QUARTER CIRCLE 10 YKMF-5

When an MP5 Isn't

- Alexander Crown

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 pistolcaliber 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.

DESIGN & OPERATION

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.

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.