Estimated reading time: 12 minutes
Why wouldn’t you want to shoot 9mm out of your AR? Ammo is cheaper than 5.56 and more than likely you can use the same ammo you use in your 9mm EDC gun in your AR. Converting your 5.56 NATO AR to 9mm is a really simple AR hack, especially if you buy a complete 9mm upper. And you will want to buy a complete upper since some are relatively inexpensive. You can have your 5.56 AR shooting 9mm as fast as field stripping your gun.
Table of contents
How Do You Convert An AR-15 to Shoot 9mm?
All you need are four parts to get running: a complete 9mm upper, buffer, magazine well adapter, and magazines. Fine, it’s five parts. Iron sights or an optic is also required. Let’s take a quick look at each of these components because there are major differences in an AR-15 chambered in 9mm and one chambered for 5.56 NATO.
Colt vs Glock AR-15 Adapter
First you need to decide if you want to feed off Colt or Glock magazines. There are some adapters that feed off Beretta 92 or S&W M&P magazines, but the Glock and Colt types are the most popular. The Colt pattern magazines are all steel; Glock style are polymer. The Colt 9mm rifle was the original AR-15 designed to utilize stick magazines that are actually modified Uzi type 32-round magazines. Glock magazines are found nearly every where, and come is variety of round counts from anemic 10-round magazines to obese 33-round magazines.
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Plus non-OEM Glock magazines are compatible, too. One thing to note if you opt for either Glock or Colt magazines, your BCG needs to be compatible with your magazine choice, which I’ll get into.
The adapter or conversion block is a part that fits inside the existing 5.56 NATO magazine well. Since 9mm magazines are smaller than standard AR-15 magazines, the adapter fills up the space. The adapter also has a built-in ejector and feed ramp.
The ProMag Quick Change Adapter, Hahn Precision Dedicated 9mm Conversion Adapter, and others are Colt-style adapters. They drop-in the top of the lower receiver and are held in place when the upper and lower are assembled. Both the ProMag and Hahn adapters lock back the BCG on last round fired. The magazine release with function that same as it does with 5.56 NATO magazines.
Mag Options
For this hack, I opted for a Glock magazine build since I have plenty of Glock magazines on hand. The Stern Defense MAG-AD9 adapter is similar to a Colt-style adapter in that it has an ejector and a feed ramp built in. The difference is that it installs from the bottom of the magwell, as if you were inserting a magazine into your AR. A set screw and the magazine catch hold the adapter in place.
The Stern Defense adapter has its own magazine release built in which is easily operated by the trigger finger of a right handed shooter. If you press the AR-15 magazine release, no worries. The Stern Defense adapter is locked it place, but easily removed by backing out the set screw.
BCG For A 9mm AR-15
The operating system for a 9mm AR-15 is different than a 5.56 NATO AR-15. Pistol caliber ARs use a direct blow back system. A 5.56 NATO AR uses a direct impingement (DI) gas system. The DI system pipes burning gases from a fired round at the muzzle through a gas tube. It goes into a gas key on the BCG to cycle the rifle. A blow back system uses the energy and the motion of the fired cartridge case to operate the BCG. There are differences in a 9mm BCG and 5.56 NATO BCG like the bolt face since the case rim of a 9mm round is different than the rim of a 5.56 NATO round.
There are three types of 9mm BCGs, those cut for Glock magazine, those cut for a Colt magazine, and a hybrid that is compatible with both Glock and Colt magazines. A Glock compatible BCG feeds from a single stack at the feed lips of the magazine. A Colt compatible BCG feeds from a double stack at the feed lips. The portion of the bolt that pushes the case head of the cartridge into the chamber varies between the two. A hybrid BCG is cut to push cartridges from either a Colt or a Glock magazine. You still need to use the compatible magwell adapter, however.
Use A Heavy Buffer
I’m converting a Palmetto State Armory PA-15 16-inch barrel AR with a carbine length gas system. A heavier buffer is needed when using the carbine spring. Stern Defense makes a 4.5 ounce buffer that works with a carbine spring. A heavier buffer offers more rearward resistance when the BCG cycles back helping to reduce felt recoil and ensure reliable functioning. A lighter carbine buffer may not be as reliable.
Complete Upper
For this hack, I used a Bear Creek Arsenal BC-9 complete upper with a right side charging handle. It had a 16-inch barrel, and a 15-inch M-LOK rail. It comes with flash hider and a heavier buffer. The M-LOK hand guard is void of Picatinny in the center gripping area. This makes it more comfortable to shoot with a thumb-over grip.
The complete upper came with the blowback BCG. At the end of the upper is knurled screw with a flat side. The flat side mates flush with the lower when the upper and lower are assembled. This screw needs to be removed along with the changing handle to remove the BCG from the upper. You can pay more for an upper, but since I wanted an inexpensive built I figured I’d try the Bear Creek Arsenal upper. I wasn’t disappointed as you will see.
Aimpoint ACRO P-2 Red Dot
The most expensive component was the optic. And in all honesty I swap the Aimpoint ACRO P-2 from rifle to rifle and between pistols when I need a good red dot. The ACRO P-2 is the latest generation of the ACRO enclosed red dot sights. It has a 3.5 MOA dot that is perfect for close to medium distance engagement. The one bitch I have with the Aimpoint is you need a special tool or a Torx wrench to adjust the reticle. Why not just use a slot so the rim of a fired case can be used? It must be a Swedish thing.
Firing Up The 9mm AR-15 Conversion
My assumption was there would be a break in period, at best, with this build. The more non-OEM components you use in a build the more chance for introducing errors. Not the case with this build. I loaded up the 33-round Glock OEM magazine, a dirt cheap KCI 33-round magazine, and a Magpul PMAG GL9 21-round magazine. For ammo I used 124-grain FMJ by Armscor and Hornady Critical Defense 115-grain FTX hollow points. Range was 15 yards and the target was a D-1 tombstone style target.
The first thing you notice is the lack of muzzle blast with the 9mm. Next you notice the rifles cycles differently. The BCG clunks back and forth because of the blowback action. DI systems cycle much smoother. The third thing you notice is how easy it is to burn through the 33-round stick magazines. I had no issues, and started to erode the 8-inch circle on the D-1.
READ MORE: Does An AR-15 Use A Direct Impingement System?
There is something about a 9mm rifle and red dot that allows you too shoot fast and easily stay on target. I needed a screw driver to remove the grin on my face. All the magazines worked perfectly. The Bear Creek Arsenal upper ran like a…bear. On last round fired the bolt locked back. Muscle memory kicked in for the first few magazines and I pressed the AR-15 magazine release. The Stern Defense adapter stayed in place. The magazine release on the Stern Defense is easily in reach with my trigger finger.
Is A 9mm AR-15 Conversion Worth It?
A 9mm AR-15 conversion allows you to get more mileage out of your AR-15 lower. A dedicated 9mm lower is the best solution for a 9mm AR-15, like a Springfield Armory Victor Saint AR15 9mm, which uses Colt-style magazine. But, if you want to shoot 9mm on the cheap out of your AR-15, a conversation kit is the way to go. Remember to buy economy packs of 9mm ammo. You’ll burn through ammo fast and have a lot of fun doing it.
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