[Review] PWS MK116 Mod 2-M Upper

Ready to move up to a piston AR-15?

I review one of the most popular models out there…Primary Weapon System‘s MK116 Mod-2M which might be everything you ever wanted.

PWS MK116 Mod 2-M
PWS MK116 Mod 2-M Upper

By the end you’ll know a little more about piston ARs…and how the MK116 Mod 2-M shoots, its accuracy, and whether it’s worth the upgrade price.

1742 at Rainier Arms

Prices accurate at time of writing

Table of Contents


Who Is It For?

PWS Piston Cutaway, Piston Highlight
PWS Piston Cutaway, Piston Highlight
  • Someone who wants to have a cleaner & cooler shooting AR system (especially when suppressed)
  • Someone who wants a long stroke piston vs a short stroke piston upper

Long Stroke Piston FAQ

You probably know the basics of piston vs direct impingement (regular ARs).

Direct Inpingement Gif
Direct Inpingement Gif

In direct impingement (DI) systems, the gas is routed via the gas block to the bolt carrier which then moves back with the force.  However…this gets all the exhaust gas and powder into your gun.

With a piston system…the gas moves a piston which moves the bolt carrier.  Therefore only a small area of the piston itself gets dirty.

PWS Long Piston System
PWS Long Piston System

Now…there’s different flavors of piston.

First up is the short stroke piston which is where the piston is separated from the bolt carrier.  It’s lighter…but with more parts and could be more susceptible to “carrier tilt.”

Adams Arms P-Series Piston Kit
Adams Arms P-Series Piston Kit

However it’s generally the more popular option since you can retrofit a normal AR into a short stroke piston.

With a long stroke piston…the piston and bolt carrier are one.

PWS MK116 Long Stroke Piston
PWS MK116 Long Stroke Piston

It’s heavier in weight and recoil…but there’s less moving parts.  Other famous guns that use a long stroke include the AK-47 and M1 Garand.

If you’re going for best possible reliability I’m going to hand it to the long stroke.  But if lightness and slightly less recoil is the name of the game…go with a short stroke.  Read more about this at The Truth About Guns.

Fit & Feel

PWS was kind enough to send me their MK116 Mod 2-M upper for testing.

PWS MK116 Mod 2-M Upper, Other Side
PWS MK116 Mod 2-M Upper, Other Side

It’s their best selling option and meets all my normal criteria for an upper:

  • 16″ barrel for each of adding muzzle devices
  • Mid-length for softer shooting
  • 12″+ M-LOK handguard

Finish was immaculate and what I expect if I’m shelling out over $1000 for an upper by itself.

Plus I love the extra design details on the upper receiver.

PWS MK116 Upper Receiver
PWS MK116 Upper Receiver

There’s no forward assist so if you’re in that camp…too bad.

I quickly added on some fun stuff…

PWS MK116 Mod 2-M with Goodies
PWS MK116 Mod 2-M with Goodies

One cool thing about the newest versions of the MK116 is that they come with PicLok free-float handguards which…you guessed it…combine both Picatinny and M-LOK.

The top rail is regular Picatinny…

PWS MK116 Front Rail
PWS MK116 Front Rail

While the other three sides are PicLok.

PWS PicLock Handguard Attachment
PWS PicLock Handguard Attachment

I also went with their 5.56 FSC Compensator…whose design has worked wonders on my AK.

PWS MK116 Side Rail and FSC556 Compensator
PWS MK116 Side Rail and FSC556 Compensator

And you’ve already seen the long stroke piston design.   But here’s the BCG again…you can see it’s connected to the piston.

PWS MK116 BCG and Piston
PWS MK116 BCG and Piston

Note that because it’s connected…it’s slightly more of a pain to remove the BCG and charging handle (which is a Radian ambi by the way).

Not that you’ll be doing that very often since I couldn’t find much trace of any blowback on the BCG when I took the rifle apart for photos.  Just oil ended up on my fingers…and this was after ~400 rounds.

There’s also an adjustable gas system that lets you choose between off, regular, and suppressed use.

PWS MK116 Adjustable Gas System
PWS MK116 Adjustable Gas System

I really wish I could get a suppressor to test it out since that’s where it would really shine.  With suppressors there’s even more blowback that gets into your breech (and sometimes your face).

A piston does wonders to clean it up.

Now…how does it shoot?


A piston gun is going to weigh a little more because you’re adding stuff to the front of the gun.

If you’re getting a piston gun…it’s likely you’re already going to have some stuff in the front.  So get ready for a little heft.

SureFire M600 Scout
SureFire M600 Scout with Steiner DBAL-A3

But remember it’s a trade-off for reliability, cleanliness, and coolness (the temperature kind).

Here’s me shooting it with the muzzle brake for our Night Vision review.

And because there’s a pretty large piston moving back and forth…there’s going to be increased recoil.

I’d chalk it up to be about 50% more than a normal AR-15 with similar specs.

But once you get used to it…still easy to nail plates at 100 yards standing.


After ~200 rounds of break-in I loaded up my standard AR testing platform.

PWS MK116 Mod 2-M Testing
PWS MK116 Mod 2-M Testing

Put the target out to 100 yards.  And shot at a leisurely pace of a round every few seconds.  Let the barrel cool to warm between each string.

PWS MK116 Mod 2-M Groups
PWS MK116 Mod 2-M Groups

Since there’s more moving parts on a piston upper…you’re likely never going to attain the accuracy from a regular DI system.

The results were bittersweet…

On one hand they weren’t the greatest even for the match Gold Medal rounds.  But the good thing was that my cheapest plinking ammo Wolf Gold fared the best at a little over 1 MOA.


  • Caliber: .223 Wylde
  • Weight: 4 lb. 8 oz.
  • Length: 25″
  • Piston System: Mid-length
  • Velocity: 2980 ft/sec
  • PWS long stroke piston operating system
  • 3-setting adjustable gas system
  • Forged upper receiver
  • Eliminated forward assist
  • FSC556 compensator
  • PicLok free float handguard
  • 1:8 barrel twist rate
  • Radian Raptor Charging Handle

By The Numbers

Reliability: 5/5

Nothing close to a failure in 400 rounds.  Plus almost no residue on the BCG.

Accuracy: 4/5

Not sub-MOA but if you’re going piston you can’t expect that for all types of ammo.  Good thing for me is that it shot the best with the cheapest ammo!

Ergonomics: 5/5

Really liked the PicLok rail where you can attach almost anything out there.

Looks: 5/5

Will turn heads with the styled upper receiver and different handguard.

Customization: 5/5

It’s an AR-15 with PicLok so you can do almost anything you want.

Bang for the Buck: 3.5/5

On the pricey side for an upper at around $1200.  But as far as I know it’s the only long stroke piston AR out there.

Overall Rating: 4/5


It was great fun to shoot the PWS MK116 Mod 2-M upper.

PWS MK116 Mod 2-M
PWS MK116 Mod 2-M Upper

And I’d wholly recommend it if you were going suppressed or want a shorter pistol/SBR length (see all lengths)…since shorter DI systems are finicky.  Or if you just want an upper-tier piston gun.

The entire rifle is also up for grabs with a matching lower that makes the whole system look even cooler.

1742 at Rainier Arms

Prices accurate at time of writing

What do you think of the PWS MK116 Mod 2-M?  Are you a fan of long stroke or short stroke piston?  Check out our other favorites in Best AR-15 Piston Uppers and Conversion Kits.

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