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[Review] PWS Diablo (MK107): 7″ AR-15 Piston Goodness

Does going super short make sense with a piston? We hands-on test the PWS Diablo for reliability, accuracy, shootability, and more...with pics and video!

Want to get real short with your AR-15 Pistol or SBR (short barrel rifle)?

Once you go below 10.5″ barrel length you start to get some problems. And at 7″ it’s a struggle.

PWS Diablo MK107 2-M Upper
PWS Diablo MK107 2-M Upper

That’s where the 7.75″ long-stroke piston Diablo MK107 series comes in.

We’ll cover why a piston AR-15 makes sense at such barrel lengths.

Plus a hands-on review of the PWS Diablo in terms of reliability, accuracy, ergonomics, worth-it-ness, and more.

at Rainier Arms

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

Piston vs Direct Impingement

We go over this in-depth in our Best Piston Uppers article but here’s the gist…

Less Dirty

A standard direct impingement (DI) AR sends its exhaust back into itself.

Direct Impingement
Direct Impingement

While a piston AR system sends its excess gas onto a piston which actuates the bolt carrier group.

Piston AR Animation, Adams Arms
Piston AR Animation, Adams Arms

All the carbon and buildup is now in a contained area (the face of the piston) which makes it easier to clean and maintain.

Heat Displacement

Guns get hot when you shoot them. And if you’re blessed enough to go full-auto…pistons will shoot much cooler.

Again, since the hot gases go onto the piston (with the excess vented)…you’re going to stay cool if you shoot a lot of rounds.


However, it’s not all hunky dory with pistons…otherwise everyone would have them.


First, they cost a lot more since pistons have more parts.

Khajit Has Wares If You Have Coin
Khajit Has Wares If You Have Coin

Oh…and these parts aren’t standardized between makers. PWS isn’t going to play nicely with LWRC or Adams Arms.

Lastly, the gun can become front heavy because of the extra piston gear.

Who Is It For?

My previous recommendation was unless you’re in really adverse environments for a long time (or really don’t like cleaning)…and aren’t shooting full-auto or suppressed…to stick with DI.

Now after shooting the Diablo…I’ve got to add if you aren’t going super short onto the list.

PWS Diablo with Aimpoint PRO
PWS Diablo with Aimpoint PRO

Because the piston part really helps with the excess gas and unburnt powder from a short barrel.

Now…enough about DI vs Piston…let’s see how the PWS Diablo fares!

Fit, Feel, & Finish

Like it’s big brother the MK116 Mod 2-M (16″ barrel)…the feel and finish was flawless.

PWS Diablo MK107 2-M Upper
PWS Diablo MK107 2-M Upper

The MK1 Mod 2 versions come with a stylized receiver and PicLOK handguards (both Picatinny and M-LOK). And an upgraded Radian Raptor charging handle.

PicMOD Rail & CQB Muzzle Device
PicLOK Rail & CQB Muzzle Device

Plus…their CQB muzzle device which adds some length and decent heft…but really forces the gasses forward.

Your shooting range buddies and teammates will thank you.

I built it as an AR pistol configuration with the SBA3 pistol brace.

PWS Diablo AR Pistol with SBA3 Brace
PWS Diablo AR Pistol with SBA3 Brace

You can check out the rest of our favorites in Best AR-15 Pistol Braces.

Going inside…you can see its long stroke piston design.

PWS Long Stroke Piston
PWS Long Stroke Piston

Makes sense right? It’s one long piston that’s attached to the BCG. That means robust parts that should theoretically break less.

Sort of like an AK’s piston…which is pretty reliable.

PSAK Bolt and Piston
PSAK Bolt and Piston

Also…there’s an adjustable gas system that lets you choose between three levels…off, regular, and suppressed use.

This picture is from the 16″ version since my lights covered it up on the Diablo.

PWS MK116 Adjustable Gas System
PWS Adjustable Gas System

Ok, ok…now let’s move on to what you’ve been waiting for…

How Does It Shoot?

Yes…just a little brush fire in the background…

But check out that CQB muzzle brake doing its job!

There’s definitely more recoil compared to a 16″ direct impingement AR. But how about compared to a shorty AR?

I’d say it was slightly less probably because of the gas venting and heavy muzzle brake.

Remember the adjustable gas system?

If you switch it to “suppressed” it shoots even softer!

However, for it to (almost guarantee) various ammo compatibility…stick with “normal.”

But even on both it shot at 100% reliability with various types of ammo and mags.


Yes…accuracy for piston guns are usually a little worse because of the heavier moving object at the front.

Let’s test it with the Diablo.

PWS Diablo Testing Rig
PWS Diablo Testing Rig

All while shooting at 8″ 100 yard targets.

PWS Diablo 100 Yard Testing
PWS Diablo 100 Yard Testing

I shot at a slow pace with the gas on normal. 10 round groups for each ammo.


PWS Diablo Accuracy Results
PWS Diablo Accuracy Results

I was getting about 6 MOA groups.

I swapped my scope setup onto my go-to AR and shot fine…so it wasn’t that.

The Diablo was just shooting not great groups.

It’s fine for steel at 100 yards…and is still minute of badguy. But 200 yards might be pushing it.

It truly is a CQB kind of gun.

PWS Diablo MK107
PWS Diablo MK107


  • Caliber: .223
  • Weight: 3 lb. 8 oz.
  • Barrel Length: 7.75″
  • Velocity: 2216 ft/sec
  • PWS long stroke piston operating system
  • CQB556 compensator
  • 1:8 barrel twist rate
  • Radian Raptor Charging Handle

By the Numbers

Reliability: 5/5

I shot about 400 rounds with a couple different types of ammo and mags…across 3 different range trips. 100% reliable.

Accuracy: 2/5

It was disappointing with ~6 MOA. If you consider it a pure close quarters gun it is fine. But for the price I’d expect at least 2 MOA.

Ergonomics: 5/5

No complaints here for the upper…loved the PicLOK rail and upgraded Radian charging handle.

Looks: 5/5

The details on the receiver were super cool. And it would look even better paired up with its lower receiver.

Customization: 5/5

It’s an AR-15 with both Picantinny and M-LOK rails so the sky is the limit.

Bang for the Buck: 3/5

The price plus the accuracy really kills it for me. But the good thing is that there’s more “affordable” models now.

Overall Rating: 3.5/5


I wanted to REALLY love the Diablo…since going short with a piston system makes sense. As I probably will never go full-auto or suppressed.

PWS Diablo AR Pistol with SBA3 Brace
PWS Diablo AR Pistol with SBA3 Brace

It was fun to shoot and I didn’t really feel the piston front-heaviness since it was so short.

It was also 100% reliable.

The accuracy is my only complaint.

But remember I said there’s more affordable options now from PWS?

If the looks don’t matter as much to you…and you just want their long-stroke piston…check out their MK1 Mod 1 offerings.

at Brownells

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

But for the original with PicLOK and CQB brake…stick with their Mod 2 options.

at Rainier Arms

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

Would you go piston in a short barrel or stick with 16″? Check out our favorite AR-15 Piston Uppers & Conversion Kits for some more inspiration.

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2 Leave a Reply

  • Commenter Avatar

    Sorry in advance... What's that hand-stop? I like that.
    P.S. nice article.

    March 28, 2019 8:54 pm
    • Commenter Avatar
      Rick Jurewicz

      Not quite sure why your accuracy results weren't satisfactory. Other reviewers have gotten 1 MOA results and no more than 2 MOA at 100 yds. No one has gotten 6 MOA results. One was using a LeadSled.

      July 10, 2019 6:37 pm
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