How to Convert an AR-15 to 9mm Caliber

| Last Updated: December 28, 2021

Changes in the popularity of various ballistics types have been constant over the years. The modularity of guns like the AR-15 makes it possible to convert the rifle to a different caliber a rather simple process.

Let’s have a look at how to go through the process and make some recommendations about the best kits available to help you through it.

In a Hurry? Here is Our Top Pick:

Easily installed on all AR-15 lowers that are mil-spec

Transform your AR-15 into a 9mm quickly and easily

The ultimate solution for converting your AR-15. 

Made from top of the line materiel. Lightweight and sleek. Hard pressed to find a better deal on the market.

Advantages of a 9mm AR-15

When you switch to 9mm you get to enjoy a few factors.

One is the lower the cost of ammunition. Obviously you’re going to put this to a different use, but shooting 9mm is going to save you a bit per shot.

The muzzle blast and noise is another factor. The smaller caliber is going to lessen the noise and maybe make it just a little more comfortable to shoot.

Compared to shooting with a pistol, you gain a bit of range and speed when shooting with an AR. There are of course other, more powerful options for shooting a 9mm, but if you like the AR platform it’s a decent choice.

Finally, if you find it practical to stock up on one type of ammo, choosing 9mm might serve you well. The cost and versatility make it a good choice when you want to have a good supply on hand for various uses in case of SHTF scenarios.

Comparison Chart of the Best 9mm Conversion Kits for the AR-15

  • Adjutable Length of Pull and Cheek Riser
  • Top Rail Makes for Easy Mount of Sights or Scopes
  • Durable carbon-fiber filler, polymer construction
View Latest Price →Read Customer Reviews
  • Fully Customizable
  • Lightweight, Yet Extremely Durable
  • Equipped With a Sling Swivel Stud
View Latest Price →Read Customer Reviews
  • Lightweight, Yet Solid Aluminum Construction
  • Picatinny Design Fits a Variety of Scopes and Sights
  • Made in the USA
View Latest Price →Read Customer Reviews
  • Durable Steel Body for Continuous Shooting
  • Covered by a Lifetime Guarantee
  • Easy to Dismantle for Cleaning
View Latest Price →Read Customer Reviews
  • Durable Leather Body for Longevity
  • Aesthetically Pleasing Design
  • Quality Construction to Withstand Pressure
View Latest Price →Read Customer Reviews
  • Versatile Design Fits Many Weapons
  • Cheek Pad Included for Proper Shooting
  • Easy and Comfortable to Use
View Latest Price →Read Customer Reviews
  • Wide Height Adjustment
  • Quality Construction to Withstand Stress
  • Adjustable Cant for Taking Difficult Shots
View Latest Price →Read Customer Reviews
  • Easy to Install on the Rifle
  • Fits Snugly and Eliminates Probable Play
  • Replaces Bolt Stop Cover Plate
View Latest Price →Read Customer Reviews
  • One-piece Construction
  • Water/Fog/Shock-Proof Design
  • Bushnell's Dead On Accurate (DOA) 600 Reticle
View Latest Price →Read Customer Reviews
  • Flip-up Design Offers Control of Use
  • Extremely Affordable
  • Made in the USA
View Latest Price →Read Customer Reviews

What to Watch For When Buying a 9mm Conversion Kit

Converting an AR-15 to a pistol caliber might sound daunting. However, 9mm conversion kits help to make it a streamlined and straightforward process. Some gunsmithing experience is useful, but even a novice or beginner shouldn’t have much of a problem completing the process. Choosing the best 9mm conversion kit is the best place to start. The following information will help you learn what to look for when shopping.

Why Would I Do This?

That question might run through your head when you consider the possibility of converting your existing AR-15. It’s a fair question to be sure. You already have a perfectly fine AR-15 chambered in your favorite rifle caliber. What’s the point? Why not just buy a separate pistol caliber carbine? Well, there are several reasons a conversion is ideal.

One of the main reasons is that you will be able to avoid a lot of the waiting time, fees, and hassle of buying a complete firearm. Another big reason is that you will be able to use your existing lower, and any upgrades you have made to it such as a nice trigger, instead of starting fresh.

Just in case you need a few more reasons, converting your AR-15 to 9mm reduces the cost and fatigue of high-volume shooting during training or range days, makes it an excellent home defense option, and allows you to pair it with your favorite handgun and even share magazines between them.

What Are My Options?

Converting an AR-15 to fire 9mm rounds can be accomplished in several ways depending on your preference. Replacing the existing bolt carrier group (BCG) with a 9mm BCG and replacing the entire upper with a complete 9mm upper is one option. This will allow you to use your existing lower.

Another option is to convert the existing upper to work with 9mm. This method is a little more involved as it requires replacing parts such as the barrel instead of just dropping in a new upper. Consider your skill level and how involved you want to get when deciding on a 9mm conversion kit.

Magazine Compatibility

One of the best parts of pairing your converted AR-15 with a pistol of the same caliber is that you will be able to share magazines between them. This is not only a time-saver when taking both guns to the range, but it is also an advantage in a home defense setup. Not all 9mm conversion kits accept the same magazines so pay attention to that when shopping. The most common magazine styles in conversion kits are Colt-style, Uzi-style, and Glock-style.

Review of the Best 9mm Conversion Kits for the AR-15

Below, we've created this brief guide to enable you to make an easier decision would you use a mag well block that converts your conventional AR-15 lower to accept either Colt or Glock magazines.

Our Top Pick:
Stern Defense MAG-AD9

The Stern Defense MAG-AD9 is in the upper price level AR-15 Conversion Kits. Originally it made from lightweight 6061 T6 aircraft grade aluminum, it’s now fabricated from 4140 alloy steel.

This 9mm AR adapter allows the Glock-style double stacked 9mm or .40 S&W magazines to be used with no required modifications to your lower or magazines.

The Stern Mag-AD9 series adapters feature a last-round hold-open feature and built-in magazine release for Glock-type magazines. The new version boasts the magazine release upgraded to be flush with the magazine well without the bulge that was present in the previous magazine latch.

These adapters require the use of a Glock cut bolt but depending on your bolt selection you may have to shave a little off the top and sides of the ejector.

With the familiar AR-15 plug-and-play system, the Mag-AD bottom-fed magazine adapters install in seconds in any mil-spec AR-15, M4 or M16 lower receiver and turn into a Glock magazine-fed lower. If you use the Stern Defense adapter, you’ll keep all of the gun’s stock components except the barrel and bolt.

Hahn Precision AR-15/M16 Conversion Block

The Hahn Precision AR-15/M16 9mm Drop-In Conversion Blocks are made in two versions, depending on the direction of mounting the block to the lower receiver.

Hahn Precision offers both bottom-loading and top-loading adapters. The bottom-loading magazine adapter inserts from the bottom of the lower receiver and secures with a non-marring tension feature, preventing accidental release.

On the other hand, the Top-Loading Block drop in from the top of the lower receiver and requires the removal of the bolt release. The drawback is that it won't be swappable in the field.

The Hahn one-piece Conversion Blocks are made from aluminum billet and are compatible with all Colt-type and modified UZI magazines.

The adapter boasts a last-round hold-open feature on a top loader. There are two types of bottom loaders: with and without the hold-open device.

The bottom-fed mag adapter without the built-in hold-open requires an extended catch to hold open on the last round. It’s weaker than the .223 version, and it can easily break during extensive use.

The third bottom-loading adapter has a built-in standard 5.56mm bolt hold-open functioning in conjunction with a feed ramp. It’s cut for reliable feeding with all types of bullets, including hollow point and frangible ball loadings.

Rock River Arms - AR-15/M16 Conversion Block

Another conversion kit comes from the Rock River Arms. It uses the Colt-style system which is very popular since they’re half the price of a genuine Colt. The Rock River Arms 9mm magazine block is inserted up into the mag well and held by the standard mag catch. You can use it with the standard Uzi magazine but without bolt hold-open feature.

The Rock River Arms Conversion Block is a drop-in solution made of aluminum and steel and fits a mil-spec small pin (.154″ dia.), .223 lower receiver. This bottom loader uses a set screw to secure the magazine block, enabling the magazines to use the standard magazine catch. The RRA conversion block will only hold open with Colt and modified UZI magazines.

The only criticism of this slightly cheaper setup is that with UZI magazines, the follower’s angle is too steep, which means that frequently the bolt will not strip the last 1-4 rounds out of the magazine.

Colt AR-15/M16 Conversion Block

The original Colt 9mm AR-15 magazine well adapter alters the standard AR lowers to use Colt 9mm style magazines. This bottom loader uses the normal AR-15 bolt catch and has a last-shot hold-open feature.

Machined from rugged aluminum, the funneled magazine well ensures smooth entry of Colt-style 9mm magazines. Its feed ramp with a steel insert is heat-treated for consistent round delivery with most bullet types.

This block for the AR-15 platform installs quickly and easily using a setscrew to lock down and secure it.

The main drawback of Colt conversion block is more expensive magazines compared to the other block-style surplus types.

Brownells AR-15/M16 W/32rd Magazine Conversion Block

The Brownells AR-15 Drop-in conversion block is one of the higher-priced adapters, but it also comes with a Colt-style 32 round magazine.

This adapter, made of an aluminum block and hardened steel ejector, converts your standard AR-15 model to accept the Colt-type magazines.

The Brownells conversion block is compatible with Colt-type, 9mm upper receivers while the feed ramp is cut for reliable feeding of all types of ammunition, including hollow-point bullets.

A spring-loaded roller retention device maintains constant tension against the lower receiver well to prevent accidental release or free play. Unlike Colt magazines, this Brownells bottom-loading model flawlessly utilizes a BHO feature. However, modified UZI magazines last-round-hold-open will not work.

This conversion block is easy to install into an existing AR-15 lower receiver. When it is paired with a 9mm bolt carrier, there is no need to change out the standard hammer.


  • Easy to Install
  • Plug-and-Play Design
  • Aesthetically Pleasing
  • Made from Aluminum
  • Bolt Hold-Open Feature


  • Might Require Fitting

The AR-15 to 9mm adapter from ATI has been made from 6061-T6 aluminum and fits any standard 5.56 lower receiver. The adapter features a built-in magazine release, suitable for Glock-style 9mm magazines.

The adapter slides inside the mag well of the upper receiver, just like a normal AR-15 mag would do. The bolt hold-open feature in the adapter locks the bolt in place after the last round is fired, which is a good safety and operational measure.

The adapter works with Glock doublestack-style magazines, plus it comes with a 10 round 9mm magazine, included in the package. The adapter has been machined from aluminum, so it is durable and strong enough to withstand shocks and pressure.

Bottom Line

The ATI AR-15 9mm adapter features a durable and simple design to convert your AR-15 into a 9mm rifle almost instantly. The adapter is comparatively less pricey than some of its counterparts, however, it still may require some fitting to eliminate play.

How Tough Is This Conversion?

Being a king of modularity, the AR-15 deserves the nickname the Lego of the firearm world. Any owner knows how easy it is to make adjustments to the gun and add a limitless array of customizations.

Gun owners can choose from several options for converting a mil-spec AR-15 to 9mm. The easiest way is to swap the upper assembly for another completed upper receiver.

You could also just use a conversion kit that replaces the bolt carrier group.

The first and more expansive approach implies a straightforward drop-in to fit your Lower Receiver. This package usually consists of an upper assembly including a barrel and a magazine well adapter. It can cost from $700 to well above 1,000 dollars.

The other method means using the AR’s standard upper receivers. You can then convert to 9mm Parabellum caliber by fitting a new barrel, new bolt, and a magazine adapter.

Tools Needed

Building a 9mm upper receiver with a conversion kit is a very simple operation if you're comfortable with a few basic tools. To replace the bolt carrier group and buffer and insert the block into the magazine well you’ll need a barrel nut wrench such as this tool from TAPCO. You’ll also need a receiver vise block like the ProMag AR-15/M16 Upper And Lower Receiver Magazine Well Vise Block set.

Parts To Be Changed

Using a conversion kit does work out to be more affordable, usually, than buying the pieces individually. Separate piece prices fall around $200 for a bolt; the 9mm barrel will set you back another $150-$200; the magazine block $50-$200 and the heavy buffer and heavier buffer spring, $50-$75.

Unlike the AR-15’s regular 20" barrel, the pistol-caliber carbine usually features a 16" barrel or shorter. They may have straight magazines with a 10 or 32 round capacity. There are also 50 and 100 round drum magazines. However, ownership of magazines with more than 10 rounds and a barrels length of less than 16" is illegal and it is subject to penalties in California, New York, New Jersey, Connecticut, Massachusetts, Colorado and Washington DC.

How to Make the Conversion

The installation of the 9mm barrel is the same as any other barrel and can be done without gunsmithing or other modifications. Since the back of the barrel is the same diameter and shape, and the barrel nut is the same, just torque the barrel nut and finish with the handguard of your choice. Just keep in mind that the 9mm barrel has different threads at the muzzle than the .223/5.56 to prevent you from installing an inadequate flash hider on your 9mm barrel.

The massive one-piece bolt is a blowback type and replaces the standard two-piece .223/5.56mm bolt. While it uses the same charging handle and buffer, for the best performance you should install the heaviest buffer.

Another modification made to the rifle would be using a special adapter to allow the use of smaller 9mm magazines. There are two types of conversion blocks: one inserted from beneath and held in position by the mag catch and the other from the top, held in by some bulge and lips. It is recommended to use the conversion adapter drop in from the top.

The barrel lengths ranging from 16 inches for carbine conversions to pistol barrels that are just 3" long!

Can the 9mm Mod Easily Be Converted Back?

Yes, of course. Changing the 9mm mod is as easy as mounting one. All you have to do is to pull out the mod from the mag well, take out the two takedown pins, change the upper, put back the pins, and you’re good to go. The 9mm mods were designed to save you time and money. It prevents you from having to buy a completely new AR-15 chamber for the 9mm.

The mods here are so simple that you’ll never need a gunsmith. If you know how to separate the upper and lower receiver, you can apply and remove the mod within seconds. However, if you don’t know how it works, it takes only a couple of minutes to learn.

The 9mm mods are just plug-and-play devices. You can shoot a 9mm from a 223 platform without changing the upper receiver, however, it is still not recommended. So most of the time spent in changing the mod is changing the upper receiver fit for the 9mm caliber, which amounts to approximately 30 seconds.


For the civilian market, being able to use 9mm ammo in both a handgun and a rifle makes sense in a lot of scenarios. Pistol-caliber carbines are good as home-defense rifles. They’re also great as plinking guns and training rifles for adults as well for youngsters. If you’re ready to make the switch, it’s not a hard adjustment - give it a try!

Josh Lewis the managing editor at Gun Mann and when he isn't writing about guns he is more than likely tinkering with them. He also enjoys hunting, fishing and spending time outdoors. As a lifelong gun owner he knows his stuff!

Leave a Comment