Primary Weapons Systems

MOD-2 6 ARC Upper Review

I finally made the choice to get into the 6 ARC platform. This took me down the road of looking for a 6 ARC upper, or just building one myself. Along comes Primary Weapons Systems (PWS) and their new 6 ARC piston upper receivers. A 6 ARC with a piston system intrigued me. PWS has been making long stroke gas piston AR’s for a long time, so if anyone could do the 6 ARC in a piston gun, I knew it would be them. They did not disappoint.

A little while later one of the Primary Weapons Systems MOD-2 6 ARC upper receiver’s showed up at my door. PWS has a MOD-1 version that has a few less frills on it, but essentially works the same way. These uppers are available in the MK118 and the MK114. The MK118 uses an 18 inch barrel, while the MK114 uses a 14.5 inch barrel. Since the 6 ARC performs decently well in shorter barrels, I chose the MK114. I also was going to run a suppressor on the rifle, so a 14.5 inch barrel would make the overall package a little shorter. Remember, if you mount the MK114 on a rifle, it must be registered as an SBR. Otherwise, it must be used on a pistol AR lower receiver. I have various SBR’s around, so that was not a problem. However, since ATF Form 1’s are zero dollars, I did fill out a new form 1 to make the PWS upper a complete SBR’d rifle.

The PWS MOD-2 upper has features that one would expect on an upper of this quality. The receiver itself is one that PWS uses in quite a few of their rifles. The MOD-2 does not come with a forward assist. It is a stripped down version with some design angles to make it more appealing to the eye. The design probably helps with strength, but I do not know that for sure. I do not care that it doesn't have a forward assist, especially since it is not a rifle that I will be using super hard. PWS installs a FRC 3 port comp on the upper’s 5/8-24 threads. This muzzle device uses the Rearden Atlas/Plan B exterior threads. This allows you to use a suppressor with that attachment system, letting you easily install and remove a suppressor. One thing that really surprised me on the MOD-2 upper was its weight. The thing only tips the scales at 4.7 pounds. The internet will tell you that piston systems are heavy. This PWS upper proves that that is not true. I weighed a 16” 556 direct impingement upper, that is pretty close to the same style, it weighed 4.5 pounds. So yes, the PWS does weigh a tiny bit more, but it is not enough to write home about.

PWS uses their PicLok handguard on the MOD-2. The PicLok handguard is a unique combination of a M-Lok handguard that has some picatinny style mounting points at the front of it. M-Lok slots are at the 3, 6, and 9 o’clock positions, with the top being a full length picatinny rail. At the front of the handguard, at the 3 and 9 positions, are short PicLok sections. Then on the bottom, there is a longer PicLok section. As one can guess, the PicLok sections are used to directly mount picatinny accessories to. But also in the PicLok sections, are threaded holes. These are used to mount M-Lok accessories. All you need to do is take the nuts off of the M-Lok mount, then screw the remaining screws directly into the holes in the PicLok rail section. It's a really cool design that works well. Since the MOD-2 has an adjustable piston, there is also a space, at the top front section of the rail, to allow you to adjust the gas system for the piston.

A long stroke gas piston system is the heart of the PWS MOD-2 upper. Piston systems can be more reliable than direct impingement systems, mainly because they run cleaner. Since the MOD-2 is a long stroke piston, that means that the operating rod and piston are attached to the bolt carrier group. When the rifle is fired, gas pushes the piston, operating rod, and the bolt carrier all at one time. This tends to give a smoother cycling over a short stroke gas piston, which literally hits the bolt carrier with the operating rod. The PWS MOD-2 upper also has a 3 position gas adjustment. This is mainly for suppressor use, but can also be used to tune the rifle for specific ammunition. The 3 positions are marked with the numbers 1, 2, and 3. Position #1 if for use without a suppressor. Positions 2 and 3 are for suppressor use. For adjustment, there is a cut-out toward the front top of the handguard. This is also where you will see what number the gas block is set to. PWS supplies a tool for adjustment, but anything that fits in the hole works. Changing the gas settings is super easy which makes it nice if you are installing and removing a suppressor often. PWS uses a 1 in 7.5 twist rate for the 6 ARC uppers. It is a good twist rate for all weights of ammunition.

As mentioned, I installed the PWS MOD-2 6 ARC upper on one of my SBR’d lowers. I also installed a Primary Arms 4-16 first focal plane optic. But for accuracy work, the 4-16 came off for a Sightmark 5-30. PWS guarantees their stuff to shoot 1 moa. I went through a bunch of ammo to try this out. Shooting 5 shot groups, most of the ammo I tried would shoot 1.25 inch groups at 100 yards all day long. Not bad, but I figured some of it was me, hence the larger scope. I then procured some 80 grain Hornady TAP. It shot a 0.75 inch group. Don't worry, the 108 grain stuff also shoots well and would be a better choice for longer distance shooting. The 6 ARC is just a fantastic round and the PWS upper shoots it very smoothly.

The PWS MOD-2 also shoots and runs fantastic when suppressed. At first I had installed a Bowers Vers 375 suppressor. This is a fairly versatile and larger suppressor. I adjusted the gas block to position #2 and it ran well, with a good ejection pattern. I did try position #3 and it would fail to feed when shooting fast. Since position #3 is the most restrictive, this didn't surprise me. A few months after receiving the MOD-2 upper, I won a Huxwrx Flow 6k suppressor. So on it went, especially since it was designed as a 6mm suppressor. The Flow 6k is also a “flow through” suppressor, unlike the Bowers can. It is also smaller than the Bowers, but suppresses really well. I proceeded to try it out on setting #2, but because of the flow though design it just wasn't ejecting very well. So back to position #1 and it worked great there. Who would have thought that a flow through suppressor actually does what it says. I did also do some bill drills with the 6 ARC, just to see how it performed. The upper ran great, but my wallet didn't. So when PWS says “suppressor optimized”, it really is true.

The 6 ARC piston upper from Primary Weapons Systems is top notch. You get the advantage of a piston driven system in the 6 ARC caliber. Which gives you a cleaner running system. It also lets you reach out to longer distances reliably, in a standard AR15 size rifle. The PWS MOD-2 also lets you run suppressors easily and adjust the upper to fit your suppressors needs. If you are looking for a great system in the 6 ARC cartridge, go check out the MOD-2 piston upper receivers from Primary Weapons Systems.

Firearms Insider Reviews - Key Points

Claim to Fame:

Piston AR upper receiver in 6 ARC

Target Market:

Anyone wanting a reliable, accurate, piston upper for their AR15

Features, Benefits, and Specifications of this product:

Sub-MOA Accuracy Guarantee: Experience Sub-MOA accuracy with high-grade ammo, thanks to our Boise made precision barrels.
PWS Long Stroke Piston System
3 way adjustable gas system
Suppressor Optimized
Caliber: 6mm ARC
Lightweight Design: The MK114 6MM ARC upper weighs just 4.7 lbs
Barrel: 41V50 steel 14.5" non-pinned barrel with a 1:7.5 twist rate, delivering exceptional accuracy and versatility when firing all types of 6MM ARC ammunition.
PWS FRC 3-Port Compensator: Compatible with the Rearden Atlas/Plan B suppressor mounting system.
Innovative PicLok™ Handguard: Combining both Picatinny and MLOK into one for mounting accessories.
Radian Raptor™ Ambidextrous Charging Handle.