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Given the fact that the ATF has $200 of my hard-earned scratch and my foot is anxiously tapping as I refresh, it’s a safe bet to say I can be counted in the short barreled rifle (SBR) fanboy club. My dedication to it isn’t cultish like our boy Leghorn, but the same deity that brought him to the table, 300 BLK, has snagged me as well. Forming a trust, paying $200 to the .gov, and sitting on your hands for 30-180 days isn’t for the weak of constitution. Buying or building a SBR has got to be worth it. Right? And after a year or so of shooting 300 BLK chambered pistols and SBRs, I think the cartridge is worth the hassle. And the CMMG PDW Pistol is a great place to get started . . .

First, a couple of qualifiers: I don’t like AR- or AK-based pistols. They’re harder to shoot than a regular semi auto at full extension and they’re far too cramped and ergonomically frustrating to shoot like a rifle. I will concede that they are normally chambered in rounds that offer much more favorable terminal ballistics than 9mm/.40/.45 and owning one comes with far less paperwork than a SBR.

But for those who want a semi auto home defense gun (CQB), an AR or AK based pistol packs more capacity and more punch than a typical handgun. At anything under 10 yards, they really aren’t much more or less accurate than a GLOCK or M&P. With a properly adjusted single point sling, they can be made to work pretty darn well.


My second qualifier on the whole issue is a recognition of the real joy of short-barreled semi-auto life. It happens when you pay the man his money and get your permission slip to put a real, honest-to-God buttstock on one. That isn’t always possible thanks to (some) onerous state regulations and it can be a bit of a fiscal stretch, but a properly equipped short-barreled rifle offers the maneuverability of the aforementioned pistol with the firm contact of a shoulder stock that enables practical accuracy beyond bad breath distance.


Lastly, 300 BLK really is as good as Nick says it is. When you take a cartridge like 5.56 NATO, designed for a twenty-inch barrel, and cut eleven inches off it, you’re basically gutting any hopes of terminal performance that it might have. I’d still never volunteer to stand in front of it, but I’d be even less inclined to stand in front of a similar length barrel shooting 300 BLK.

Our own Jim Barrett did a piece two years ago that lends some further data to support that feeling. I’d also challenge you to find a caliber that offers 110 gr to 220 gr loadings using commonly available AR-15 parts with as much factory support as 300 BLK. It’s the best thing we’ve got going.


So lets talk about this CMMG gun, shall we? My first introduction to a CMMG gun (way back in Obama’s first term) wasn’t a good one. A friend won a rifle in a raffle and it was a proper pile of crap. The edges of everything were rough, it seemed to be filled with sand, and it would not shoot reliably or accurately.

The accuracy issue was finally tracked down to a loose barrel nut. My friend bailed on the gun, offloading it to a different gun store from the one that facilitated the raffle and bought something different with the proceeds.


My second experience with a CMMG gun, this pistol, was a bit better. Moving from the back to the front, the knurling on the buffer tube is aggressive and even. The upper and lower mate up well and without wiggle. Moving towards the business end, the controls are where they’re supposed to be, and surfaces on the upper and lower are free of defects.

The PDW’s bolt carrier group is a parkerized number that’s fairly consistent with a milspec build kit. The junction between the hand guard and the upper is without gaps and appears to be solidly constructed. The KeyMod hand guard, as best I can tell, is made by CMMG as it lacks any other manufacturer’s rollmarks or symbols.

There’s a DI pistol-length gas system under the handguard which ends just short of the muzzle threading. The pistol ships with a standard A2-style birdcage flash hider, which I promptly lost when I unscrewed it to test this gun with a silencer. As such, some photographs show a Black River Tactical Covert Compensator Flash Hider. My bad.

Overall, the finish of the CMMG is definitely not the highest quality I’ve ever seen, but it seems functional and falls in line with what you’d expect at this price point. Scuffing the finish off the handguard, buffer tube and lower receiver was fairly easy. This is not a gun that will come through hell looking no worse for the wear. It is built with the mil(est) of mil spec parts, and that design decision shines through after a day or two at the range when it shows scratches and scuffs character marks.

Again, this is what you’d expect at this price point. Speaking of price points, the CMMG Mk4 PDW in 300 BLK lists for $999.95 on the CMMG website, but can be found in the wild in the $800 to $900 range. The complete upper is listed at $749.94 on the CMMG website but can be found online for something closer to $600. That’s a pretty decent price for a completed upper assuming it shoots well and runs reliably.

cmmg 300 BLK target 1


I tested the PDW off a supported shooting position (front and rear) with an optic at 50 yards. This is my de facto shooting distance for the 300 BLK cartridge. I realize that accurate loadings are capable of much further shots, but this allows me to stay on target across the diversity of bullet weights and velocities of 300 BLK  and represents what I feel is a typical distance for engagement with this round. Results of my testing in order of bullet weight are below.

I tested this gun with ten different types of ammo, nine of which I put on paper for groups. The tenth was the ammo that broke Nick’s Liberty can and I didn’t do any accuracy testing while I was wrecking his gear. Since each box of ammo was 20 rounds and I tried to make an effort to test as many types of ammo as the budget allows, most of the ammo listed above got at least two, five-shot groups. Usually five rounds are reserved for getting it on paper, five are reserved for rapid fire testing, and ten reserved for slow fire for groups. That’s enough to allow some trends to emerge.


For the most part, this is what I’d call a 2-3 MOA gun. It seemed to appreciate lighter weight bullets like your 110 gr V-Max and 110 gr TAC-TX. It has a special place in its heart for Remington 120 gr.

Once I got into the 147 & 150 gr. zones, though, accuracy started to fall apart. I’m not certain that can be attributed to all of the middle weight loadings, or just the two that I used. It started to clean up a bit with the only two subsonic loads I used with the caveat that there was a can in place. My recollection of sighting the gun in with the 220 gr. SIG ammo without a can was that it performed worse than when it had a silencer in place, but I have no concrete data to back that assertion.

These aren’t exactly encouraging results, but in a lot of reading on the internet, chatting with Nick, and considerable thought on my part, I just don’t see the .300 BLK cartridge out of a 8″-10″ barrel AR being capable of much better than 2 MOA no matter what. I’m happy to entertain opposing data, but everything I’ve ever collected says that I shouldn’t expect much better.



On the reliability front, the CMMG was at least predictable in what it would eat. With a can screwed to the end, I couldn’t get it to choke on anything at all. It was 100% reliable as long as there was additional back pressure provided by a silencer.

Once the can came off, things would get a little weird. As I mentioned above, I tested ten different types of ammo. Every subsonic load would fail to fully cycle the gun without a silencer aboard, and every single round made by PNW had the same problem. I’ve experienced similar issues with PNW’s 150 gr loading in other .300 BLK guns, but I didn’t realize until the gun started short-stroking that the “Team Never Quit” brand of ammo is a PNW brand as well. Too bad, since that stuff was fairly accurate.

The lesson here is to avoid PNW-branded ammo and subsonics unless you’re using a silencer. All other ammo I tested cycled perfectly.



The trigger is, you guessed it, a mil spec piece of garbage. It breaks somewhere between 5 – 3/4 lbs and 6 – 1/4 lbs on my trigger scale and the road to get there is filled with grit, false breaks, and vague feelings of unhappiness. I think it actually takes the cake for being the worst trigger I’ve ever tested. By the time I got done with a night of accuracy testing, my right forearm was bulging from the workout of using the CMMG trigger. Get it? It sucks.

The rest of the controls are functional, but not necessarily glamorous. There’s some grit evident in flicking off the safety or pulling the takedown pins, and in cycling the bolt. It’s important to remind oneself that this is a but a budget gun, and budget guns don’t usually have controls that feel like two pieces of oiled glass rubbing against each other. Everything but the trigger is totally serviceable.



As you can see above, the CMMG pistol weighs in at a touch over four and a half pounds. That seems to be about right. The piston-driven, SIG Brace-equipped PWS MK109 I tested out awhile back tipped the scales at over five pounds, so it makes sense that a DI gun without the brace would weigh less.

On the range, it behaves like all the other semi-auto rifle based pistols. The buffer tube is too short for a six foot man like me to comfortably shoulder, so you get a very cramped shooting position. It can be made to work, but it isn’t any fun whatsoever.

Shot at full extension like a pistol, those 4.5 pounds add up quick, and relegate you to usage of a red dot optic exclusively as most iron sights were meant for a “nose to charging handle” style of shooting. If you place your support hand on the foregrip for added stability, you’ll need to retract your shooting hand a bit to make everything comfortable, and the whole thing feels a little awkward.

The best way I’ve found to shoot one of these guns is to throw on a single point sling and adjust it so that it creates tension between your body and the gun at a point slightly back of full arm extension. That provides a decent amount of stability and a somewhat pleasant ergonomic experience. You should assume that position for as long as it takes your Form 1 paperwork to clear. After that, toss the pistol buffer in the trash and put a proper buttstock on this gun.


Specifications: CMMG Mk4 PDW Pistol – .300 BLK

  • Caliber: 300 Blackout
  • Barrel Length: 8 inch
  • Barrel Twist: 1:7
  • Muzzle: A2 comp., threaded 5/8-24
  • Hand Guard: CMMG RKM7 KeyMod hand guard
  • Furniture: Magpul MOE pistol grip
  • Receivers: Forged 7075-T6 AL M4 type upper, AR15 type lower
  • Trigger: Single stage mil-spec style trigger
  • Advertised Weight: 5.3 lbs (unloaded)
  • Measured Weight: 4.6 lbs (unloaded)
  • Length: 25 inches”
  • Gas Port Location: Pistol
  • MSRP Full Pistol as tested: $999.95
  • Street Price: roughly $800 – $900
  • MSRP Complete Upper: $749.94
  • Street Price: roughly $600

Ratings (out of five stars):

Fit, Finish, Build Quality * * * 
All of the pieces fit together very well. And while there was a distinct lack of high-end finish, everything was functional for the most part. I’m knocking off two stars for the overall finish of the gun which seemed to be a fairly rough Parkerization vs. the anodizing seen on some other guns. I was able to easily scuff the surface with day-to-day use.

Reliability * * * 
I have had issues with PNW ammo in nearly every 300 BLK gun I’ve run, and the short-barreled guns rarely like subsonic 300 BLK unless there’s a silencer on the end, so I hesitate to knock too many points off on the reliability front. All of the failures were related to being undergassed, so I’d imagine that an intrepid user could fiddle with the buffer system a bit to get everything dialed in. Either way, it ran everything else flawlessly so the issues with reliability are at least understood.

Accuracy * * * *
I gave PWS’s MK109 a four star rating for accuracy even though it pushed a couple of higher-end types of ammo above 3 MOA. It shot Remington ammo better than this gun, but the CMMG seems to shoot pretty much everything OK. This is a 2-3 MOA gun that occasionally farts out a group that starts with a one. Given what I’ve read and experienced about 300 BLK out of short barrels like this one, I’m not entirely surprised by the results.

Overall * * * *
This is where our subjective testing methodology starts to break down a bit. The biggest reason the PDW gets four stars: bang for the buck. The aforementioned PWS gun got a three-star rating because it battled the same reliability and accuracy issues, but cost something like $1300 for just the upper.

I’m confident that I could build a better 300 BLK upper with higher quality parts that would make me happy accuracy wise, but I doubt I’d spend less the $600 it costs to go out and acquire a complete CMMG upper that achieves “pretty good” functionality for that money. Similarly, a street price of $800-$900 is pretty good for a functional, slightly boutique gun. Would I pay $800 for a bare bones M4gery? Not at all. But they don’t lob .30 cal bullets out of a compact package.

I’m a guy who can’t stand poor quality finishes and and “meh” accuracy out of guns, so the CMMG isn’t for me per se. But…I totally understand that my feelings aren’t recognized by others in the community so I’m happy to call something a deal when I see it. The CMMG pistol is everything you’d expect out of a $800-$900 gun that lobs .30 cal rounds out of an AR-30015 platform.

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  1. Given how there’s nothing “special” about this thing, the price seems steep when “no name” 300 upper builds can be had for $400-$500 all day long from many vendors.

  2. I was looking at one of these the other day. Nice looking piece.

    Thought: An AR pistol with a brace is illegal to shoot from the shoulder. Would it be legal if a shouldering device were part of your apparel instead of the firearm? Maybe a rubber block attached to a vest with a divot for the buffer tube?

  3. In my experience if you want to cycle heavy subs un-suppressed try a kink style break like the Barking Spider from Spikes. Works great on an 8.5″ CMMG 300BLK pistol gas length barrel. It took a bored out to .125″ carbine length gas port for a 16″ barrel to cycle subs reliably. I’ve heard 16″ barrels with pistol gas ports work for subs too. A current favorite for cheap sub shooting is a 203 gr polymer coated lead bullet.

  4. “I just don’t see the .300 BLK cartridge out of a 8″-10″ barrel AR being capable of much better than 2 MOA no matter what. I’m happy to entertain opposing data, but everything I’ve ever collected says that I shouldn’t expect much better”

    Well I hate to be ‘that guy’, but I can get 1 inch groups at 100 yards, out of an AAC MPW 9″ with it’s Geissele G2 trigger, shooting Barnes TacTX 110’s factory loads. You even mentioned that the trigger sucks, but didn’t account for that at all in your accuracy comment above? Also, barrel length has nothing to do with accuracy as I’m sure you know (but seem to infer otherwise above?). The AAC barrel is a real gem. I imagine a bit better than a CMMG? So if you add that in with the trigger variable, I’m sure it’s not much of a stretch to imagine that you can get 1 inch out of a 9″ 300 Blk. I don’t have to imagine, I can do it at the range, on a good rest.

    Sorry, I really hate being the ‘whining contrarian’ type, and I liked the review. I just had to jump in there and say my peace. And yes, it was a surprise to me too and seems almost counter-intuitive, being such a short barrel and not exactly a high speed round. But from what the ‘precision-shooting’ dudes at the range say, it’s more about the quality/whip of the barrel and the consistent speed and quality of the projectile than anything else – hence the move towards shorter sniper barrel’s these days. These dudes at the range are shooting 16 and 18″ precision rigs and are getting .5″ groups and less consistently, at range. I will say though that 300 Blk drops off quickly after 100, 150 though… But is less susceptible to wind than 5.56, so if you can manage the drop well, you can still work it out pretty far efficiently. I know, now you want me to do a post to prove it. Maybe I can drag a camera with me next time and get some data. I’m guessing I’m not the only however that will claim 1″ groups at a 100 with 300 Blk out of a (good) 9″ FF barrel with a good trigger and good ammo.

    Anyway, please take that for what it is. I do not claim to be an expert and respect your experience. I guess in the end, all of our mileage varies. and I have a few miles on me by now (aint no spring chicken these days lol).


    • Thanks for taking the time to a.) not flame me to death and b.) explain the setup. As a man of science, I’m VERY open to data that challenges my previously held positions.

      I use 1/4″ graph paper with a 1″ Shoot n See dot for all my targets. Most are shot indoors or on calm days and all are from a position of support front and rear. I take my time taking five shots and then run the targets through OnTarget.

      If you’d like, next time you’re at the range, run that and send it to [email protected] with a note that its for me. I’d be very interested to see the data.

      • OK sounds cool, Tyler. Yeah, didn’t want to come off like an ass there, so appreciate your response. Next time I go to the range, I’ll take the 300 and get some real data to send over…

        1/4″ graph paper with a 1″ Shoot n See dot. 5 shots, calm day, support front and rear. All good there.

        and have to admit, I have never used OnTarget before, but I just did the free download. Very cool! (how is it I didn’t know about this before lol). I guess for the most part, I don’t really shoot ‘precision’. But if we’re really going apples to apples here, that seems to be a great tool to employ the scientific method and see what I come up with. Good stuff man…

        • The Barnes works out well for my 16″ stainless 300 BLK build with a single stage trigger set at 4.5-5 pounds, too. Different animal, I know, but I’ve always had a hard time beating 1.5 MOA with Mil-Spec triggers. That trigger isn’t as good as a Timney or Geissele, but still helps.

          I have targets as well, but don’t have your target analysis program. They’re just cell phone photos with various objects as reference points. Not sure if that would be helpful or not.

  5. >> hen you take a cartridge like 5.56 NATO, designed for a twenty-inch barrel, and cut eleven inches off it, you’re basically gutting any hopes of terminal performance that it might have.

    While I agree that in a 8″ SBR/pistol .300 BLK is definitely the way to go, this claim is not entirely correct. There are 5.56 loads and bullets that are designed to work reasonably well from similar-length bullets. Barnes TSX especially, which relies on expansion rather than fragmentation, and will reliably expand out of 8″ barrel even out to 150 yards.

  6. Call me old school even though I’m young or a traditionalist even though I’m new to the firearm world but AR and Ak pistols don’t do much for me. Never had experience with them but other than fun range guns they don’t seem like they are good for much. They look like settling for the next best thing when you can’t get a sbr.

  7. I have a Frankenstein Build AR300 Blackout that gets 1MOA.
    I also load my own bullets and have tested over 10 different bullets with different powder at different OAL. So, in my quest to build a ALL AROUND GO-TO PISTOL, I didn’t take any shortcuts.
    Here is my build:
    10.5″ ODIN Match SS Barrel, adjustable GB
    10″ MI MLok HG
    Nickel Teflon Upper, BCG,p and CH
    Black Sheep Tactical Milled Lower
    Spikes LPK and Hipperfire EDT trigger
    Phase 5 Hex Buffer, Tube and Spring
    Sig Brace Gen 1
    Vortex Strike Eagle 1-6×24 on Vortex 2″offset Mount

    Here are my Favorite Bullets/Loads
    110gr Nosler Varment and Hornady Varment and vMax loaded to (20gr H110)

    125gr Nosler Baliistic Tip and Hornady SST (17.8gr H110)
    Hope this helps. Use at your own risk (IANAL but I feel I need to say that). Fix your trigger, get a new barrel and lose that red dot. At least for me, I can’t shoot well past 50 without a red dot.

    • Their appeal is that for home defense, cqb and hunting on the East Coast for deer and wild hog they are light, accurate and pack enough Foot Pounds of Energy (in 300 blackout) to take down most game and threats that can be thrown at you. They are not the END ALL – BE ALL weapons for every scenario and are rarely the only tool in the box for most people that own them. Myself, I have a 16″ AR10 in 308 that would be the weapon I grabbed for more broad or more specific engagements, or a Remington 597 in 22LR for others. I wouldn’t recommend an AR Pistol in Blackout to beginners or people looking for one weapon for every task. But for enthusiasts with a bit of time, money and love for the sport, it is my favorite weapon I own. If I had to spend more than a week in the bush, hunting and hiking (if the good Lord allows me my foot back) for either survival or say Ginsing hunting, it would be the rifle on my back. Ammo is lighter, so I can carry more of it. Barrel is shorter, yet I get complete powder burn in 8″. Accurate as the dickens. And reliable. What’s not to get?

  8. I purchased this in the 9 mm version. It is the biggest disappointment of my gun / pistol buying career. Mag & charging handle issues right out of the box. CMMG has not returned any of my E-mails. My phone calls are only met with: “Someone will call you back about your problem.” After 7 calls to date, No one has called back. I’ve been put On-Hold, only to have the line go dead after about 10 minutes. I’ve had the pistol a week & have only been able to get 3 rounds off & it was by manually loading the round into the chamber by hand

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