A 9mm pistol caliber carbine that takes MP5 magazines? That intrigued me enough to build one. This QC5 came as a fully-built pistol with arm brace. I took it apart and replaced some components: 1) Replaced SB Tactical arm bracewith a Gearhead Works Tailhook, 2) replaced QC10 handguard with a sawed-off Centurion Arms rail, 3) replaced A2 type flash hider with a Spike's Tactical Barking Spider 2, 4) added front and rear sights, 5) replaced A2 type pistol grip with Magpul MOE K2, 6) Added sling.



Some observations:
- Both upper and lower fit and finish are excellent.
- Captured spring and detent for rear receiver retaining pin are a nice touch.
- The bolt and carrier are one unit.
- I tried a Vltor MUR-1A upper and found that mags wouldn't seat. The QC5 upper has bevels that allow MP5 mags to insert all the way to the bolt.
- The bolt has a plug that adds weight, probably for proper cycling...

The plug is secured by a roll pin. I asked QC10 if the plug could be removed in order to accomodate a Law Tactical Folder Stock Adapter extension. Danny at QC10 said that the plug weight can be removed, but that I should cut it to length and reinsert it to accommodate the Law Tactical extension while retaining part of the original weight of the bolt.
- During function tests, I found that the Geissele Super Dynamic 3-Gun trigger doesn't work. Danny said that the SSA and SSA-E triggers are the only ones that have been found to work with the QC5 setup.
- A fully-loaded MP5 mag won't seat without considerable force in a bolt-forward condition.
- There is no last round bolt hold open (LRBHO). Danny said that's because MP5 mags weren't designed for LRBHO.
- The QC10 proprietary buffer is heavier than a regular AR-15 H3.

The lack of LRBHO is a bit of a deal breaker for me. Having to load up less than full mags just so they can seat into the mag well when the bolt is foward is another. Off to the range this weekend! I'll have to figure out some funky manual of arms for the QC5.