Building a 9mm AR-15 Pistol

 

In this post I want to talk about a 9mm AR-15 pistol that I built.  This isn't going to be a complete, in depth article about how to assemble a lower receiver, but will talk about the parts and specifics of building an AR-15 in 9mm.  While 9mm may not be most people's first choice for an AR-15, there are several benefits.  Let's take a look at why 9mm and what you need to build one:

 

Why 9mm?

First off, why 9mm?  Before building the 9mm, I had AR-15s in 223 Rem/5.56mm, 6.8 SPC II, 300-221 Fireball (like a 300 Whisper), and lastly, my "Thumper" 450 Bushmaster.  I picked each of those calibers for specific reasons and will probably write more about my experiences with each at a later time.  

The one thing I noticed is that each caliber was more expensive and/or more labor intensive than 5.56mm.  With the 300 Whisper I was making brass from 223 brass.  While the overall cost was fairly low, it requires a lot of labor.  The 6.8 SPC II and 450 Bushmaster have significantly more expensive brass.  

I had an extra lower receiver laying around and wanted to build another rifle.  I thought of several different calibers, but wanted to build something that would be cheap to shoot.  I thought about a .22 Long Rifle, but wasn't too impressed with many of the options.  I also have a Ruger 10/22 and really enjoy shooting it.  Ultimately I started looking into 9mm.  I like the fact that 9mm can be loaded inexpensively with lead bullets.  It has low recoil in a rifle, but can still be used in self defense, if necessary.  

While thinking about the 9mm, I thought that a 16" barrel seemed a little unnecessary.  I started thinking about a short barreled rifle and decided to build a 7" barreled pistol.  That would allow me to install a short barrel and shoot it now, but if I decided to register it in the future, I could add a stock and convert it to a rifle.  

Specific parts for a 9mm AR-15: Barrel, bolt assembly, buffer, magazine block.

9mm Specific Parts

The 9mm requires a few specific parts compared to a standard 5.56mm AR-15. But, as you can see in the video above, it's not too complicated.  From the top down, you will start with a 9mm bolt.  The 9mm bolt is a one piece bolt and carrier, unlike the two part 5.56mm bolt and carrier assembly.  Next, is a 9mm barrel.  The one above was from JSE Surplus.  It is contoured to accommodate a gas block if you use a pistol length handguard.  You will note that there is no gas port.  The pistol round is blow back operated, so it doesn't need a gas tube.  Next is the buffer, which I will talk about below.  Lastly, you will need a magazine block.  The converted UZI magazine pattern that fits in Colt-style 9mm AR-15s is much smaller than a standard AR magazine.  The magwell needs to be changed to fit the 9mm magazine.  The magazine block also has feed ramps and a fixed ejector.

Upper: 9mm long buffer, Lower: standard H buffer.

Because of the blow back action of the 9mm, a heavy buffer needs to be installed to keep the bolt locked in place long enough for pressure to dissipate.  Several companies make heavier buffers.  I chose a Yankee Hill Machine 9mm buffer.  Not only is it a heavy buffer, it is slightly longer than a standard buffer, as you can see in the picture above.  The extra length keeps the buffer from moving too far back in the receiver.  If it goes too far back, ejected brass could potentially fall into the hammer area of the lower receiver.

9mm magazine block installed.  Note the feed ramps and ejector.

Above is a picture with the magazine block installed.  You can see the ejector sticking out.  At the front are the feed lips that aid in getting the bullets from the magazine to the barrel.  This particular magazine block doesn't have any accommodation for the bolt stop.  Unfortunately, that means my magazines don't hold the bolt back after the last round.

Installing the barrel.

The picture above shows the barrel installed in the receiver.  The focus is near the receiver because I want to point out that you want to make sure the pin on the barrel is properly lined up in the receiver.

Yankee Hill barrel nut attached, ready for the handguard.

Next, I installed the barrel nut that comes with the Yankee Hill Machine(YHM) diamond free float handguard.  You'll also notice the jamb nut screwed against the receiver.  With the 9mm barrel I didn't have to worry about lining up the holes in the barrel nut with the gas tube hole in the receiver.  The Yankee Hill forearm uses screws to lock the handguard in place, preventing it from rotating during use.  I decided to line up one of the holes with the receiver gas tube hole anyway.  Just to make sure the anti rotation screws lined up properly.

Install the flash hider before the handguard.

Next I installed the flash hider.  The barrel nut won't slide over the flash hider, but there is no problem installing the handguard over the flash hider.  Since I chose a handguard length that would come to the very end of the barrel, I wanted to make sure the flash hider was installed before I blocked the wrench flats on it with the handguard.  

Handguard installed, note the free float.

As you can see above, the handguard has plenty of clearance around the barrel and flash hider!  If you don't want to have the gaping hole, Yankee Hill makes a cap the screws into the end.  I added it to one of my forearms, but not this one.

Side view of the Yankee Hill diamond pattern.

Here is a profile shot of the diamond free float tube from the side.  Some of the features I like about the handguard are the continuous top rail, anti-rotation screws and laser engraved rail markings.  I also like the look of the diamond patter, it looks better than their standard lightweight tube, which you can partially see in the bottom left corner of the picture below.

Complete 9mm pistol with MOE grip, Magpul MBUS and Yankee Hill diamond handguard.

Above is the completed pistol.  So far the gun has ran great with both FMJ ammo and hollow points.  Below is a short video of me shooting it at the range.  

 

In the picture above you can see the parts from my 6.8 SPC rifle.  I had is disassembled so that I could paint it with Brownells Alumahyde II in coyote.  You can see my comparison between Alumahyde II and Magpul FDE here.  You can also see my easy and inexpensive guide to installing your front take down pin here.

Complete Yankee Hill Diamond Freefloat installation video

 

 

9mm AR Pistol in Action

 





So did this help you with choosing 9mm, or building one for yourself?  Anything else you want included, or do you have additional questions?  Let me know in the comment section below.  

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