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Quarter Circle 10: 9mm AR Pistol Build and Review

They’ve been called “legos for adults” and for good reason, the now ubiquitous AR15 is one of the easiest things to build you’ll come across in the firearms arena. The building, or more precisely assembling, a 16″ AR15 in the common 5.56 caliber and with all ‘mil-spec’ parts is relatively easy and fairly straight forward.

You’ll have a ton of different options but for the most part, everything is fairly plug and play. Once you start changing up calibers and barrel lengths things can start to get dicey. Gas length system really becomes an issue, buffer weights, adjustable gas blocks, what magazines will work in the lower I’ve chosen, wait, what type receiver am I using? 

Think of it this way, the 16″ 5.56 AR is like a Honda but the 7.5″ 7.62x51mm or custom 9mm build you plan on running suppressed is like a Ferrari- you shouldn’t just start grabbing any part off the shelf and expect it to run well. You’re going to have to precisely tune it if you want it run properly but if you use the correct parts up front there will be minimal, if any, tweaking to do on the back end.

The standard AR-15 wasn’t meant to run a short 9mm cartridge, nor function as a blowback firearm. If you really want one to run, you should look into dedicated 9mm receivers and corresponding parts.

For this build, we started with the Side Charging Glock Small Frame (GSF) Builders Kit from QC10. Below is a list of the parts that are included in the kit.

  • Glock Small Frame Stripped Lower
  • Side Charging Upper Receiver
  • 1/2×28 threaded barrel
  • G9 bolt assembly
  • Adjustable buffer kit
  • Parts needed for use of 9mm Glock mags

To complete the build you’ll need to pick up the most common items people like to customize such as the brace (or stock if building an SBR), grip, trigger, and rail.

The upper receiver will be the easiest to complete, you’ll only need a rail and corresponding barrel nut. Initially, my plan was to use Geissele MK8 rail in DDC but I completely forgot that the side charging (and forged) uppers don’t usually play well with anti-rotation tabs for on that rail. This was my fault and luckily I have a very sweet 300 Blackout build that I was looking to upgrade, so you’ll see that rail in an upcoming video.

As I went about finding a new rail to use on this build I found myself back at the QC10 website, they have specifically cut rails to match perfectly with their barrel lengths. After I ordered the new rail it was less than a week later it was on the porch and I was ready to finish the build.

The lower, while it contains the key 9mm components, is still technically a “stripped” lower so you’ll need a lower parts kit to finish it off, we picked one up without a trigger to save some cash. 

For the trigger, we selected the Advanced Combat Trigger (ACT) from ALG Defense. 

Trigger selection on a 9mm AR is critical to good performance. Your favorite AR trigger might not be the best choice to interface with a 9mm bolt. Go with the proper profile and save yourself some grief.

Remember above how I alluded to building a Ferrari? Timing is everything on a 9mm AR and selecting a trigger with a profile that isn’t ideal means you will be having a negative effect on the speed of your bolt in BOTH directions. You can try and offset that by tweaking with the variable weight buffer that comes with the QC10 Builder Kit or… you can get out ahead of it from the start and save yourself some grief.

Getting this correct, right out of the gate, means you shouldn’t have to lower the buffer weight. Why does that matter if an adjustable buffer is included? If you add a silencer then you might have to change the buffer weight. The same goes for when you remove it. It’s easier just to get it right the first time.

This leads us to the buffer spring. I chose a Sprinco Red Buffer spring. Again, purpose-made for Pistol Caliber Carbines (PCC). 

Buffer weight. In our case, I didn’t have to tinker with the buffer weight at all. The gun runs flawlessly with or without a silencer. Instructions are included with the kit on how to properly change weights and use the included spacers. If you need to adjust them, it’s very easy, just remember to follow the spacer placement guide.

9mm AR build
Every single component was purposefully selected to run in a 9mm build.

Brace, stock or slick buffer tube? There is no wrong answer here but if you have a barrel under 16″, you must have your SBR tax stamp in hand before you attach a stock. We selected the SB-PDW Brace from SB-Tactical. It is a 3 position adjustable brace that doesn’t just look good, it’s also highly functional. 

Parts List: Quarter Circle 10 9mm AR

Total build cost $1983.75

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