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t seems like most blackrifle manufactures are now adding AR pistols offerings to their rifle lines. Boise, ID based Primary Weapon Systems is no different. They offer both direct impingement and piston driven pistols. This author is no stranger to PWS and have been using their complete upper receivers for many moons now. I recently reviewed their MK216 .308 battle rifle and was looking forward to taking a look at the other end of the spectrum. Instead of a full sized .308 fire breather, I wanted to look at their smallest offering, the MK107 pistol.

The MK107 is an extremely short AR pistol with an overall length of 23.75 inches and only weighs 5 lbs. 4 oz. The MK107 pistol is nearly identical to its NFA cousin the PWS MK107 Diablo. For those who don’t want the NFA hassle or who live in states that can’t own a SBR, the pistol version is the next best thing. In place of a stock, the pistol features a Sig Sauer Arm Brace which is not classified as a stock by the BATFE. Other than that, the MK107 is the same bad ass Diablo.

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The MK107 upper can be purchased separately for your own pistol project or registered SBR lower receiver. The MK107 upper features a 7.75 inch chromoly button rifled barrel that is QPQ treated. Chambered in .223 Wylde with a 1:8 twist. PWS turns their barrels in house which allows them to keep a close eye on quality control measures. One thing that is unique about their barrels is that they have threads right in front of the gas block seat. This allows the gas block to be installed then captured with a proprietary nut.

The MK107 barrels come topped with the PWS TRIAD flash suppressor which boasts flash suppression and some compensation abilities. As I was going to suppress this AR pistol, I removed the TRIAD and installed a CRUX brake that works with their Nemesis 30 series of sound suppressors.

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The long stroke gas piston system PWS is famous for is also included on the MK107 but much smaller due to the barrel length. For those not familiar, the long stroke system is what the AK47 uses. PWS uses the same concept and successfully incorporated this technology into the AR15 platform. The long stroke piston features a floating piston head that is removable. This aids in maintenance as well as allows the user to separate the charging handle from the bolt carrier group. The piston op rod is secured to the bolt carrier at the same location the gas key would be in a traditional gas gun. The bolt carrier group is machined out of tool steel then Nickle-Teflon coated for increased lubricity and durability. PWS machined their BCG to minimize friction points while increasing mass to increase dwell time and delay the bolt from unlocking from the barrel extension which allows the pressure in the barrel to decrease prior to extraction. The bolt also includes spring assist which is a nice feature.

The long stroke piston system works hand in hand with the adjustable gas block. The gas block features four firing positions and an off position which are adjusted from the top of the rail with a provided tool that also doubles as a front sight adjustment tool. The positions range from normal operation to suppressed. During my review of the MK216, I didn’t have the opportunity to shoot it suppressed. That wouldn’t be the case with the MK107 so the adjustable gas block would be important.

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The rail the MK107 uses is free floating with a continuous top 1913 rail. The remaining three sides feature KeyMod attachments for rail sections and integral quick detach (QD) attachment points for slings. Unlike other handguards, the anti-rotation tabs are located on the bottom instead of the top. The pistol does come with (2) two inch rail sections for lights and other accessories. Don’t even think about attaching a vertical grip to the handguard unless you want to file some NFA paperwork…

The upper receiver itself is machined from a 7075 aluminum forging. After machining it receives a Type-III hard coat anodizing treatment. It features a forward assist, brass deflector and dust cover. The remaining features of the upper include the tried and true Bravo Company USA Gunfighter charging handle and the economical Magpul MBUS back up sights.

The lower is made from the same aluminum as the upper and the two fit together very well. The pistol lower is marked “Pistol” on the left hand side. It comes with a standard bolt catch and safety, Magpul MOE grip and enhanced trigger guard. The trigger is a Mil-Spec quality trigger with a crisp break. The SIG Brace is attached to the lower via the PWS pistol buffer extension tube which houses the PWS H2 buffer. This buffer has the PWS logo on the face and is Nickel Teflon coated. This tube is the same as their rifle extension tube with integral QD points however it cannot accommodate a stock. It is basically a slick tube. The SIG Brace stays on via friction and quite frankly probably isn’t the best solution. The brace slips fore and aft and side to side very freely on the tube.

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I took the MK107 to a local outdoor range for our first date. I knew she would be loud thanks to the CRUX brake I had installed. I wanted to get a few groups without the suppressor on. I was using an Aimpoint T2 in a Fortis mount with an Aimpoint 3x magnifier in a Larue flip mount behind it. I was sighting in at 50 yards with 62 grain M855. The first shot was absolutely obnoxious, but in a good way. Meaning, it looked absolutely awesome, or at least I think it did! All I saw was flame. Sight picture absolutely destroyed. I was able to hear the other people at the range say, “Back up, stay away from that guy.” I lit off the remaining four rounds to complete my five shot string then had to give my ears a rest. She was LOUD and PROUD! The group spanned 1.5 inches at 50 yards which wasn’t my greatest but considering the barrel length, trigger and optic choice it was acceptable.

It should also be noted this is a pistol and it was not fired from the shoulder which will also degrade accuracy to some extent (at least for me). This is a close and personal weapon and I was looking for a solid MOM (Minute of Man). In order to be courteous to my fellow range attendees I screwed on the CRUX Nemesis 30 sound suppressor and continued my evaluation. Thanks to the adjustable gas block, I was able to tame the blowback. Yes, even piston guns have blowback but it is nice to have the ability to tune the gas settings. My groups tightened up some with the suppressor attached and the muzzle flash phenomenon was eliminated. I finished the day with zero malfunctions and left the range satisfied.

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For my next session I wanted to spice it up a bit. I removed the upper from the lower and put the upper on a Joe Bob Outfitters Spartan Billet lower equipped with a Geissele Super Dynamic 3 Gun Trigger, Battle Arms Development short throw, ambi-safety, Magpul MIAD grip and Odin Works XMR II enhanced magazine release. I am pretty quick with this trigger and I wanted to get the MK107 purring real good. But the main reason I chose this lower is it has a Law Tactical folding adapter with Shockwave Technologies Stabilizing blade and KAK extension tube with Fortis Manufacturing QD endplate.

This would allow the already compact pistol to fold up into a nice truck or pack gun and the Blade is lighter and rock solid compared to the bulky, loose and somewhat awkward SIG Brace. I found out really quick the BCG on the PWS has a smaller opening in the rear end and will not accommodate the Law Tactical extension adapter. Without the adapter, the folder will not work. I called up the fine folks at Law Tactical and low and behold they had an adapter specifically for BCGs with smaller openings. A few days later it showed up on my door step. Problem solved!

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The upgrades really enhanced the shooting experience and made for a more functional pistol. With the Law Tactical and KAK Shockwave kit, the AR pistol became extremely compact and easily fit into my Triple Aught Design Fast Pack EDC. As I suspected, my groups improved drastically. I was actually surprised when I pulled the target and measured .95 inches at 50 yards. That group was achieved with M855 62 grain green tip while suppressed with the CRUX Nemesis. Optic used was the Aimpoint T2 with Aimpoint 3X magnifier. I’ll take it!

Overall, I really am fond of this little blaster. If you are looking for a really fun compact AR Pistol check out Primary Weapon System. The MK1 pistol series can be had for $1,949.95 regardless if you go with a 7.75, 9.75 or 10.75 inch barrel.

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Steve has been a firearms enthusiast for over 20 years and is currently an NRA lifetime member. In 1996 he joined the United States Navy and served as a Special Warfare Combat Crewman (SWCC) at Special Boat Unit 12 (Now renamed Special Boat Team 12). He made two tours during his time of service and spent most of his time in southeast Asia and the Middle Eastern theaters. Upon his Honorable Discharge in 2000, Steve spent the next 10 years earning his Masters Degree and state license as an Architect. Steve brings a unique perspective from both his tactical and design background and is a reviewer and contributor for Guns & Tactics Magazine, Defense Marketing Group and other media outlets.

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