Side Cocking AR That Feeds From MP5 Magazines: Quarter Circle 10/ Strike Industries 9mm AR Build

The new gun at SHOT 2019 was the MP-5. Importers and American companies are supplying clones and new AR pistol caliber carbines now run MP-5 magazines. Quarter Circle 10 (QC10) and Strike Industries (SI) are at the front of that movement and plug the excellent MP-5 magazine into the All-American AR to make a world-class 9mm carbine.

This is my current favorite gun. No matter what I am shooting, the QC5 goes along. I can shoot handgun steel and ring plates at 200 yards. It shoots cheap 9mm and is great for new shooters.

It all starts with the QC10 Side Charging QC5 9MM Builders Kit. What I love about the builders kit is that it gives you all the critical compatible parts you need, and nothing else.

The Builder’s Kit includes the core components for building your own Quarter Circle 10 Carbine or Pistol:

  • QC5 Stripped MP5 Magazine Frame Lower Receiver with proprietary parts
  • QC5 Billet Upper Receiver
  • 9x19mm barrel in 5.5″, 7.5″, 10.5″, or 16″ length
  • G9 Bolt Assembly
  • QC10 Adjustable Buffer Kit
  • Made For the AR Platform- Not Modified
  • MSRP $949.95

QC10 had their Builders Kits and some custom builds at SHOT. State of the art Cerakote makes a great build even better.

One thing that is very different and counter-intuitive about pistol caliber carbines is buffer weight. It seems logical that 9mm has less energy than .223 so an AR buffer should work, right? Well, no.

Quarter Circle 10’s new Adjustable Pistol Caliber Buffer Assembly sets a new standard for buffers. Adjustable quick change weight management provides optimum reliability in multiple configurations.

The Package Includes 4 aluminum weights, 4 steel weights, 4 tungsten weights, 4 rubber spacers, buffer body, and Allen key. The rubber spacers cut noise and reduce bolt bounce in rapid fire.

These can be mixed and matched to achieve the precise weight that works with your current configuration. Add a can? Change bullet weight? By removing a single screw, this QC10 buffer adapts to provide exactly what you need.

Quarter Circle 10 Adjustable Pistol Buffer Assembly: The Perfect Tool For Tuning Your Pistol Caliber AR

QC10 Adjustable Pistol Caliber Buffer Assembly

I have seen custom AR builders make their own buffers like this to determine the best weight for their rifles. QC10 makes that kind of scientific tuning available to the little guy. It is fast and simple. The roll pin in the regular buffer has been replaced with a simple screw.

There are many companies making pistol caliber ARs. Even among QC10 offerings, there are possible parts combinations which won’t work. The builder’s kit gives you a compatible upper, lower, barrel and their incredible tune-able buffer. Any other AR parts you want to hang on your gun should fit. I went with Strike Industries to fill in the spaces and get the ergonomics and performance I wanted.

Here is a breakdown of the Strike Industries parts I used:

Strike Industries Oppressor

With short barrelled firearms, muzzle blast is always a concern. It is distracting when shooting, inconsiderate in a team environment and possibly unsafe around barricades. The Strike Industries Oppressor represents the next evolution in concussion reduction devices.

Designed with computer-aided flow dynamics, the SI Oppressor provides a no-compromise solution for redirecting the overpressure produced by aggressive brakes and compensators.

Strike Industries AR Pistol Stabilizer and AR Carbine Length Pistol Receiver Extension Buffer Tube in Black

HK Factory OEM magazines have always been expensive, hard to find and prone to corrosion. Many American manufacturers, including QC10 are providing KCI USA magazines.  Here’s why you need them for your gun.


MP-5 Mags from KCI USA: 21st Century Tech at 20th Century Prices

One of the most important and most overlooked parts of any build is the magazine. Well made magazines which don’t malfunction are boring and often go unnoticed. Magazine problems are often attributed to other causes, making a good build look bad.
Conventional wisdom is that 80% of weapons malfunctions are caused by magazines. Too often, shooters overlook this basic truth and continue to run poor magazines far past their useful life. Invest in quality magazines and track their performance.

KCI USA invests in high tech quality control. Their all-steel construction and a curved mag body provide reliable feeding of all 30 rounds. One of the smartest things KCI USA has done is shoot their magazines, a lot. They are located in Las Vegas, home of the biggest machinegun rental ranges in the world. These guys shoot millions of rounds a month. KCI USA provides many of the magazines for these ranges and takes endurance testing to new levels.

MP-5 Mags from KCI USA: 21st Century Tech at 20th Century Prices

HK magazine on left, KCI USA on right after 5% saltwater exposure.

The biggest single advantage the new magazines have is a 21st-century electro-plating process which is slick for fast loading and highly resistant to corrosion. Any steel magazines are subject to rust. KCI USA has done extensive testing on their own products and their competitors. Their magazines are better by design. You can get your very own KCI USA magazines HERE for $34.99

For a short handy gun like this, a red dot sight is an obvious answer. Nikon’s P-Tactical SPUR reflex sight is built for close and medium range applications. Tiny but mighty, it is rugged enough to handle abuse and recoil of direct blowback. The machined aluminum body measures 1.8 inches x 1.1 inches x 1 inch. SPUR weighs only 1.1-ounces.

The SPUR’s 1X magnification makes shooting with both eyes open simple with an extra-wide parallax free field of view and unlimited eye relief. The 3 MOA dot is centered in an extra-wide 27mm x 16mm sight window for fast target acquisition. Nikon’s “TRUCOLOR” coating lets you see the target without the unnatural bluish tint present in other reflex sights.

I found the SPUR easy to set up and zero. Windage and elevation are adjusted with a tiny screwdriver provided with the sight. There are distinct clicks as you make adjustments. Every click of the adjustment screw moves the strike of the round 1 inch at 100 yards in the direction indicated on the screw.

The operation is intuitive and simple. It provides a quickly adjustable dot which is big enough to pick up quickly and small enough to be precise. It is ideal as a primary sight with the tiny Strike Industries 45 degree offset backup iron sights.

The 3 MOA illuminated red dot has 10 intensity settings that are adjustable using the plus and minus buttons located on the left side of the sight. Press the plus button to turn on, to turn off, hold the minus button down for three full seconds.

There is an automatic shutoff after 12 hours. The two lowest settings are for night vision, settings 3–10 are for daylight. Throughout the day from low light to bright midday sun, the dot is adjustable for a perfect sight picture.

First Look: Nikon P-Tactical SPUR Reflex Sight

Spur brightness controls. Photo courtesy of Nikon.

SPUR has a long battery life running for 15,000 hours on a single CR1632 lithium battery at the lowest setting. A new lithium battery is provided with the SPUR. The battery compartment is accessible from the top and the battery can be replaced easily without removal of the sight or changing the settings.

Nikon P-Tactical Spur Red Dot Sights help you to rapidly address targets and reacquire for follow up shots. Nikon’s top quality optics and rugged housing provide great performance. The price tag provides excellent value. You can get your very own SPUR from one of the dealers HERE  Suggested retail price $219.95.

I have run over 3000 rounds through this build over the last two months. Letting friends shoot it (with their ammo), it has run a dozen different loads, both ball and hollow point. The only functional issues were some failures to fire with re-manufactured ammo. This was completely an ammo issue.

Most of what I shot was 9mm SIG SAUER Elite Performance 115gr full metal jacket. At 1185 FPS, it is flat shooting out to 100 yards and affordable enough for high volume shooting. The quality and accuracy of this ammo provide great value.

The combination of the QC-5 with SPUR and Strike Industries sights was a minimalist wonder. Small and light, the QC-5 pointed well and was effortless moving from target to target or rapidly placing multiple rounds to a hit zone.

Ringing Steel at 100 yards is no challenge for the QC-5.

The finishing touch on this minimalist build is the Streamlight TLR-8. This super bright 500 lumens tactical light is low profile yet high performance. The low-profile design prevents snagging and a “safe off” feature prevents accidental activation, saving batteries.

A red laser gives the TRL-8 an edge over other lights for a wide variety of tactical situations. The TLR-8 enables shooters to identify a potential threat and accurately engage it, even from an awkward position.

  • 500 lumen white C4® LED, impervious to shock with a 50,000 hour lifetime.
  • 4,300 max candela, 131m beam; runs 1.5 hours (light only or light/laser combo)
  • Custom TIR optic for a concentrated beam with optimum peripheral illumination.
  • 3 Modes: Laser only, LED only, both Laser and LED; selectable by pressing both switches.
    Double tap either switch within ¼ second activates strobe (if enabled).
  • Borofloat high-temperature glass lens
  • Red laser 640-660nm Runs 18 hours (laser only)
  • Length 2.15 inches (5.46 centimeters) Weight 2.64 ounces (74.84 grams)
  • User-enabled strobe function.
  • Low profile ambidextrous switch
  • Safe off feature to prevent accidental activation
  • Rail grip clamp system securely attaches/detaches quickly and safely with no tools
  • Operating temperature: -20°F to +120°F
  • IPX4 water-resistant

Tougher than actual nails, the body and face cap is made of 6000 Series machined aircraft aluminum with a black anodized finish. The switch housing is impact and chemical resistant engineered polymer with elastomeric over the mold. You can get your very own Streamlight TRL-8 HERE MSRP $350.

No matter what kind of shooting you like, AR pistol caliber carbines are a fun way to shoot fast and accurately, even for beginners. Bringing the best from Quarter Circle 10 (QC10), Strike Industries (SI), Nikon and Streamlight, I have put together a minimalist carbine that can defend the house or knock down steel. MP-5 magazines work better than re-purposed handgun mags angled to fit in a pistol grip. Break free and build your own All-American AR.

About the author: Mark Miller is a former Customs Agent and a Green Beret who served in Afghanistan and a number of other live fire locations. A student of firearms and shooting, he is an FFL and a SOT. The guiding philosophy of his life is that terrain and situation dictate tactics and the enemy always gets a vote on any plan.

{ 6 comments… add one }
  • bjd May 18, 2019, 1:41 am

    hi mark, great article. wondering if your qc5 utilizes a last round round bolt hold open or is it manual? thx

    • Mark Miller May 18, 2019, 10:18 am

      Sadly, the MP-5 magazine does not support a last round bolt hold open.

  • Scotty Gunn May 14, 2019, 8:53 pm

    Should use a spacer in the buffer tube,too. Keep the bolt from getting a big run at the bolt stop and breaking it out of the receiver.

    • Mark Miller May 23, 2019, 2:02 am

      The QC10 PCC buffer is longer than an AR carbine buffer. No need for spacers.

  • B R Kurtz May 13, 2019, 8:57 am

    First Im a huge fan of PCCs and Quarter Circle 10; but as for this build I only find it mildly interesting. A true HK type side handle (on the forearm) would be something that got me excited. A side handle on the receiver is rather Meh… HK mags do “look” good. Im waiting for KCI mags to prove themselves.

    The gun seems to scream for a paddle mag release AND a PDW (Honeybadger) style stock.

    If all that seems to negative, I do like the adjustable buffer–THAT is genius!

    • Mark Miller May 23, 2019, 2:07 am

      KCI mags are better than HK mags. The coatings are slick and resist rust. My KCI USA mags function as reliably as my rusty old H&K mags.

      The PDW stocks that I have don’t work with the longer PCC buffer. I would love to find one that does.

      Paddle mag releases are slow forcing mag retention. If you prefer the MP-5, Zenith makes a great one that has everything you want.

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