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[Review] Quarter Circle 10 YKMF-5: The MP5 That Isn’t


Pistol-caliber carbines are all the rage nowadays and Quarter Circle 10 is no stranger to the game.

They offer a full line of products dedicated to pistol-caliber carbines that allow users to put together dedicated platforms instead of having to modify existing parts to work correctly. 

Quarter Circle 10 was one of the first to market dedicated receiver sets for 9mm, .40 S&W, and .45ACP into AR platforms. Here we examine their latest offering, the YKMF-5, a 9mm side-charging pistol that looks like a PCC had a wild night with an MP5. 


The YKMF-5 is a special build that takes popular aspects of AR pistols and mates them with the Heckler & Koch MP5. 

To explain further, let’s analyze two popular operating systems for pistol-caliber firearms. Many 9mm AR platforms use what’s called a direct blowback design in which, once a cartridge is fired, the gases that propel the bullet down the barrel are also pushing the bolt backward to operate the system. 

This is a very simple and reliable system that has worked for decades in submachine guns and other platforms. The downside to this system is that heavy bolts and springs are necessary to slow down the bolt to ensure proper function and timing. 

Quarter Circle 10 YKMF-5 (7)
Dead Air KeyMicro mount and 12 o’clock mounted SureFire.

Another is the roller delated blowback system, which is commonly found in the MP5. This design uses rollers that are built into the bolt that lock into positions in the firearm. 

When the gases that push the bullet down the barrel begin to act rearward on the bolt, these rollers prevent immediate release and create a delay. This delay is timed so that the bullet has left the gun before another is fed. 

The upside to this design is the lack of heavy bolts and springs; the downside is it can be more expensive and complicated to produce. 

The YKMF-5 is a direct blowback design. It comes equipped with a side-charging handle, which is uncommon among AR-type platforms and a nice touch. We expected the side charger to be stiff given the direct blowback’s need for heavy springs but were surprised to find it was very manageable. 

This is a non-reciprocating charging handle as well and is machined with large purchase. If you’re a fan of AR-style controls, and chances are you’re at least familiar with them, then the YKMF-5 has you covered, Quarter Circle 10 was smart when they decided to include the Reptilia Corp. CQG Grip and Phase 5 Ambidextrous Selector Lever. The grip is a great size for this compact gun. 

The lower also features a standard AR-style bolt hold open and a push-button magazine release opposed to the paddle type commonly found on MP5s. In our opinion, the push button is a better and more ergonomically pleasing solution. On the subject let’s talk about magazines. 


Quarter Circle 10 opted for MP5-style magazines, which are universally known to be reliable and of high quality; however, they’re expensive and not what we’d consider readily available. 

With the incredible number of manufacturers offering Glock magazines in all shapes and sizes and at all price ranges, they’ve become a standard for 9mm. CZ Scorpion mags also have a robust aftermarket. The MP5 has excellent magazines, and they perform well, plus they look sexier than the others. 

Quarter Circle 10 YKMF-5 (6)
Unlike an MP5, the YKMF-5 breaks down exactly like an AR-style firearm.

Quarter Circle 10 did some homework before launching this platform. Small guns like this are very often adorned with the SB Tactical PDW brace, and it allows adjustability and a solid shooting platform while keeping overall length to a minimum without sacrificing any reliability. 

Just like everything from Quarter Circle 10, the bolt and buffer are beautifully machined. The bolt has a built-in weight for taming the blowback system and tips the scales at 15.2 ounces. The beefy buffer and spring are another 8.7 ounces. 

The front end of the gun has a 5.5-inch barrel and includes a Dead Air Micro Brake for attaching silencers that have the Dead Air KeyMicro system. 

This is nice for someone who may already have one of the Dead Air silencers, but for anyone who doesn’t, this device will likely have to be replaced or removed. The threads are ½-28, however, for virtually every other 9mm can on the market. 

The handguard on the YKMF-5 is from Bowden Tactical feels sturdy and well-made. It’s 4.25 inches long and mimics the old MP5 style. The drawback to this handguard is the complete lack of attachment points minus the 12 o’clock rail. 

Quarter Circle 10 YKMF-5 (4)
SB Tactical is practically an industry standard for clean lines and compact overall length.

This means no hand stops, no flashlight, no pressure pad — nothing. The handguard simply looks like an MP5, that is all.


The YKMF-5 functioned flawlessly on the range. Not a single hiccup. 

The Dead Air KeyMicro brake tamed a majority of the felt recoil of the blowback system. Something of note is that MP5 magazines don’t have a last round bolt hold open, so at the end of the mag the bolt closes and you receive a “click” letting you know you’re empty. 

Quarter Circle 10 YKMF-5 (1)
Quarter Circle 10 is known for high-quality parts and they continue that reputation with the dedicated bolt and heavy buffer.

This may or may not be beneficial, depending on who you ask. In our opinion, it’s a step in the wrong direction. The trigger was crisp with little play and appears to be a standard Mil-spec trigger found in most AR-style firearms and, given that this is in 9mm a lighter trigger, doesn’t really feel necessary. 

Accuracy was perfectly acceptable on the 25-yard line, giving us a nice, ragged hole in the center of the target. 


There’s no doubt that Quarter Circle 10 has produced a high-quality 9mm pistol. 

Basing this design around the MP5 magazine is an interesting choice given the plethora of other equally reliable and cheaper magazines on the market. 

It stands possible that the target customer for this is someone who already has numerous MP5 magazines but wants an AR-style control group, or maybe a department that used to have MP5s but is going in another direction. 

The YKMF-5 comes equipped with many features shooters would’ve added after the fact and some they may not have. 

This coupled with the well-made parts commands a price tag of $1,775 — not a small amount of money and about equal to some of the MP5 clones on the market and other high-end 9mm ARs. You wouldn’t be amiss in selecting the YKMF-5 if it fits your exact needs, but we wouldn’t kick an MP5 out of bed for it. 

Quarter Circle 10 YKMF-5 (3)
SB Tactical is practically an industry standard for clean lines and compact overall length.


  • Quarter Circle 10 YKMF-5
  • Caliber: 9mm
  • Barrel length: 5.5 inches
  • Overall length: 21 3/8 inches 
  • collapsed; 23 7/8 extended
  • Weight: 5.14 pounds
  • Magazine capacity: 30
  • MSRP: $1,775
  • Website: 


  • OuterImpact adjustable Co-witness mount
  • Vortex Viper red dot
  • SureFire X300U 


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