Previous Post
Next Post


I’m a recent SBR convert. For the longest time, I wasn’t on board with the whole idea. Sure, I thought the NFA was stupid, but I really thought I’d get a can before I got a lower receiver serialized. But now? I think a honest to goodness SBR would be a really nice thing to have in the safe. And if my perfect AR platform SBR exists, it’ll be chambered in .300 BLK. And piston driven. Enter the MK109 from Primary Weapons Systems . . .


Nick has this really great .300 BLK page that he’s been updating over the years, and before you read much further, I’d ask that you head over there and, at a minimum, skim the work he’s put together. Once back from that, we can talk about how the MK 109 deviates slightly from the original design specs. Specifically around the gas system.


The original .300 BLK uppers that Kevin Brittingham and crew at AAC built were 9″ barreled uppers driven by a direct impingement gas system. They were designed to run all four shooting modes (subsonic, supersonic, subsonic suppressed, supersonic suppressed) without use of a switchable gas block or other gas volume/flow modifying device. Looking back on the design goals for the .300 BLK project, it seemed like a lofty goal, but having shot Nick’s 9″ gas impinged SBR on many occasions using all of the modes described above, I can attest to the fact that the designers succeeded.

The problem, as it always ends up being, is that the process of firing a projectile results in a tremendous amount of crap getting blown back towards the internals running the gun. This residual crap made up of lead, copper jacket, and powder serves to gum up the works and eventually will lead to a failure. But the debate forever rages on about when that failure will happen. Some say a thousand rounds, others say that if you just keep lubing, a DI gas gun will run forever, and of course, tons of people pepper the flyover country between those two opinions. For example, the guys at E.A.G Tactical have been running a BCM without much maintenance for 40,000 (!!!) rounds with very little maintenance and report very few failures. But for some, there will never be anything as good as a piston driven gun.


But there again, we’ve got some problems. Because not all pistons are driven the same way. Your short stroke guys believe that a short, hard tap on the bolt is all that’s needed to get things moving. The long stroke guys — Mikhail Kalashnikov, his holiness John C. Garand, and the fine folks at PWS — think differently. Given that the the first two guys designed two of the most iconic (and reliable) guns of the last century, I’m willing to lump PWS in with the likes of John and Mikhail.

PWS is kind of known for their long stroke piston guns. I tested the MK107 and found it to be utterly awesome, especially with the four position gas block for the four shooting positions I mentioned above. The MK109 lacks the regulator function as Kevin and AAC originally intended, but instead of using a DI system that gums up the works, they use a long stroke piston.

There’s also claims that a long stroke piston system tames the recoil impulse, a claim I’m not set up to dispute or prove. The MK109 is a joy to shoot for sure, but the .300 BLK cartridge has never been known as a shoulder breaker. So down to the nuts and bolts of any good gun review. Ergonomics, reliability, and accuracy. 


You can purchase the MK109 package two ways. First, as a standalone upper. PWS put a MSRP of $1449.95 on it, but they can be found for a hair under $1300 most places. The second option is to purchase the entire gun (as a pistol called the MK109P) with the SIG Brace. MSRP there is $1949.95 but it can be spotted in the wild for $1500-$1600. Since I used the same lower from the MK107 review, for the sake of brevity, I’ll direct you that way for the particulars on triggers, SIG Braces, etc. Suffice it to say, the trigger is good, the Magpul grip is great as always, and the SIG Brace, while not as ergonomically pleasing as a true collapsible carbine-length stock does fine in a pinch.

As to the MK109 itself, it features PWS’s Keymod rail with Keymod openings at the 3:00, 6:00, and 9:00 positions. The top rail is all Picatinny. I mounted several Keymod sections all over, and had no issues making it work with PWS branded Keymod sections or a variety of other aftermarket types. I didn’t find the hand guard to be overly chunky in my hands, though there are handguard models out there that very well might be slimmer.

My MK109 test model came with PWS’ Triad 30 which allows the connection of any suppressors that use an A2 style attachment system. It is not pinned/welded either, so feel free to swap it out for something more in line with your suppressor needs. If you plan on running the gun without a can, I can confidently say that the Triad 30 does an admiral job of reducing muzzle rise and flash. It’s a gun so it’s still loud.


On to reliability. I don’t wish to besmirch the fine name of PWS when it comes to reliability of the MK109, but I’m giving it a solid “meh” when it comes to gun food. It likes some ammo and doesn’t like others. For example, no matter what buffer system I ran, including the stock pistol one, it refused to eat PNW 147 gr. With a rifle-length buffer (and yes, I understand that’s not the design parameter but our job is to do dumb things that people might do in the hopes of providing comprehensive reviews), it would eject the spent case, but wouldn’t go far back enough to pick up the next round. Using the pistol system, it would happen several times through the course of a magazine. PNW was honestly the biggest offender as it ran six other types of ammo fairly well using the pistol lower. Though the Gorilla 208 gr. A-Max would fail to cycle the action completely every now and again with the pistol lower. Running with a suppressor, these problems went away as there was a bit more back pressure.

On to doublefeeds as you see above. Holy moly. It only did this with Freedom Munitions 150 gr., and I’m actually willing to put the blame on Freedom since it happened across multiple magazines and not with any of the other brands of ammo I tried. When I got a set of calipers out, I found that the Freedom ammo treated specs as a polite suggestion. I fired 500 rounds of mixed ammo for this test, and had several failures to cycle which I’m pinning on the gun being under gassed. The double feeds seemed to be ammo-specific, and therefore I can’t fault PWS for those failures. That said, that double-feed picture is pretty sweet.

A note on cleanliness: Unless I’m dealing with a precision rifle of some sort, I run test guns as they come out of the box. My rationale being that some of our readers are going to take the gun out of the box, and start shooting without a stop at the bench to clean, lube, and tune. Our goal here at TTAG is to simulate real world conditions, and a lot of folks are going to go directly from box to range. So after 500 rounds of mixed ammo, some of it good, some of it bad, and no cleaning, what do the guts look like?








Truth be told, I was pleasantly surprised to see how clean the action was after 500 rounds. Sure, there was some grit and powder. I don’t think the piston makes a “perfect” seal so there’s certainly going to be some blow-by that you can see in the photos above. But, the PWS piston system delivers on cleanliness versus a DI gas system. PWS uses a nice coating on the internals so a KG 1 Carbon remover shower, and a wipedown with a clean rag were all I needed to restore the guts to factory cleanliness. Same went for the hammer and lower receiver guts. However, when I it came to the business end of the piston system, I was shocked at all the crud I found.


I assumed that I was dealing with some baked-on carbon, not unexpected given the operating conditions. But after much soaking and scrubbing with KG1, I found a much more insidious buildup of copper. Lots of it.


I’ll leave it up to our gunsmiths and technical wizards to tell me how that much copper accumulated on the end of the piston, but what I can tell you is that 20 minutes later, panting from exertion (mostly joking), I had finally managed to scrape away the copper leaving a clean piston. If you buy a MK109, I’d suggest you keep tabs on the end of the piston to see what your buildup looks like. It never seemed to affect the function of the gun, but it was certainly an oddity of sorts.


Ergonomics and reliability out of the way, let’s chat accuracy. All groups were shot off a solid rest front and rear using a 3 lb Timney trigger and the Bushnell SMRS set at 8.5X. Some of the groups were shot at 50 yards, some at 60 yards, but I took my sweet time on a windless day to produce these. Afterwards, I plugged them into OnTarget to get some very objective readings on the accuracy potential of this gun. I’ve ranked them from worst to best.

Freedom Munitions 150 gr

PWS MK109 Barnes 110 gr

Gorilla 208 gr

gorilla 220 gr

PNW 147 gr

Gorilla 125 Sierra
Remington 125 gr OTM

In case those images don’t resolve enough for you, the info is below in a handy bulleted list with links to the particular ammo in question.

As I said earlier in the reliability section, it likes some rounds, but not others. This holds true for accuracy as well. However, I do want to deliver a harsh dose of reality when it comes to these numbers, especially as it relates to accuracy and usable distance. It’s a point I made in my review of the MK107 and one I’ll beat to death in any future short barrel reviews.

As gun owners who spend more time on the internet than in the field, we’re horrified to hear that a gun shoots ~3.5 MOA, but given that the effective distance of this cartridge should be <250 yards or so, we’re talking about a cone of fire with a diameter less than 9 inches with even our worst performer. That said, it really likes Remington 125 gr. OTM, so if ultimate accuracy is your thing, shoot that and be happy. It’s not a sub MOA gun in my hands, but just barely. Give me a case of that ammo and an afternoon at the range, and I’ll pump out a sub MOA group eventually.


Specifications: PWS MK109

  • Advertised Weight (MK109P): 5 lbs, 8 oz.
  • Advertised Weight (MK109): 3 lbs, 7 oz.
  • Overall Length: 25.75”
  • Barrel Length: 9.75”
  • Chamber: .300 BLK
  • Barrel Twist: 1:8
  • Price (MK109P): $1949.95
  • Price (MK109): $1449.95

Ergonomics and Fit * * * *
As a pistol with the SIG Brace, this is not the most comfortable gun I’ve ever gotten behind. Then again, it’s a blunt instrument of CQB awesomeness. It’s not supposed to be super comfortable. The hand guard is nicely shaped especially for those with elfin hands like the author.

Fit, Finish, and Build Quality * * * * *
With a magnifying glass and a few hours, I’m sure you could find some imperfections, but I sure wasn’t able to find a damn thing wrong with it. The coatings are evenly applied, the machining work is flawless, and everything felt sturdy and well built. This is not the lightest .300 BLK upper in the world, but there’s a feeling of quality that goes along with the weight that makes it feel alright.

Accessorization * * *
The handguard uses the Keymod system so the world is your oyster as it relates to bolt-ons. Further, it uses a stock charging handle and a common thread-type muzzle device. But that’s the limit of what you can swap out. Because the gas system requires the barrel to be threaded around the gas port, you’re stuck with the barrel you get. So I hope you like the MK109’s accuracy, because you can’t upgrade the barrel later on down the road for more precision.

Reliability * * *
Five stars is reserved for guns that go bang every time, in any configuration, with any ammo. Four stars is reserved for guns that go bang every time in the stock configuration with any ammo. Three stars is for guns like the MK109 that are just a little picky about what they like to eat and have to be run in the stock config. The gun is under-gassed as far as I can tell, which might have been a nod to those planning to run a suppressor. And threading a can on the end does resolve the problem of being undergassed.

However, the original spec for .300 BLK rounds was a 9-inch barrel without the need to switch gas block settings. I’m honestly curious why PWS didn’t include at least a two-position gas block with this gun since their execution on the MK107 was so flawless. Maybe future revisions will have this feature. For now, feel free to use my work to help make ammo selections that will run in this gun.

Accuracy * * * *
Five star ratings are for guns that go sub MOA with good ammo, while four stars can be had by going sub 1.5 MOA with at least one brand of factory ammo. In this case, Remington 125 gr. OTM saved the day and allowed this upper to snag a four star rating. Given my experience running hand loads vs. Factory Match, I don’t doubt the gun can go sub MOA, but for now, it gets four stars in the accuracy department.

Overall * * * 
There’s not much out there in the world of ~9 inch, piston driven, .300 BLK AR uppers. Adams makes a short stroke kit, the MCX uses a short stroke, and PWS has their long stroke. I might be wrong, but I think PWS is the only long stroke .300 BLK upper maker out there so a comparison against anything in the market that matches spec for spec is impossible. I’ve run the MCX a handful of times, and it seemed to go bang with whatever I threw at it, but I have no experience with the Adams kits. Expanding the circle of competitors to DI kits, and there’s a great deal more to choose from that I’ve actually put hands on.

I’m going to preface my next series of statements with the disclaimer that my experience with .300 BLK is limited vs. what I know about .233/5.56. It might be very possible that the accuracy I saw is just what you get when you chop a barrel down to 9 inches and push a .30 cal bullet through it. So take what I’m about to say with that in mind.

If this upper had a price tag closer to the $1000 mark, I’d award it four stars easily. And truly, this is one of those scenarios where I miss our half star ratings because this is better than three stars, but at this price, it isn’t quite there. Given that the upper alone costs an eyewatering $1450 from the factory, $1300 if you look around online, I have a hard time giving it the full four star treatment when it gives me max spread accuracy numbers that start with a two using most of the off-the-shelf ammo I tried. And I know I waxed poetic earlier about “practical” accuracy, and you’ll have to pardon me if I sound like an elitist, but if I pony up $1300 for an upper, it better eat everything I throw at it while sending bullets through nearly the same hole each time I squeeze the trigger. And since the PWS gas system uses a proprietary barrel, you gets what you get. There’s no upgrading to a precision barrel later on down the road.

On the reliability front, if this had a gas block that could be adjusted, I would have gone four stars. It was truly that close.

For those who want to keep the guts of their gun clean, and don’t mind consistent ~2.5 MOA with off the shelf ammo, the MK109 is a four star gun. But for me, the savings between this and a DI upper gun that will shoot better can be spent on more frequent cleaning sessions.

That’s a painful thing to say because I genuinely like PWS and what they are doing. I think the long stroke piston has merit, and I think the world would be a better place with more long stroke ARs running around. And I do think that if you place a bigger value on keeping a gun clean vs. accuracy, the quality of this gun make the money worth it. But as is, I just can’t give it the full four stars.

Previous Post
Next Post


  1. Excellent article.

    I noticed some mistakes on velocity readings near the end of the article. The Barnes 110 grain was listed at 3114 FPS, and the Remington 125 grain was listed at 2623 FPS. That would yield muzzle energy ratings of 2368 and 1909 FPE. That’s way high for the 300 BLK.

    I just shot a decent sized whitetail deer with the Barnes 110 grain 300 BLK load. Although I didn’t chronograph the latest load, I wasn’t impressed with the tiny little holes it made in the deer. The other members of our party were using .308’s with a 20″ barrel and 16″ barrel. They were putting 2″ holes on entry and exit. Next year I’ll probably step up to a bigger caliber for chasing corn-fed WI whitetail.

      • That still seems very high, but I believe that you got those numbers, and it’s clear that you know how to shoot. My buddy has either the same build or one that is very similar. I just got him a chrono for his birthday (I got 12, 15, and 18 year scotch set), so we’ll have to check that out. I’m curious what my 16″ 416R build will do, because it sure doesn’t seem to hit as hard as a .308 or 6.8.

        • My wife and I got in a minor argument as she was calling out the numbers from the chrono.

          I said “that’s impossible” and she said “it’s right there!”

          So I power cycled it. And still. 3000+. I figured I must be going crazy.

          Send us your chrono results. Well post em when you do.

        • Great review.
          According to the box, PWS 300BLK with 110gr Barnes TAC-TX through a 12.75 inch barrel should clock out at 2355 fps. . . . . . . . . .
          I have the 16-inch PWS 300BLK upper and it has never experienced any sort of malfunction over the past 3+ years while eating a lot of the PWS 110 Barnes TAC-TX, Lehigh Defense 300 Whisper 110gr Controlled Chaos, Lehigh Defense 140gr Controlled Frag, Gorilla 208gr subsonic, Remington UMC 220gr subsonic (noisy though), or PNW 220gr subsonic.
          My 16-inch barreled version does have the adjustable gas block, although I just leave it in “0” all the time. Good ejection to 4 o’clock with everything except the Gorilla 208 which goes more to 3 o’clock.

      • Having loaded a lot of recipes for .300 BLK, I can tell you that the highest velocity I have seen was in the high 2600sFPS. That was out of a bolt gun with a 20″ barrel. Even Barnes says the max you can get out of a 16″ is mid 2400FPS. I have some experience loading about max pressures too. Both the Barnes and Remingtons are not correct, especially from a short barrel.

        I have had velocities read like crazy before, and what we discovered was that with certain powders(pretty sure it was H110), you need to move the chronograph back to 20-30′. You are registering the gas and unburned powder on the chrono.

        The fastest powder I’ve had success with is PP 300-MP. You can exceed the velocities of achieved with H110/296, 2400, 1680, and lil’ gun with these lighter bullets. (Seems for the same pressure I can get an extra 100-200 FPS over a normal H110 load).

  2. I have yet to see a successful build with a pistol length .300blk barrel and the Adams Arms kit. Wonder what size the gas port is on the PWS?

  3. Modified gas key piston systems? Is that shit still legal?

    Listen, LWRC, Sig, HK and other high-tier manufacturers have pretty much moved us past the ghetto-rigging of BCGs. The price you pay for piston reliability in an AR is giving up the modularity of the bolt/BCG.

    If you’re going to make a piston AR, craft the damn thing like it’s what you meant to build in the first place rather than something you decided on halfway through the process. Not having a dedicated ground-up bolt/piston system designed for the weapon feels cheap.

  4. Any chance you can run it with a can for a couple hundred rounds along side a DI system in a comparable build? I’ve avoided the piston setup for the 300 blk because it seems to defeat the purpose of a suppressed build if cleanliness is the goal.

  5. Question of the day? Who buys a new firearm and doesn’t clean it before first range trip? New handguns get the cleaning, 100 rounds, clean then sight in if needed. Rifles get home and cleaned, optic mounted if needed, bore sighted, shot once check that its on paper then if its rimfire start shooting, if it’s centerfire 1 shot patch, repeat 9 times, clean and then zero.

    Sub moa accuracy is fair, I hope my build is 1-2 moa.

  6. This is a neat weapon, and a good review, bit I was wondering if TTAG will get back to reviewing guns that averag joes can afford? I mean, not always, just more frequently than never.

  7. That looks like a lot of copper fouling indeed, interesting. Do you think a suppressor would mitigate that in any way or could it make it even worse? Still looks like a cool gun, but I’m disappointed it seems to be really optimized for suppression. I guess that is what .300 BLK is for though.

    There is still the Galil ACE pistola for those of us who can’t get a suppressor and it will be a lot cheaper to feed. Presumably less copper fouling with the bi-metal jacketed bullets too.

    • yeah, definitely keeping my eye on the Ace. Maybe I settle for the ‘pick up truck’ vs sports car on this one, and know that when I hit the gas, it will go. Every time. High maintenance is best left for hotties.

    • I talked with PWS today specifically about the copper buildup, and it is a result of the nitride treatment on the barrels before they get broken in. To mitigate/eliminate the problem, they recommended following a break in procedure, or just use something as simple as Dawn dish soap and scrub the heck out of the barrel. The nitride process basically leaves microscopic salt-based crystals inside the bore that initially shave a little copper off as the bullet passes down the bore. I had this issue initially with other PWS Mod1 rifles I own and when I would periodically clean my guns, I would just be at my bench and quickly hit the piston with a wire wheel. That solved the problem early on, but it is no longer an issue. Maybe there is a difference between 5.56 and 300blk, but I am picking up a 300blk next week and will post my results after the barrel scrub and several hundred rounds.

  8. Still waiting for my Rock River Arms LAR-PD in .300 AAC 🙁
    With no buffer tube it would be very compact, get a stamp and put on a side folder, still compact.

    • Hmm, would be nice if they’d do an adapter for a sig brace. Love my RRA Entry Tactical and love the company/product. That looks promising with the adjustable block too. I’ll have to keep my eye on that one…

  9. Thanks a ton for the review, Tyler. I’ve been anticipating this since you gave us a preview back in July. Comparing this to my considerable experience with a (DI) AAC 9″ MPW, it’s hard to see enough of a clear advantage to drop the $1200 to swap uppers right now… Basically, instead of the gunk being in the guts on a DI platform, it looks like just gets moved to the piston. One area I think the AAC DI clearly wins is accuracy. It’s almost shockingly accurate for a 9″ barrel. it’s light as heck too. The KAC rail is awesome, lower comes with a Geissele G2, etc… But it sure does get gunked up quick, which is my one real area of concern.

    I would love to see a full review on the new Adams Arms 300 BLK 9.5” pistol/upper with their new XLP adjustable gas block. I can’t find any info out there on it yet, but it sure looks interesting (although I can’t see how to adjust the gasblock being so far under the rail?). And of course, short stroke piston may also have it’s issues and without and honest review, it’s hard to know the real deal without dropping the dime and going through the motions yourself- which I’m hoping to avoid this time. Done that a few times already and seem to get burned about half the time. (another reason I love TTAG!)

    My fallback, and perhaps where I end up with this, is the new IWI Ace Galil pistol. If the ‘POU’ is a PDW for CQB in a SHTF-type scenario (like that?), then reliability/low maintenance is the most critical factor – and it might just be worth compromising a bit on the ergo’s and benefits of an AR platform vs ‘AK’ platform and go with the more known-quantity. Think you lose a little something with the manual of arms (slower mag changes, no bolt hold open), as well as the sights/optics options, ability to accessorize, etc – but in return get a side folder with a more common (and much less expensive) cartridge (with somewhat similar performance) and of course — the reliability of the good old AK system. So those are the next two reviews I’m hoping for. Thanks again for the straight-up review. I love PWS and think this looks pretty cool, just not blowing the doors off of a good DI, or in the same league with an AK as far reliability and low-maintenance.

    One quick note: it’s interesting to see that LWRC, DD (and perhaps others) are going with DI for 300 Blk SBR pistol/uppers. Think there could be a reason for that.

  10. Personally i’ve regulated my freedom munitions 150gr ammo to just “cleanup crew” status. Just firing off couple rounds unsuppressed to blow out any excess carbon/grit in the rifle and mags before packing things up. At least they’re not completely useless lol.

  11. “Mikhail Kalashnikov, his holiness John C. Garand”

    I think you meant to say “Their holinesses, Mikhail Kalashnikov and John C. Garand.”

  12. Nice review. I own the same gun and have been waiting on somebody to comment on the copper build-up on the piston. I have had better results with ammo than you (zero malfunctions but only shot Hornady 110gr, Barnes 110gr, and Remington OTM 125gr). The copper is a bit bothersome but has yet to cause any problems other than a cleaning nightmare.

  13. I bought this gun last summer, shot about 1,000 rounds through it before the snow fell. I had the same problem with PNW 147gr. On the PWS website now is a disclaimer: “For unsuppressed use, stick with 110gr to 130gr full power ammo and for suppressed you can use anything from 110gr all the way to 220gr sub sonics. 147gr is not an actual part of the 300BLK SAAMI spec submitted by AAC and will typically not function fully without the added back pressure of a suppressor.”

    It feeds Hornady and Barnes 110gr. fine, and I’m generally very happy with it after they fixed an odd glitch for me: some of the built-in sling swivel holes in the handguard didn’t reliably hold a QD swivel. Third swap fixed it. Got my Form 1 approved in 7 weeks (thank you efile) and then had to pay for a new buffer and tube, which are not cheap, so screw me. Getting the pistol buffer tube off required making a custom wrench out of a 2×4 to go around the proprietary castle nut thing. Maybe I’m just stupid, but it finally worked.

    Ordered a GMT-300BLK can for it, which will live on it full time. By the time I’m done, might as well have just gone with a Spike’s Tactical Compressor or maybe even a NEMO, but going with the pistol I was able to get it right away and start shooting, so that was my (perhaps overly short-sighted) motivation.


Please enter your comment!
Please enter your name here