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The mailman cometh and he hath brought me an upper! PWS apparently didn’t take it too hard that we took so long to get a review out for their MK 107 pistol, as a new .300 BLK upper showed up at my doorstep today. I’ll be slapping this bad boy on the SigBrace-equipped pistol lower that shipped with the MK 107 and hitting the range soon . . .


Like any review, we’ll test for function with 250 or so rounds of ammunition from Freedom Munitions, and about six or seven different types of premium ammo from 110 gr. Barnes all the way up to 220 gr. subsonics from Gorilla. Additionally, Nick has been nice enough to loan me his 7.62 SDN for use in testing suppressed function.


In case you weren’t familiar, the MK 107 is a piston driven, nine inch barreled, AR upper chambered in Nick’s favorite cartridge, .300 BLK. Because .300 BLK was originally designed for a nine inch barrel, this is supposed to be the bee’s knees when it comes to .300 BLK. Nick spent a lot of time and money coming up with his perfect .300 BLK rifle detailing all the steps necessary in a post last fall. You’ll notice that he ended up back at nine inches and stuck a 7.62 SDN on it. This pistol isn’t exactly like his perfect rifle because SBR paperwork takes a long time, but it is going to be pretty close to the real deal.


As always, you can expect an honest and thorough review of this gun in the coming weeks. Stay tuned, we’re about to have some fun.

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      • As do I.

        The .300 BLK cartridges are so cute; they look like miniature ballistic missiles, ready to launch from a tiny silo…

        • “The .300 BLK cartridges are so cute; they look like miniature ballistic missiles, ready to launch from a tiny silo…”

          Well, once they leave the loving embrace of the rifling lands, they _are_ballistic…

  1. I never got .300 Blackout other than the fact that it functions a lot better out of a 16″ barrel. Care to elaborate on the cartridge?

  2. My big question is how did they get it to function with a piston? Most attempts with an Adams or other kit seem to end up in failure, or only work with a few loads.

    Can you discuss, as much as possible, the operating group and any differences with their standard 5.56 parts? Spring and buffer weight, mods to the bolt carrier and such? And if they will let you, the size/location of the gas port.

  3. I’m sure you guys know more on the subject than I do, although I do have the same rig as Nick, but I think he needs to be with you when you’re using that can, in which case you may as well use his registered SBR lower. If you wish to remain legal, I mean. Other than that, the guys at the Silencer Shop are of the opinion that the op rod type action is less reliable than the direct impingement when working with 300 Blk, so I’ll be looking forward to your report. Try out the Freedom Munitions 150g Hornady FMJ remanufactured ammo, I just bought a bunch to discover it is BY FAR the filthiest burning ammo I’ve ever used. Shoots fine, but the first case comes out nearly unrecognizable as brass it’s so dirty, and the rifle took forever to clean, after 40 rounds.

    Or, hell, tell you what-since I see you live in Austin, give me a shout and I’ll bring mine and we can compare back-to-back, I’ll bring the 150g (dirty) ammo!

    • Those guys are wrong (IMO) because when you use a suppressor you increase the backblast. So it is better to have a piston take all the fouling than your chamber/bolt.

      In regards to reliability it is the same as long as you have a gas regulator.

      • If that works out, it would actually be fun to see Tyler blaze away with that upper and those 150g bullets, the main problem I had was the bolt/bolt carrier/upper receiver seeming to have fired 20K rounds after only 40. I’d guess this unit would be nearly spotless!
        OTOH, “those guys” were quoting (now defunct) AAC, who developed the round, as well as far too much of their own fun shooting all variations. Tyler is gonna tell us, don’t forget to try function both suppressed and not.

  4. This one looks like the version to get since the .300 is optimized for short barrels. My understanding is that it can match Kalashnikov ballistics from roughly half the barrel length? Maybe you could measure the decibels relative to the 5.56 version(s)?

    • Hey, does anyone know if an op rod reduces noise of operation? See, the 300 Blk subsonic ammo/762-SDN combo is seriously quiet, further reducing the signature would just be COOL.

    • Not quite, it matches 7.62×39 ballistics (marginal differences). Subsonic 300 and x39 have the same ballistics. Load 7.62×39 with the same powder as 300 BLK and you will have pretty much the same results.

      • Is that right? 300BLK uses pistol/shotgun powder, H110 and the like. I see 4198 powder as popular for x39. Most of the popular 300BLK powder seems to be too fast for x39, and x39 powder too slow.

  5. I’ve got two PWS rifles (Mod 0 and 1) and couldn’t be happier. Been looking at the MK109 as an SBR (now that they’re legal in WA) and look forward to the review.

    @hasdrubal, only the barrel is different, everything else should function the same, between the two calibers. The Mod 1 rifles have adjustable gas blocks so various ammo or a suppressor shouldn’t be an issue.

    • According to PWS’s website the mk109 does not have an adjustable gas block: Due to its specialized nature, the MK109 does not feature an adjustable gas block.

      That seems strange given the obvious benefits of running 300BLK both suppressed and unsuppressed. Also, all of the other MK1 uppers seem to have adjustable gas blocks (visible as a small break in the Picatinny). I know for a fact the mk107 in 7.62×39 does.

  6. Looks like an awesome pistol. The 300 BLK is much better than a 5.56 out of a short barrel. I want one.

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