentioned above was Primary Weapon System or PWS for short. They operate out of Boise, Idaho and have been making piston guns for quite some time now. They manufacture accessories like muzzle devices, extension tubes, rails, and other related pieces. They have also entered the direct impingement market and sell that series of rifles under their Modern Musket brand. PWS also recently started making an aftermarket slide for the Glock series of pistols.

My first experience with PWS was many years ago. The actual number of years escapes me, however I was still on my piston gun kick and read a lot of good things about their DC-16 upper receiver. The DC-16 was the predecessor to their MK1 series of rifles and it utilized the long stroke piston system PWS has become famous for. I bought the upper from one of their online dealers and promptly broke it down. Compared to all the other piston guns I had previously dealt with, this was the oddest looking bolt carrier group I had seen. The BCG had proprietary components. It used an operating rod keyed to where the gas key would normally be on a DI gun. The tip of the op rod had a piston attached to it. The piston was mated to the op rod with a pin and cup providing a flexible joint. The charging handle was fixed around the op rod yet allowed the rod to move freely during the firing cycle. The only way to swap out the charging handle for an aftermarket selection was to separate the op rod and piston by driving out a pin which was cumbersome to say the least. Regardless of how it looked, the BCG was getting really favorable reviews and I was ready to give it a go.

My first outing at the range was with a few buddies who wanted to shoot the PWS. After about 200 rounds in, the DC-16 had a stoppage. Perplexed, my buddy tried to charge the gun and clear it. After the round was removed, he tried to load another round and we heard what sounded like broken glass getting chewed up inside the upper. Not good. Not good at all. Clearing the rifle, I broke it down to discover the op rod and the piston connection had sheared and the glass noise we were hearing were all the parts cascading out of the gun. So much for that. When I got home, I immediately contacted PWS. They were just as perplexed as I was. Supposedly this was a first for them and they told me to send it back so they could diagnose the problem. Before I even had the upper boxed up, I got a tracking number saying my new upper was on the way. Talk about customer service! The broken upper and the new upper passed each other like two UPS trucks in the night. Upon receiving the new upper, I didn’t experience any of the issues I had previously, but eventually I sold the upper. To the best of my knowledge, the DC-16 is no longer available and is not listed on the PWS website.

Fast forward to present day. During SHOT Show 2015, I hooked up with the fine folks at PWS to see what they had been up to. They now offer a whole new line of piston driven rifles to include .308 caliber variants known as the MK2 series. I really wanted to get my hands on one so after the SHOW they sent a MK216 for me to play around with.
The MK2 series can be had in barrel lengths of 12", 14", 16" or 20". All NFA rules apply for sub 16” barrel lengths. The MK216 is their 16 inch barreled version of their .308, long stroke piston rifles. At first glance, the PWS offering looks very similar to other AR10 type rifles I have used, however I would soon realize it was indeed unique.

I am not a machinist with an expert eye, however I was very impressed with the fit and finish right out of the box. I later had a friend of mine (who is a machinist) rave about how good the craftsmanship is on the PWS rifles. The MK216 came with a hard case, some “propaganda”, a sample of Gun-Ease and a Magpul 20 round, SR25 style magazine.


The MK216 weighs in at 8 lbs. 10 oz. and has an overall length of 35.6". The upper and lower are made from 7075-T6 aluminum forgings and the KeyMod, free float handguard is made from 6061 aluminum extrusions. The handguard features a continuous 1913 Picatinny top rail with a cut out above the gas block for adjusting the five gas settings.

The gas settings can be adjusted with a gas knob adjustment tool which is stored in the grip. The tool allows the user to optimize the settings depending on ammo used or if a sound suppressor is attached. The fifth setting shuts off the gas. The tool also doubles as a front sight adjustment tool. The gas can also be adjusted with a bullet tip or any other pointed object. I used an allen wrench with success. The handguard also has KeyMod interface points along the 3, 6 and 9 o’clock positions for custom mounting options. At the end of the handguard were two short rail section that could be configured for whatever light or accessory desired at just about any position on the rail. The hand guard felt very nice in the hand and the profile allowed for a nice "thumbs forward" placement above the 3 and 9 o’clock positions.

The MK216 sports a hefty chrome moly barrel that is Isonite QPQ treated for hardness. The treatment resists corrosion well. It is chambered in .308 Match with a 1:10 twist. The business end is tipped with the PWS FSC30 muzzle device which is very effective in stabilizing the muzzle when sending .30 caliber projectiles down range.

While the barrel is visible through the handguard, I wanted to see it up close so I removed the handguard by extracting the six hex screws at the base of the handguard. The handguard was mated to the upper pretty tightly but with a little elbow grease and a rubber mallet, I was able to slide it off. The barrel and gas block were now completely exposed revealing that the gas block was keyed to the barrel.

This key was a nipple at the rear of the gas block that mated with a notch in the gas block seat of the profiled barrel. This prevented the block from rotating. Instead of pins or set screws to keep the block on the barrel, there was a nut that was tightened down at the front of the block holding it in place against the seat in the barrel. Quite the simple solution, really. The gas block was mated to a tube that protected the operating rod and piston all the way back to the receiver. I also noticed the gas relief port was in front of the block, venting gasses out of the block and away from the shooters face.

Separating the upper receiver from the lower receiver is just like you would expect if you are familiar with the AR platform. Push out the pivot pin and the take down pin and you are in business.
The lower is very nice with softer lines than some of the .308 ARs on the market today. The magazine well was elegantly flared and the oversized trigger guard was round. The PWS logo and serial number are deeply embossed on the left side of the magazine well.

There are fire control markings on both the left and right side of the lower, however the safety lever is not ambidextrous. It is simply a Mil-Spec safety as is the bolt catch and magazine release. The fire control group is the ALG Defense QMS which is basically a tuned Mil-Spec trigger. The lower also features the PWS enhanced buffer tube which is really a work of art.

I have one on an AR15 and have been very happy with it. It actually eliminates the castle nut, features two QD attachment points and has drainage holes throughout. The tube is fluted to allow water and debris to fall free and not bind up the stock. The Magpul MOE stock that comes with the rifle slides smoothly along the tube with only a hint of wobble. Within the buffer tube is the PWS H2 buffer which looks bad ass. It has the PWS logo proudly displayed on the front in all its glory.

Moving to the upper, there was something that just didn’t look right. After staring at it for a while I realized it was missing the brass deflector, sort of. The forward assist had been moved forward and was in the position of the brass deflector. Apparently it is designed to pull double duty. Interesting.

Removing the bolt carrier group was also a little different than the standard AR10 I was used to. As I had previous experience with the DC-16 I thought I knew what to expect, however I was pleasantly surprised. The bolt, bolt carrier group with op rod and piston attached all slides out with the BCM 7.62 charging handle.

The bolt carrier group is beefy and both the carrier, bolt and op rod are Nickel-Teflon coated. The OP rod is staked in place of where the traditional gas key would be. The forward assist notches in the right side of the bolt carrier are few as the forward assist is forward of the traditional location. The bolt has 7 massive lugs and a large extractor and ejector.

I was pleased to discover, that unlike the DC-16, the Mk216 op rod and piston were easily separated. This was achieved via a slotted key. As the entire assembly rides in a tube above the barrel, there isn’t room for it to fall apart during the firing cycle. It also allowed the charging handle to be easily separated from the BCG. This is a vast improvement over the DC-16! It does require a little finagling to get everything put together in all the right slots, but with a little patience it is doable. As a final touch, the bolt carrier group has the PWS logo etched on the side and is visible with the dust cover down.


Range time is always a blast for me. That being said, this was not going to be a torture or endurance test. I was limited to what ammo I had on hand which was 100 or so rounds. Not a ton of trigger time, I know, but .308 ain’t cheap and I ain’t loaded. My goal was to test for function and accuracy. The first thing I did to the rifle was remove the MBUS BUIS. I have never liked them and I wouldn’t be using them for the test so I deep sixed them. In its place I installed a Leupold ER/T 8.5-25×50 scope with M5 dials with the front focal H-58 Horus Vision reticle. The optic was mounted in a Warne RAMP mount. This mount features 30mm rings and left and right offset mounts for micro red dots. On the right side I mounted an Aimpoint T2 for close up work. I wrote about the T2 in an earlier Guns & Tactics post. I also mounted a Harris bipod to the front of the hand guard. This set up took the rifle from its stock 8 lbs. 10 oz. weight and bumped it up to 11 lbs. 8 oz.

During my range time I cycled rounds through the provided Magpul magazine as well as two Lancer System L7 Advanced Warfighter magazines and a 50 round XProducts X25 drum magazine. All magazines fed the rounds just fine, however I did have some issues inserting the XProducts drum into the mag well. For whatever reason I was unable to just insert the drum as normal. It would hit the magazine latch inside the well and stop. I had to depress the magazine release to get the magazine past the catch. After that it seated fine and functioned normally.

To zero the scope, I used American Eagle 150 grain full metal jacket boat-tail ammo. Cold bore shots were at about .75 inches to an inch at 100 yards. As the barrel heated up, the groups spread to a solid inch.

The ALG trigger is anything but a precision trigger. I feel if I had a crisper trigger, match ammo and I was a better shooter (honesty hurts sometimes) I would have reduced the group size. I did notice after the second magazine the groups really spread out to 2 and 3 inches. I was pretty perplexed until I realized the mount had worked itself loose. Great. At least I wasn’t going crazy. The MK216 is a solid 1MOA rifle. The Leupold glass was outstanding. Even with the magnification only cranked up half way it felt like I was standing directly in front of the target. The Horus reticle is killer. This was my first time using this retical and I had to do a little research on their website to learn how to best use it. It made zeroing the target stupid simple. The reticle was clear and easy to read. The knobs were solid and turned smoothly with a tactile click at each tenth of a mil.

Transitioning from longer shots to closer engagements was simple with the offset Aimpoint T2. Simply flip the wrist 45 degrees, place the dot on the target and light it up. While PWS says the FSC 30 muzzle device has flash suppression features, I was still able to catch muzzle flash during the day.

The FSC 30 did a good job taming the .308 beast and follow up shots were relatively quick. The weight of the rifle also added in dampening some of the felt recoil as well.

After I was done shooting I broke down the rifle to inspect it. The bolt carrier group was still shiny and the little bit of carbon that was on it wiped off easily thanks to the Nickel-Teflon coating. The end of the piston showed some signs of carbon build up but at least it wasn’t all over the bolt. The forward assist took all the brunt of the brass and showed the scars but wore them well.


Of course nothing is perfect and opinions on how something should work are just that; opinions. So here are mine for those that care. This is a top quality piece of firearms engineering, but it is fitted with economical furniture, sights and trigger. The MOE series of grips and stocks are a great bargain but I would expect more on a rifle of this caliber. Maybe upgrade the grip to a MIAD and the stock to a STR or UBR. If you want really light weight go with the Mission First Tactical Minimalist stock. Like I said before, I am not an MBUS fan. Either don’t include them or throw on a pair of Troy BUIS or Magpul MBUS PRO offset sights or similar. Just my 2 cents. The only other thing I had concern about was the ease in which the op rod and the piston separated. In one regard I loved it as it freed the charging handle and was much more robust than the DC-16 rendition. On the flip side if it is lost out in BFE during a field strip, the rifle would be rendered useless. Maybe some sort of ring or clip to lock it together better. I am sure the folks a PWS could figure it out.

Overall my experience with this rifle has been great. I wish I had the time and the ammo to put more rounds through it. I would like to also shoot it suppressed to test out the gas settings but that will have to wait. I have no doubt that it would hold up to years of hard use and reliable service. The rifle retails for $2,600.00 which isn’t cheap, but isn’t out of this world for a semi auto .308. Many rifles in the same class run closer to $3K. The quality is outstanding and the engineering appears to be cutting edge. Bravo Zulu PWS.

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