The AR platform is flexible enough to support such a conversion. Also, since there are so many quality aftermarket upgrades, the AR platform will allow me to customize this 9mm AR to my exact needs. I've been wanting to do this for awhile now but it was difficult to find a 5" 9mm AR barrel that was threaded in 1/2-28TPI at the muzzle. This is important to me because I want to use my existing 9mm suppressor. With the current state of affairs, it was darn near impossible to find ANY AR parts, let alone specialized 9mm AR parts. As fate would have it, I received 4 separate emails from various vendors on the same day and was able to find all the parts for my build. I found the elusive 1/2-28TPI barrel, an upper that was actually marked 9mm. It's made by YHM, which is the same maker of my free float forend. So, here we go!
We start out with the YHM (Yankee Hill Machine) 9mm marked upper. The upper is no different than any other .223 upper, but I like that it was actually marked "9mm". This came from Gunbroker.com.
* I'm reusing the free float forend from AR Build #2, since it's also a YHM brand item.
* The 9mm bolt carrier is from CMMG. I chose this one because the bolt is designed to use a standard .223 hammer. Most others require you to change out the hammer to a dedicated 9mm hammer. By going this route, I can quickly swap back and forth from 9mm to .223 and not have to break out my armorer's kit. The 9mm is a blowback design, so no need for a gas tube or a front gas block.
* 5.5" 9mm barrel, threaded 1/2-28TPI came from ADCO.
* Standard AR bolt charging handle.
* Hahn's Precision bottom install 9mm mag adaptor. There are many companies that make a 9mm magwell adaptor block. Hahn's make the best and the other good ones just copied their design. Hahn's makes it in two version. Top install and bottom install. Top install is a "drop-in" and quicker to switch calibers but, I lose the "Last Round Bolt Open" feature. The bottom install needs an Allen key to lock it in place. It take a few seconds and I have to carry a tool, but it allows for the "Last Round Bolt Hold Open".
Receiver goes into the vice block, barrel is installed and barrel nut is torqued to 40 ft-LB. The jam-ring is threaded on and installed all the way to the back of the barrel nut. Next, free float railed forend goes on.
Once the free float is screwed in as far as it will go, I back it off and align it to the top top rail of the flat top receiver, then finger tight the jam-ring. I use a picatinny scope riser to align the flat top receiver and the top rail of the forened. Basically, I install it across both, and snug down the knobs. This will hold both of the rails evenly, so I now they are level with one another. Once the picatinny scope riser is tighened, I turn it over and put the riser into the vice, then tighten the jam ring with a hammer and flat head screwdriver.
BooYa! This turned out to be pretty sweet looking! Now, I can share the 9mm suppressor between my MP5, this 9mm AR, and any 9mm handgun in my collection. So HAPPY! Since the barrel is shorter than the handguard, it "should" only be shot with the suppressor installed. Otherwise, I should get a long flash hider so it sticks out past the handguard.
It's really hard to see how this Hahn's Precision bottom install mag adaptor works. Basically, you insert it from the bottom (like you're loading a magazine). It will click in place just like a magazine. If your press on the magazine release button, it will fall right out. So, to make it say inside the mag well, there is an adjustment bolt. You insert an allen key into the front hole and turn the bolt. This puts force on the roller that extends forward and "wedges" the mag adaptor into the mag well, keeping it from falling out. It takes only 3/4 of a turn to get it to lock. Now that the mag well adaptor is locked in place, the magazine catch will actually function as a magazine catch. The 9mm AR mags will click in place and release when you press the magazine release button.
The mag adaptor has it's own feedramps, ejector, and bolt catch lifter to lock open after the last round is fired.
I hand cycled some dummy rounds and it seems to be fine. Put on some Magpul MBUS sights in the front and rear, and called it done.
Project #3 is done.
Note: A 9mm AR uses a heavier recoil buffer than a .223. In most cases, you can get away with using a heavier buffer on a .223. It will actually eat up some of the felt recoil and slow the bolt down. As you move up in weight , eventually you will move up so much that the bolt will not come back far enough to pull a new bullet out of the magazine. It will eject, but it won't feed the next round. At that point, you will need to back some of the weight off.
In the case of a piston gun, the heavy buffer is a no-no. There is NOT enough gas blowing backwards to cycle the bolt carrier, and a heavier buffer will cause the rifle to have a Failure to Feed. The trick is to find a combination of a slightly overweight recoil buffer and an overpowered recoil spring. This will be an ongoing experiment. I can also cheat. I can use a heavy buffer with a standard recoil spring when shooting 9mm. When I switch to the piston 223, I can just run it under 100% power instead of 50%. That'll work, too. For now, I will be lazy and go this route. :)
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I decided not to do a write-up on Project #4. It's an upper receiver swap, just like in Project #1 and #3. The steps are identical for removing the traditional upper and install the DPMS upper from Project #1. The stiffness and weight of the DPMS will serve this 20" Varminter well. It should increase the accuracy by making the upper as stiff as the lower.
Well, not a bad way to spend 5 hours. The garage was not as hot since it was overcast all day. Now, just need to wait for cooler weather and take all of these rifles out to resight them in again.